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97-438: 5243 18671 ENSG00000085563 ENSMUSG00000040584 P08183 P21447 NM_000927 NM_011076 NP_000918 NP_001335873 NP_001335874 NP_001335875 NP_035206 P-glycoprotein 1 ( permeability glycoprotein , abbreviated as P-gp or Pgp ) also known as multidrug resistance protein 1 ( MDR1 ) or ATP-binding cassette sub-family B member 1 ( ABCB1 ) or cluster of differentiation 243 ( CD243 )

194-415: A lipid bilayer , made up of two layers of phospholipids with cholesterols (a lipid component) interspersed between them, maintaining appropriate membrane fluidity at various temperatures. The membrane also contains membrane proteins , including integral proteins that span the membrane and serve as membrane transporters , and peripheral proteins that loosely attach to the outer (peripheral) side of

291-574: A P-gp inhibitor. P-gp was discovered in 1971 by Victor Ling . A 2015 review of polymorphisms in ABCB1 found that "the effect of ABCB1 variation on P-glycoprotein expression (messenger RNA and protein expression) and/or activity in various tissues (e.g. the liver, gut and heart) appears to be small. Although polymorphisms and haplotypes of ABCB1 have been associated with alterations in drug disposition and drug response, including adverse events with various ABCB1 substrates in different ethnic populations,

388-419: A fraction of the lipid in direct contact with integral membrane proteins, which is tightly bound to the protein surface is called annular lipid shell ; it behaves as a part of protein complex. Cholesterol is normally found dispersed in varying degrees throughout cell membranes, in the irregular spaces between the hydrophobic tails of the membrane lipids, where it confers a stiffening and strengthening effect on

485-501: A host target cell, and thus such blebs may work as virulence organelles. Bacterial cells provide numerous examples of the diverse ways in which prokaryotic cell membranes are adapted with structures that suit the organism's niche. For example, proteins on the surface of certain bacterial cells aid in their gliding motion. Many gram-negative bacteria have cell membranes which contain ATP-driven protein exporting systems. According to

582-444: A large quantity of proteins, which provide more structure. Examples of such structures are protein-protein complexes, pickets and fences formed by the actin-based cytoskeleton , and potentially lipid rafts . Lipid bilayers form through the process of self-assembly . The cell membrane consists primarily of a thin layer of amphipathic phospholipids that spontaneously arrange so that the hydrophobic "tail" regions are isolated from

679-479: A large variety of protein receptors and identification proteins, such as antigens , are present on the surface of the membrane. Functions of membrane proteins can also include cell–cell contact, surface recognition, cytoskeleton contact, signaling, enzymatic activity, or transporting substances across the membrane. Most membrane proteins must be inserted in some way into the membrane. For this to occur, an N-terminus "signal sequence" of amino acids directs proteins to

776-405: A limited variety of chemical substances, often limited to a single substance. Another example of a transmembrane protein is a cell-surface receptor, which allow cell signaling molecules to communicate between cells. 3. Endocytosis : Endocytosis is the process in which cells absorb molecules by engulfing them. The plasma membrane creates a small deformation inward, called an invagination, in which

873-452: A lipid bilayer. In 1925 it was determined by Fricke that the thickness of erythrocyte and yeast cell membranes ranged between 3.3 and 4 nm, a thickness compatible with a lipid monolayer. The choice of the dielectric constant used in these studies was called into question but future tests could not disprove the results of the initial experiment. Independently, the leptoscope was invented in order to measure very thin membranes by comparing

970-471: A membrane is the rate of passive diffusion of molecules through the membrane. These molecules are known as permeant molecules. Permeability depends mainly on the electric charge and polarity of the molecule and to a lesser extent the molar mass of the molecule. Due to the cell membrane's hydrophobic nature, small electrically neutral molecules pass through the membrane more easily than charged, large ones. The inability of charged molecules to pass through

1067-427: A minute amount of about 2% and sterols make up the rest. In red blood cell studies, 30% of the plasma membrane is lipid. However, for the majority of eukaryotic cells, the composition of plasma membranes is about half lipids and half proteins by weight. The fatty chains in phospholipids and glycolipids usually contain an even number of carbon atoms, typically between 16 and 20. The 16- and 18-carbon fatty acids are

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1164-402: A plasma membrane and an outer membrane separated by periplasm ; however, other prokaryotes have only a plasma membrane. These two membranes differ in many aspects. The outer membrane of the gram-negative bacteria differs from other prokaryotes due to phospholipids forming the exterior of the bilayer, and lipoproteins and phospholipids forming the interior. The outer membrane typically has

