The plasma membrane is liberally studded with a variety of proteins and glycoproteins involved in (1) ion and metabolite transport, (2) fluid-phase and receptor- mediated uptake of macromolecules, and (3) cell–ligand, cell–matrix, and cell–cell interactions. Proteins interact with the lipid bilayer by one of four general arrangements (Fig. 1.7B): #79 в контекст | | |
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Most proteins are transmembrane (integral) proteins, having one or more relatively hydrophobic α-helical segments that traverse the lipid bilayer. Integral mem- brane proteins typically contain positively charged amino acids in their cytoplasmic domains that anchor the proteins to the negatively charged head groups of membrane phospholipids. #81 в контекст | | |
Proteins may be synthesized in the cytosol and post- translationally attached to prenyl groups (e.g., farnesyl, related to cholesterol) or fatty acids (e.g., palmitic or myristic acid) that insert into the cytosolic side of the plasma membrane. #82 в контекст | | |
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Attachment to membranes can occur through glyco- sylphosphatidylinositol (GPI) anchors on the extracel- lular face of the membrane. #84 в контекст | | |
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Extracellular proteins can noncovalently associate with transmembrane proteins, which serve to anchor them to the cell. #86 в контекст | | |
Many plasma membrane proteins function together as larger complexes; these may assemble under the control of chaperone molecules in the RER or by lateral diffusion in the plasma membrane. The latter mechanism is charac- teristic of many protein receptors (e.g., cytokine receptors) that dimerize or trimerize in the presence of ligand to form functional signaling units. Although lipid bilayers are fluid in the two-dimensional plane of the membrane, membrane components can nevertheless be constrained to discrete domains. This can occur by localization to lipid rafts (dis- cussed earlier), or through intercellular protein–protein interactions (e.g., at tight junctions) that establish discrete boundaries; indeed, this strategy is used to maintain cell polarity (e.g., top/apical versus bottom/basolateral) in epi- thelial layers. Alternatively, unique domains can be formed through the interaction of membrane proteins with cyto- skeletal molecules or an extracellular matrix (ECM). #87 в контекст | | |
The extracellular face of the plasma membrane is dif- fusely studded with carbohydrates, not only as complex oligosaccharides on glycoproteins and glycolipids, but also as polysaccharide chains attached to integral membrane proteoglycans. This glycocalyx functions as a chemical and mechanical barrier, and is also involved in cell–cell and cell–matrix interactions. #88 в контекст | | |
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Small, nonpolar molecules such as O2 and CO2 readily dis- #90 в контекст | | |
solve in lipid bilayers and therefore rapidly diffuse across them, as do hydrophobic molecules (e.g., steroid-based molecules such as estradiol or vitamin D). Similarly, small polar molecules (<75 daltons in mass, such as water, ethanol, and urea) readily cross membranes. In contrast, the lipid bilayer is an effective barrier to the passage of larger polar molecules, even those only slightly greater than 75 daltons, such as glucose. Lipid bilayers also are impermeant to ions, no matter how small, because of their charge and high degree of hydration. We will next discuss specialized mechanisms that regulate traffic across plasma membranes. #91 в контекст | | |
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For each of the larger polar molecules that must cross #93 в контекст | | |
membranes to support normal cellular functions (e.g., for nutrient uptake and waste disposal), unique plasma mem- brane protein complexes are typically required. For #94 в контекст | | |
Cellular Housekeeping 9 low-molecular-weight species (ions and small molecules #95 в контекст | | |
up to approximately 1000 daltons), channel proteins and carrier proteins may be used (although this discussion focuses on plasma membranes, it should be noted that similar pores and channels are needed for transport across organellar membranes). Each transported molecule (e.g., ion, sugar, nucleotide) requires a transporter that is typi- cally highly specific (e.g., glucose but not galactose): #96 в контекст | | |
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Channel proteins create hydrophilic pores that, when open, permit rapid movement of solutes (usually restricted by size and charge; Fig. 1.8). #98 в контекст | | |
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Carrier proteins bind their specific solute and undergo a series of conformational changes to transfer the ligand across the membrane; their transport is relatively slow. #100 в контекст | | |
In many cases, a concentration and/or electrical gradi- ent between the inside and outside of the cell drives solute movement via passive transport (virtually all plasma mem- branes have an electrical potential difference across them, with the inside negative relative to the outside). In other cases, active transport of certain solutes against a concentra- tion gradient is accomplished by carrier molecules (not channels) using energy released by ATP hydrolysis or a coupled ion gradient. Transporter ATPases include the notorious multidrug resistance (MDR) protein, which pumps polar compounds (e.g., chemotherapeutic drugs) out of cells and may render cancer cells resistant to treatment. #101 в контекст | | |
Because membranes are freely permeable to water, it moves into and out of cells by osmosis, depending on rela- tive solute concentrations. Thus, extracellular salt in excess of that in the cytosol (hypertonicity) causes a net movement of water out of cells, whereas hypotonicity causes a net movement of water into cells. The cytosol is rich in charged metabolites and protein species, which attract a large number of counterions that tend to increase the intracel- lular osmolarity. As a consequence, to prevent overhydra- tion cells must constantly pump out small inorganic ions (e.g., Na+)—typically through the activity of membrane ion-exchanging ATPases. Loss of the ability to generate energy (e.g., in a cell injured by toxins or ischemia) there- fore results in osmotic swelling and eventual rupture of cells. Similar transport mechanisms also regulate intracel- lular and intraorganellar pH; most cytosolic enzymes prefer to work at pH 7.4, whereas lysosomal enzymes func- tion best at pH 5 or less. #102 в контекст | | |