1261-438: A polarized cell is the surface of the plasma membrane that forms its basal and lateral surfaces. It faces outwards, towards the interstitium , and away from the lumen. Basolateral membrane is a compound phrase referring to the terms "basal (base) membrane" and "lateral (side) membrane", which, especially in epithelial cells, are identical in composition and activity. Proteins (such as ion channels and pumps ) are free to move from

1358-403: A porous quality due to its presence of membrane proteins, such as gram-negative porins , which are pore-forming proteins. The inner plasma membrane is also generally symmetric whereas the outer membrane is asymmetric because of proteins such as the aforementioned. Also, for the prokaryotic membranes, there are multiple things that can affect the fluidity. One of the major factors that can affect

1455-453: A universal mechanism for cell protection and development. By the second half of the 19th century, microscopy was still not advanced enough to make a distinction between cell membranes and cell walls. However, some microscopists correctly identified at this time that while invisible, it could be inferred that cell membranes existed in animal cells due to intracellular movement of components internally but not externally and that membranes were not

1552-430: Is a pathway for internalizing solid particles ("cell eating" or phagocytosis ), small molecules and ions ("cell drinking" or pinocytosis ), and macromolecules. Endocytosis requires energy and is thus a form of active transport. 4. Exocytosis : Just as material can be brought into the cell by invagination and formation of a vesicle, the membrane of a vesicle can be fused with the plasma membrane, extruding its contents to

1649-424: Is a single polypeptide chain that crosses the lipid bilayer seven times responding to signal molecules (i.e. hormones and neurotransmitters). G-protein coupled receptors are used in processes such as cell to cell signaling, the regulation of the production of cAMP, and the regulation of ion channels. The cell membrane, being exposed to the outside environment, is an important site of cell–cell communication. As such,

1746-414: Is also used to differentiate transitional B cells from naive B cells. Dyes such as rhodamine 123 and MitoTracker dyes from Invitrogen can be used to make this differentiation. It has been suggested that MDR1 inhibitors might treat various diseases, especially cancers, but none have done well in clinical trials. Single Nucleotide Polymorphism rs1045642 (3435T>C or 3435C>T) is important for

1843-446: Is an ATP-dependent drug efflux pump for xenobiotic compounds with broad substrate specificity. It is responsible for decreased drug accumulation in multidrug-resistant cells and often mediates the development of resistance to anticancer drugs. This protein also functions as a transporter in the blood–brain barrier . Mutations in this gene are associated with colchicine resistance and Inflammatory bowel disease 13. Alternative splicing and

1940-589: Is an important feature in all cells, especially epithelia with microvilli. Recent data suggest the glycocalyx participates in cell adhesion, lymphocyte homing , and many others. The penultimate sugar is galactose and the terminal sugar is sialic acid , as the sugar backbone is modified in the Golgi apparatus . Sialic acid carries a negative charge, providing an external barrier to charged particles. The cell membrane has large content of proteins, typically around 50% of membrane volume These proteins are important for

2037-445: Is an important protein of the cell membrane that pumps many foreign substances out of cells. More formally, it is an ATP -dependent efflux pump with broad substrate specificity. It exists in animals, fungi, and bacteria, and it likely evolved as a defense mechanism against harmful substances. P-gp is extensively distributed and expressed in the intestinal epithelium where it pumps xenobiotics (such as toxins or drugs) back into

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2134-404: Is capable of lowering intracellular concentrations of otherwise beneficial compounds, such as chemotherapeutics and other medications, to sub-therapeutic levels. Consequently, P-gp overexpression is one of the main mechanisms behind decreased intracellular drug accumulation and development of multidrug resistance in human multidrug-resistant (MDR) cancers. P-gp was first characterized in 1976. P-gp

2231-531: Is first moved by cytoskeleton from the interior of the cell to the surface. The vesicle membrane comes in contact with the plasma membrane. The lipid molecules of the two bilayers rearrange themselves and the two membranes are, thus, fused. A passage is formed in the fused membrane and the vesicles discharges its contents outside the cell. Prokaryotes are divided into two different groups, Archaea and Bacteria , with bacteria dividing further into gram-positive and gram-negative . Gram-negative bacteria have both

2328-462: Is found underlying the cell membrane in the cytoplasm and provides a scaffolding for membrane proteins to anchor to, as well as forming organelles that extend from the cell. Indeed, cytoskeletal elements interact extensively and intimately with the cell membrane. Anchoring proteins restricts them to a particular cell surface — for example, the apical surface of epithelial cells that line the vertebrate gut — and limits how far they may diffuse within

2425-414: Is incorporated into the membrane, or deleted from it, by a variety of mechanisms: The cell membrane consists of three classes of amphipathic lipids: phospholipids , glycolipids , and sterols . The amount of each depends upon the type of cell, but in the majority of cases phospholipids are the most abundant, often contributing for over 50% of all lipids in plasma membranes. Glycolipids only account for

2522-1583: Is linked to the daily dose of warfarin required to maintain the INR to a target of 2.5. Patients with the GT or TT genotypes of the 2677G>T SNP require around 20% more warfarin daily. Common pharmacological inducers of P-glycoprotein include carbamazepine , dexamethasone , doxorubicin , nefazodone , phenobarbital , phenytoin , prazosin , rifampicin , St. John's wort , tenofovir , tipranavir , trazodone , and vinblastine . Substrates of P-glycoprotein are susceptible to changes in pharmacokinetics due to drug interactions with P-gp inhibitors or inducers. Some of these substrates include colchicine , ciclosporin , dabigatran , digoxin , diltiazem , fexofenadine , indinavir , morphine , and sirolimus . Decreased P-gp expression has been found in Alzheimer's disease brains. Altered P-gp function has also been linked to inflammatory bowel diseases (IBD); however, due to its ambivalent effects in intestinal inflammation many questions remain so far unanswered. While decreased efflux activity may promote disease susceptibility and drug toxicity, increased efflux activity may confer resistance to therapeutic drugs in IBD. Mice deficient in MDR1A develop chronic intestinal inflammation spontaneously, which appears to resemble human ulcerative colitis . P-gp efflux activity

2619-490: Is usually 6–7 m (20–23 ft) long (post mortem), about two-fifths of which (about 2.5 m (8.2 ft)) is the jejunum. The interior surface of the jejunum—which is exposed to ingested food—is covered in finger–like projections of mucosa, called villi , which increase the surface area of tissue available to absorb nutrients from ingested foodstuffs. The epithelial cells which line these villi have microvilli . The transport of nutrients across epithelial cells through

2716-732: The ABCB1 gene. P-gp is a well-characterized ABC-transporter (which transports a wide variety of substrates across extra- and intracellular membranes) of the MDR / TAP subfamily. The normal excretion of xenobiotics back into the gut lumen by P-gp pharmacokinetically reduces the efficacy of some pharmaceutical drugs (which are said to be P-gp substrates ). In addition, some cancer cells also express large amounts of P-gp, further amplifying that effect and rendering these cancers multidrug resistant . Many drugs inhibit P-gp, typically incidentally rather than as their main mechanism of action ; some foods do as well. Any such substance can sometimes be called

2813-525: The PI3K/Akt pathway and the Wnt/β-catenin pathway were reported to positively regulate the expression of P-gp. Mitogen-activated protein kinase (MAPK) signaling includes three pathways: the classical MAPK/ERK pathway , the p38 MAPK pathway , and the c-Jun N-terminal kinase (JNK) pathway, all of which were reported to have implications in the regulation of the expression of P-gp. Studies suggested that

2910-527: The Raf kinase inhibitor protein (RKIP); alternatively, miR-27a can also directly bind to the promoter of the P-gp gene, which works in a similar way with the mechanism of action of transcriptional factors. The expression of P-gp is also regulated by post-translational events, such as post-transcriptional modification , degradation , and intracellular trafficking of P-gp. Pim-1 protects P-gp from ubiquitination and

3007-500: The United States National Library of Medicine , which is in the public domain . Cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane , and historically referred to as the plasmalemma ) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extracellular space). The cell membrane consists of

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3104-438: The abdomen . It also contains circular and longitudinal smooth muscle which helps to move food along by a process known as peristalsis . The jejunum contains very few Brunner's glands (found in the duodenum) or Peyer's patches (found in the ileum). However, there are a few jejunal lymph nodes suspended in its mesentery. The jejunum has many large circular folds in its submucosa called plicae circulares that increase

3201-414: The cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other cells to hold them together to form tissues . Fungi , bacteria , most archaea , and plants also have a cell wall , which provides a mechanical support to the cell and precludes the passage of larger molecules . The cell membrane is selectively permeable and able to regulate what enters and exits

3298-418: The endoplasmic reticulum , which inserts the proteins into a lipid bilayer. Once inserted, the proteins are then transported to their final destination in vesicles, where the vesicle fuses with the target membrane. The cell membrane surrounds the cytoplasm of living cells, physically separating the intracellular components from the extracellular environment. The cell membrane also plays a role in anchoring

3395-419: The fluid mosaic model of S. J. Singer and G. L. Nicolson (1972), which replaced the earlier model of Davson and Danielli , biological membranes can be considered as a two-dimensional liquid in which lipid and protein molecules diffuse more or less easily. Although the lipid bilayers that form the basis of the membranes do indeed form two-dimensional liquids by themselves, the plasma membrane also contains