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Uptake of fluids or macromolecules by the cell, called endo- #105 в контекст | | |
cytosis, occurs by two fundamental mechanisms (Fig. 1.8). Certain small molecules—including some vitamins—are taken up by invaginations of the plasma membrane called caveolae. For larger molecules, uptake occurs after binding to specific cell-surface receptors; internalization occurs through a membrane invagination process driven by an intracellular matrix of clathrin proteins. Clathrin is a hexamer of proteins that spontaneously assembles into a basketlike lattice to drive the invagination process. We shall come back to these later. #106 в контекст | | |
The process by which large molecules are exported from cells is called exocytosis. In this process, proteins synthe- sized and packaged within the RER and Golgi apparatus are concentrated in secretory vesicles, which then fuse with the plasma membrane and expel their contents. #107 в контекст | | |
Transcytosis is the movement of endocytosed vesicles between the apical and basolateral compartments of cells; this is a mechanism for transferring large amounts of intact proteins across epithelial barriers (e.g., ingested antibodies in maternal milk across intestinal epithelia) or for the rapid movement of large volumes of solute. In fact, increased transcytosis probably plays a role in the increased vascular permeability seen in tumors. #108 в контекст | | |
We now return to the two forms of endocytosis men- tioned earlier. #109 в контекст | | |
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Caveolae-mediated endocytosis. Caveolae (“little caves”) are noncoated plasma membrane invaginations associ- ated with GPI-linked molecules, cyclic adenosine monophosphate (cAMP)-binding proteins, SRC-family kinases, and the folate receptor. Caveolin is the major structural protein of caveolae. Internalization of caveo- lae with any bound molecules and associated extracel- lular fluid is denoted potocytosis—literally “cellular #111 в контекст | | |
sipping.” Although caveolae likely participate in the transmembrane delivery of some molecules (e.g., folate), they also appear to contribute to the regulation of trans- membrane signaling and/or cellular adhesion via the internalization of receptors and integrins. Mutations in caveolin are associated with muscular dystrophy and electrical abnormalities in the heart. #112 в контекст | | |
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Pinocytosis and receptor-mediated endocytosis (Fig. 1.8). Pinocytosis (“cellular drinking”) is a fluid-phase process. The plasma membrane invaginates and is pinched off to form a cytoplasmic vesicle; after delivering their cargo, endocytosed vesicles recycle back to the plasma membrane (exocytosis) for another round of ingestion. Endocytosis and exocytosis are tightly balanced and highly active, as a cell typically pinocytoses 10% to 20% of its own cell volume each hour, or about 1% to 2% of its plasma membrane each minute. Pinocytosis and receptor-mediated endocytosis begin with the formation of a clathrin-coated pit containing the ligand to be inter- nalized (by itself or bound to the receptor), which rapidly invaginates and pinches off to form a clathrin-coated vesicle. Thus, trapped within the vesicle is a gulp of the extracellular milieu, as well as receptor-bound macro- molecules as described below. The vesicles then rapidly uncoat and fuse with an acidic intracellular structure called the early endosome, which progressively matures to late endosomes and ultimately fuses with lysosomes. #114 в контекст | | |
Receptor-mediated endocytosis is the major uptake mechanism for certain macromolecules, as exemplified by transferrin and low-density lipoprotein (LDL). These macromolecules bind to receptors that localize to clathrin-coated pits. After binding to their specific recep- tors, LDL and transferrin are endocytosed in vesicles that mature into early and late endosomes. In the acidic environment of the endosome, LDL and transferrin release their bound ligands (cholesterol and iron, respec- tively), which then exit into the cytosol, and the LDL receptor and transferrin receptor subsequently recycle to the plasma membrane. Defects in receptor-mediated transport of LDL are responsible for familial hypercho- lesterolemia, as described in Chapter 7. #115 в контекст | | |
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The ability of cells to adopt a particular shape, maintain polarity, organize the intracellular organelles, and move about depends on the intracellular scaffolding of pro- teins called the cytoskeleton (Fig. 1.9). In eukaryotic cells, there are three major classes of cytoskeletal proteins: #117 в контекст | | |
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Actin microfilaments are fibrils 5- to 9-nm in diameter formed from the globular protein actin (G-actin), the most abundant cytosolic protein in cells. G-actin mono- mers noncovalently polymerize into long filaments (F-actin) that intertwine to form double-stranded #119 в контекст | | |
helices. In muscle cells, the filamentous protein myosin binds to actin and moves along it, driven by ATP hydro- lysis (the basis of muscle contraction). In non-muscle cells, F-actin assembles via an assortment of actin- binding proteins into well-organized bundles and net- works that control cell shape and movement. #120 в контекст | | |
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Intermediate filaments are fibrils 10-nm in diameter that comprise a large and heterogeneous family. Members include lamins A, B, and C, which contribute to the struc- ture of nuclear lamina. Individual types of intermediate filaments have characteristic tissue-specific patterns of expression that are useful for identifying the cellular origin of poorly differentiated tumors. #122 в контекст | | |
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Vimentin: Mesenchymal cells (fibroblasts, endothe- lium) anchoring intracellular organelles #124 в контекст | | |
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Desmin: Muscle cells, forming the scaffold on which actin and myosin contract #126 в контекст | | |
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Neurofilaments: Axons of neurons, imparting strength and rigidity #128 в контекст | | |