3492-404: The liquid crystalline state . It means the lipid molecules are free to diffuse and exhibit rapid lateral diffusion along the layer in which they are present. However, the exchange of phospholipid molecules between intracellular and extracellular leaflets of the bilayer is a very slow process. Lipid rafts and caveolae are examples of cholesterol -enriched microdomains in the cell membrane. Also,

3589-410: The paucimolecular model of Davson and Danielli (1935). This model was based on studies of surface tension between oils and echinoderm eggs. Since the surface tension values appeared to be much lower than would be expected for an oil–water interface, it was assumed that some substance was responsible for lowering the interfacial tensions in the surface of cells. It was suggested that a lipid bilayer

3686-429: The transcriptional level, the expression of P-gp has been intensively studied, and numerous transcription factors and pathways are known to play roles. A variety of transcription factors, such as p53 , YB-1 , and NF-κB are involved in the direct regulation of P-gp by binding to the promoter regions of the P-gp gene. Many cell signaling pathways are also involved in transcriptional regulation of P-gp. For example,

3783-415: The 1970s. Although the fluid mosaic model has been modernized to detail contemporary discoveries, the basics have remained constant: the membrane is a lipid bilayer composed of hydrophilic exterior heads and a hydrophobic interior where proteins can interact with hydrophilic heads through polar interactions, but proteins that span the bilayer fully or partially have hydrophobic amino acids that interact with

3880-540: The MAPK/ERK pathway is involved in the positive regulation of P-gp; the p38 MAPK pathway negatively regulates the expression of the P-gp gene; and the JNK pathway was reported to be involved in both positive regulation and negative regulation of P-gp. After 2008, microRNAs (miRNAs) were identified as new players in regulating the expression of P-gp in both transcriptional and post-transcriptional levels. Some miRNAs decrease

3977-637: The absorption rate of nutrients. Localized decoupling of the cytoskeleton and cell membrane results in formation of a bleb . The content of the cell, inside the cell membrane, is composed of numerous membrane-bound organelles , which contribute to the overall function of the cell. The origin, structure, and function of each organelle leads to a large variation in the cell composition due to the individual uniqueness associated with each organelle. The cell membrane has different lipid and protein compositions in distinct types of cells and may have therefore specific names for certain cell types. The permeability of

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4074-410: The attachment surface for several extracellular structures, including the cell wall and the carbohydrate layer called the glycocalyx , as well as the intracellular network of protein fibers called the cytoskeleton . In the field of synthetic biology, cell membranes can be artificially reassembled . Robert Hooke 's discovery of cells in 1665 led to the proposal of the cell theory . Initially it

4171-863: The basal to the lateral surface of the cell or vice versa in accordance with the fluid mosaic model . Tight junctions join epithelial cells near their apical surface to prevent the migration of proteins from the basolateral membrane to the apical membrane. The basal and lateral surfaces thus remain roughly equivalent to one another, yet distinct from the apical surface. Cell membrane can form different types of "supramembrane" structures such as caveolae , postsynaptic density , podosomes , invadopodia , focal adhesion , and different types of cell junctions . These structures are usually responsible for cell adhesion , communication, endocytosis and exocytosis . They can be visualized by electron microscopy or fluorescence microscopy . They are composed of specific proteins, such as integrins and cadherins . The cytoskeleton

4268-564: The bilayer. The cytoskeleton is able to form appendage-like organelles, such as cilia , which are microtubule -based extensions covered by the cell membrane, and filopodia , which are actin -based extensions. These extensions are ensheathed in membrane and project from the surface of the cell in order to sense the external environment and/or make contact with the substrate or other cells. The apical surfaces of epithelial cells are dense with actin-based finger-like projections known as microvilli , which increase cell surface area and thereby increase

4365-656: The cell because they are responsible for various biological activities. Approximately a third of the genes in yeast code specifically for them, and this number is even higher in multicellular organisms. Membrane proteins consist of three main types: integral proteins, peripheral proteins, and lipid-anchored proteins. As shown in the adjacent table, integral proteins are amphipathic transmembrane proteins. Examples of integral proteins include ion channels, proton pumps, and g-protein coupled receptors. Ion channels allow inorganic ions such as sodium, potassium, calcium, or chlorine to diffuse down their electrochemical gradient across

4462-415: The cell membrane results in pH partition of substances throughout the fluid compartments of the body . Jejunum The jejunum lies between the duodenum and the ileum and is considered to start at the suspensory muscle of the duodenum , a location called the duodenojejunal flexure . The division between the jejunum and ileum is not anatomically distinct. In adult humans , the small intestine

4559-469: The cell membrane, acting as enzymes to facilitate interaction with the cell's environment. Glycolipids embedded in the outer lipid layer serve a similar purpose. The cell membrane controls the movement of substances in and out of a cell, being selectively permeable to ions and organic molecules. In addition, cell membranes are involved in a variety of cellular processes such as cell adhesion , ion conductivity , and cell signalling and serve as

4656-604: The cell membrane. Some common pharmacological inhibitors of P-glycoprotein include: amiodarone , clarithromycin , ciclosporin , colchicine , diltiazem , erythromycin , felodipine , ketoconazole , lansoprazole , omeprazole and other proton-pump inhibitors , nifedipine , paroxetine , reserpine , saquinavir , sertraline , quinidine , tamoxifen , verapamil , and duloxetine . Elacridar and CP 100356 are other common P-gp inhibitors. Zosuquidar and tariquidar were also developed with this in mind. Lastly, valspodar and reversan are other examples of such agents. ABCB1

4753-442: The cell, as well as getting more insight into cell membrane permeability. Lipid vesicles and liposomes are formed by first suspending a lipid in an aqueous solution then agitating the mixture through sonication , resulting in a vesicle. Measuring the rate of efflux from the inside of the vesicle to the ambient solution allows researchers to better understand membrane permeability. Vesicles can be formed with molecules and ions inside

4850-463: The cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be achieved by either passive transport , occurring without the input of cellular energy, or by active transport , requiring the cell to expend energy in transporting it. The membrane also maintains the cell potential . The cell membrane thus works as a selective filter that allows only certain things to come inside or go outside

4947-433: The cell. The cell employs a number of transport mechanisms that involve biological membranes: 1. Passive osmosis and diffusion : Some substances (small molecules, ions) such as carbon dioxide (CO 2 ) and oxygen (O 2 ), can move across the plasma membrane by diffusion, which is a passive transport process. Because the membrane acts as a barrier for certain molecules and ions, they can occur in different concentrations on

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5044-465: The description of the cell membrane bilayer structure based on crystallographic studies and soap bubble observations. In an attempt to accept or reject the hypothesis, researchers measured membrane thickness. These researchers extracted the lipid from human red blood cells and measured the amount of surface area the lipid would cover when spread over the surface of the water. Since mature mammalian red blood cells lack both nuclei and cytoplasmic organelles,

5141-501: The differential activity of the P-gp pump. Homozygous subjects, identified with the TT genotype , are usually more able to extrude xenobiotics from the cell. A Homozygous genotype for the allele ABCB1/MDR1 is capable of a higher absorption from the blood vessels and a lower extrusion into the lumen . Xenobiotics are extruded at a lower rate with heterozygous (CT) alleles compared to homozygous ones. This article incorporates text from

5238-417: The ectoplast ( de Vries , 1885), Plasmahaut (plasma skin, Pfeffer , 1877, 1891), Hautschicht (skin layer, Pfeffer, 1886; used with a different meaning by Hofmeister , 1867), plasmatic membrane (Pfeffer, 1900), plasma membrane, cytoplasmic membrane, cell envelope and cell membrane. Some authors who did not believe that there was a functional permeable boundary at the surface of the cell preferred to use

5335-412: The entropy of the system. This complex interaction can include noncovalent interactions such as van der Waals , electrostatic and hydrogen bonds. Lipid bilayers are generally impermeable to ions and polar molecules. The arrangement of hydrophilic heads and hydrophobic tails of the lipid bilayer prevent polar solutes (ex. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across

5432-603: The equivalent of a plant cell wall . It was also inferred that cell membranes were not vital components to all cells. Many refuted the existence of a cell membrane still towards the end of the 19th century. In 1890, a revision to the cell theory stated that cell membranes existed, but were merely secondary structures. It was not until later studies with osmosis and permeability that cell membranes gained more recognition. In 1895, Ernest Overton proposed that cell membranes were made of lipids. The lipid bilayer hypothesis, proposed in 1925 by Gorter and Grendel, created speculation in

5529-480: The exception of fat, which goes to the lymph) pass from the enterocytes into the enterohepatic circulation and enter the liver via the hepatic portal vein, where the blood is processed. In fish , the divisions of the small intestine are not as clear and the terms middle intestine or mid-gut may be used instead of jejunum. Jejunum is derived from the Latin word jējūnus (iēiūnus) , meaning " fasting ." It

5626-404: The expression of P-gp. For example, miR-200c down-regulates the expression of P-gp through the JNK signaling pathway or ZEB1 and ZEB2 ; miR-145 down-regulates the mRNA of P-gp by directly binding to the 3'-UTR of the gene of P-gp and thus suppresses the translation of P-gp. Some other miRNAs increase the expression of P-gp. For example, miR-27a up-regulates P-gp expression by suppressing

5723-486: The first half of the polypeptide. In 2009, the first structure of a mammalian P-glycoprotein was solved (3G5U). The structure was derived from the mouse MDR3 gene product heterologously expressed in Pichia pastoris yeast. The structure of mouse P-gp is similar to structures of the bacterial ABC transporter MsbA (3B5W and 3B5X) that adopt an inward facing conformation that is believed to be important for binding substrate along

5820-478: The fluidity is fatty acid composition. For example, when the bacteria Staphylococcus aureus was grown in 37 C for 24h, the membrane exhibited a more fluid state instead of a gel-like state. This supports the concept that in higher temperatures, the membrane is more fluid than in colder temperatures. When the membrane is becoming more fluid and needs to become more stabilized, it will make longer fatty acid chains or saturated fatty acid chains in order to help stabilize

5917-454: The fluidity of the membrane. Cholesterol is more abundant in cold-weather animals than warm-weather animals. In plants, which lack cholesterol, related compounds called sterols perform the same function as cholesterol. Lipid vesicles or liposomes are approximately spherical pockets that are enclosed by a lipid bilayer. These structures are used in laboratories to study the effects of chemicals in cells by delivering these chemicals directly to

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6014-409: The following degradation in the proteasome . Small GTPases Rab5 down-regulates the endocytotic trafficking of P-gp and thus increases the functional P-gp level on the cell membrane ; while Small GTPases Rab4 work in an opposite way: Rab4 down-regulates the exocytotic trafficking of P-gp from intracellular compartments to the cell membrane, and therefore decreases the functional P-gp level on

6111-410: The inner leaflet of the membrane. Additional structures (3G60 and 3G61) of P-gp were also solved revealing the binding site(s) of two different cyclic peptide substrate/inhibitors. The promiscuous binding pocket of P-gp is lined with aromatic amino acid side chains. Through Molecular Dynamic (MD) simulations, this sequence was proved to have a direct impact in the transporter's structural stability (in

6208-411: The intensity of light reflected from a sample to the intensity of a membrane standard of known thickness. The instrument could resolve thicknesses that depended on pH measurements and the presence of membrane proteins that ranged from 8.6 to 23.2 nm, with the lower measurements supporting the lipid bilayer hypothesis. Later in the 1930s, the membrane structure model developed in general agreement to be

6305-409: The intestinal lumen , in liver cells where it pumps them into bile ducts , in the cells of the proximal tubule of the kidney where it pumps them into urinary filtrate (in the proximal tubule), and in the capillary endothelial cells composing the blood–brain barrier and blood–testis barrier , where it pumps them back into the capillaries. P-gp is a glycoprotein that in humans is encoded by

6402-429: The jejunum and ileum includes the passive transport of sugar fructose and the active transport of amino acids , small peptides , vitamins , and most glucose . The villi in the jejunum are much longer than in the duodenum or ileum. The pH in the jejunum is usually between 7 and 8 (neutral or slightly alkaline ). The jejunum and the ileum are suspended by mesentery which gives the bowel great mobility within

6499-527: The lipid bilayer of the membranes; they function on both sides of the membrane to transport molecules across it. Nutrients, such as sugars or amino acids, must enter the cell, and certain products of metabolism must leave the cell. Such molecules can diffuse passively through protein channels such as aquaporins in facilitated diffusion or are pumped across the membrane by transmembrane transporters . Protein channel proteins, also called permeases , are usually quite specific, and they only recognize and transport

6596-431: The lipid bilayer through hydrophilic pores across the membrane. The electrical behavior of cells (i.e. nerve cells) is controlled by ion channels. Proton pumps are protein pumps that are embedded in the lipid bilayer that allow protons to travel through the membrane by transferring from one amino acid side chain to another. Processes such as electron transport and generating ATP use proton pumps. A G-protein coupled receptor

6693-444: The membrane, but generally allows for the passive diffusion of hydrophobic molecules. This affords the cell the ability to control the movement of these substances via transmembrane protein complexes such as pores, channels and gates. Flippases and scramblases concentrate phosphatidyl serine , which carries a negative charge, on the inner membrane. Along with NANA , this creates an extra barrier to charged moieties moving through

6790-539: The membrane. Bacteria are also surrounded by a cell wall composed of peptidoglycan (amino acids and sugars). Some eukaryotic cells also have cell walls, but none that are made of peptidoglycan. The outer membrane of gram negative bacteria is rich in lipopolysaccharides , which are combined poly- or oligosaccharide and carbohydrate lipid regions that stimulate the cell's natural immunity. The outer membrane can bleb out into periplasmic protrusions under stress conditions or upon virulence requirements while encountering

6887-407: The membrane. Membranes serve diverse functions in eukaryotic and prokaryotic cells. One important role is to regulate the movement of materials into and out of cells. The phospholipid bilayer structure (fluid mosaic model) with specific membrane proteins accounts for the selective permeability of the membrane and passive and active transport mechanisms. In addition, membranes in prokaryotes and in

6984-408: The membrane. The ability of some organisms to regulate the fluidity of their cell membranes by altering lipid composition is called homeoviscous adaptation . The entire membrane is held together via non-covalent interaction of hydrophobic tails, however the structure is quite fluid and not fixed rigidly in place. Under physiological conditions phospholipid molecules in the cell membrane are in

7081-417: The membrane. Additionally, the amount of cholesterol in biological membranes varies between organisms, cell types, and even in individual cells. Cholesterol, a major component of plasma membranes, regulates the fluidity of the overall membrane, meaning that cholesterol controls the amount of movement of the various cell membrane components based on its concentrations. In high temperatures, cholesterol inhibits

7178-436: The membranes were seen but mostly disregarded as an important structure with cellular function. It was not until the 20th century that the significance of the cell membrane as it was acknowledged. Finally, two scientists Gorter and Grendel (1925) made the discovery that the membrane is "lipid-based". From this, they furthered the idea that this structure would have to be in a formation that mimicked layers. Once studied further, it

7275-430: The mitochondria and chloroplasts of eukaryotes facilitate the synthesis of ATP through chemiosmosis. The apical membrane or luminal membrane of a polarized cell is the surface of the plasma membrane that faces inward to the lumen . This is particularly evident in epithelial and endothelial cells , but also describes other polarized cells, such as neurons . The basolateral membrane or basolateral cell membrane of

7372-401: The most common. Fatty acids may be saturated or unsaturated, with the configuration of the double bonds nearly always "cis". The length and the degree of unsaturation of fatty acid chains have a profound effect on membrane fluidity as unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus decreasing the melting temperature (increasing the fluidity) of

7469-435: The movement of phospholipid fatty acid chains, causing a reduced permeability to small molecules and reduced membrane fluidity. The opposite is true for the role of cholesterol in cooler temperatures. Cholesterol production, and thus concentration, is up-regulated (increased) in response to cold temperature. At cold temperatures, cholesterol interferes with fatty acid chain interactions. Acting as antifreeze, cholesterol maintains

7566-433: The non-polar lipid interior. The fluid mosaic model not only provided an accurate representation of membrane mechanics, it enhanced the study of hydrophobic forces, which would later develop into an essential descriptive limitation to describe biological macromolecules . For many centuries, the scientists cited disagreed with the significance of the structure they were seeing as the cell membrane. For almost two centuries,

7663-402: The nucleotide-binding domains) and defining a lower boundary for the internal drug-binding pocket. P-gp is expressed primarily in certain cell types in the liver, pancreas, kidney, colon, and jejunum . P-gp is also found in brain capillary endothelial cells . Substrate enters P-gp either from an opening within the inner leaflet of the membrane or from an opening at the cytoplasmic side of

7760-406: The plasma membrane is the only lipid-containing structure in the cell. Consequently, all of the lipids extracted from the cells can be assumed to have resided in the cells' plasma membranes. The ratio of the surface area of water covered by the extracted lipid to the surface area calculated for the red blood cells from which the lipid was 2:1(approx) and they concluded that the plasma membrane contains

7857-507: The protein, so that the process can start again. The protein belongs to the superfamily of ATP-binding cassette (ABC) transporters . ABC proteins transport various molecules across extra- and intra-cellular membranes. ABC genes are divided into seven distinct subfamilies (ABC1, MDR/TAP, MRP, ALD, OABP, GCN20, White). This protein is a member of the MDR/TAP subfamily. Members of the MDR/TAP subfamily are involved in multidrug resistance . P-gp

7954-425: The protein. ATP binds at the cytoplasmic side of the protein. Following binding of each, ATP hydrolysis shifts the substrate into a position to be excreted from the cell. Release of the phosphate (from the original ATP molecule) occurs concurrently with substrate excretion. ADP is released, and a new molecule of ATP binds to the secondary ATP-binding site. Hydrolysis and release of ADP and a phosphate molecule resets

8051-449: The results have been majorly conflicting, with limited clinical relevance." P-gp is a 170 kDa transmembrane glycoprotein , which includes 10–15 kDa of N-terminal glycosylation. The N-terminal half of the protein contains six transmembrane helixes, followed by a large cytoplasmic domain with an ATP-binding site, and then a second section with six transmembrane helixes and an ATP-binding domain that shows over 65% of amino acid similarity with

8148-401: The role of cell-cell recognition in eukaryotes; they are located on the surface of the cell where they recognize host cells and share information. Viruses that bind to cells using these receptors cause an infection. For the most part, no glycosylation occurs on membranes within the cell; rather generally glycosylation occurs on the extracellular surface of the plasma membrane. The glycocalyx

8245-422: The substance to be transported is captured. This invagination is caused by proteins on the outside on the cell membrane, acting as receptors and clustering into depressions that eventually promote accumulation of more proteins and lipids on the cytosolic side of the membrane. The deformation then pinches off from the membrane on the inside of the cell, creating a vesicle containing the captured substance. Endocytosis

8342-427: The surface area for nutrient absorption. The plicae circulares are best developed in the jejunum. There is no line of demarcation between the jejunum and the ileum. However, there are subtle histological differences: The lining of the jejunum is specialized for the absorption by enterocytes of small nutrient particles which have been previously digested by enzymes in the duodenum . Once absorbed, nutrients (with

8439-414: The surrounding medium. This is the process of exocytosis. Exocytosis occurs in various cells to remove undigested residues of substances brought in by endocytosis, to secrete substances such as hormones and enzymes, and to transport a substance completely across a cellular barrier. In the process of exocytosis, the undigested waste-containing food vacuole or the secretory vesicle budded from Golgi apparatus ,

8536-510: The surrounding water while the hydrophilic "head" regions interact with the intracellular (cytosolic) and extracellular faces of the resulting bilayer. This forms a continuous, spherical lipid bilayer . Hydrophobic interactions (also known as the hydrophobic effect ) are the major driving forces in the formation of lipid bilayers. An increase in interactions between hydrophobic molecules (causing clustering of hydrophobic regions) allows water molecules to bond more freely with each other, increasing

8633-507: The term plasmalemma (coined by Mast, 1924) for the external region of the cell. Cell membranes contain a variety of biological molecules , notably lipids and proteins. Composition is not set, but constantly changing for fluidity and changes in the environment, even fluctuating during different stages of cell development. Specifically, the amount of cholesterol in human primary neuron cell membrane changes, and this change in composition affects fluidity throughout development stages. Material

8730-430: The two sides of the membrane. Diffusion occurs when small molecules and ions move freely from high concentration to low concentration in order to equilibrate the membrane. It is considered a passive transport process because it does not require energy and is propelled by the concentration gradient created by each side of the membrane. Such a concentration gradient across a semipermeable membrane sets up an osmotic flow for

8827-471: The use of alternative promoters results in multiple transcript variants. P-gp transports various substrates across the cell membrane including: Its ability to transport the above substrates accounts for the many roles of P-gp including: It is inhibited by many drugs, such as amiodarone , azithromycin , captopril , clarithromycin , cyclosporine , piperine , quercetin , quinidine , quinine , reserpine , ritonavir , tariquidar , and verapamil . At

8924-547: The vesicle by forming the vesicle with the desired molecule or ion present in the solution. Proteins can also be embedded into the membrane through solubilizing the desired proteins in the presence of detergents and attaching them to the phospholipids in which the liposome is formed. These provide researchers with a tool to examine various membrane protein functions. Plasma membranes also contain carbohydrates , predominantly glycoproteins , but with some glycolipids ( cerebrosides and gangliosides ). Carbohydrates are important in

9021-433: The water. Osmosis, in biological systems involves a solvent, moving through a semipermeable membrane similarly to passive diffusion as the solvent still moves with the concentration gradient and requires no energy. While water is the most common solvent in cell, it can also be other liquids as well as supercritical liquids and gases. 2. Transmembrane protein channels and transporters : Transmembrane proteins extend through

9118-449: Was believed that all cells contained a hard cell wall since only plant cells could be observed at the time. Microscopists focused on the cell wall for well over 150 years until advances in microscopy were made. In the early 19th century, cells were recognized as being separate entities, unconnected, and bound by individual cell walls after it was found that plant cells could be separated. This theory extended to include animal cells to suggest

9215-445: Was found by comparing the sum of the cell surfaces and the surfaces of the lipids, a 2:1 ratio was estimated; thus, providing the first basis of the bilayer structure known today. This discovery initiated many new studies that arose globally within various fields of scientific studies, confirming that the structure and functions of the cell membrane are widely accepted. The structure has been variously referred to by different writers as

9312-423: Was in between two thin protein layers. The paucimolecular model immediately became popular and it dominated cell membrane studies for the following 30 years, until it became rivaled by the fluid mosaic model of Singer and Nicolson (1972). Despite the numerous models of the cell membrane proposed prior to the fluid mosaic model , it remains the primary archetype for the cell membrane long after its inception in

9409-503: Was shown to be responsible for conferring multidrug resistance upon mutant cultured cancer cells that had developed resistance to cytotoxic drugs. The structure of mouse P-gp, which has 87% sequence identity to human P-gp, was resolved by x-ray crystallography in 2009. The first structure of human P-gp was solved in 2018, with the protein in its ATP-bound, outward-facing conformation. Radioactive verapamil can be used for measuring P-gp function with positron emission tomography . P-gp

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