Peripheral protein

Singer and Nicolson also pointed out that proteins can be associated with the surface of this bilayer or embedded in the bilayer to varying degrees (Figure 9.6). They defined two classes of membrane proteins. The first, called peripheral proteins (or extrinsic proteins), includes those that do not penetrate the bilayer to any significant degree and are associated with the membrane by virtue... 

C2 domains (phosphokinase C conserved 2 domains) mediate membrane targeting of diverse peripheral proteins. A C2 domain consists of approximately 130 residues and was first discovered as the Ca2+-binding site in conventional phosphokinase Cs. 

Membranes Contain Integral Peripheral Proteins (Figure 41-7)... 

Figure 41-7. The fluid mosaic model of membrane structure. The membrane consists of a bimolecu-lar lipid layer with proteins inserted in it or bound to either surface. Integral membrane proteins are firmly embedded in the lipid layers. Some of these proteins completely span the bilayer and are called transmembrane proteins, while others are embedded in either the outer or inner leaflet of the lipid bilayer. Loosely bound to the outer or inner surface of the membrane are the peripheral proteins. Many of the proteins and lipids have externally exposed oligosaccharide chains. (Reproduced, with permission, from Junqueira LC, Carneiro J Basic Histology. Text Atlas, 10th ed. McGraw-Hill, 2003.)...

Molecules released by exocytosis fall into three categories (1) They can attach to the cell surface and become peripheral proteins, eg, antigens. (2) They can become part of the extracellular matrix, eg, collagen and glycosaminoglycans. (3) They can enter extracellular fluid and signal other cells. Insulin, parathyroid hormone, and the catecholamines are all packaged in gran-... 

Fig. 7 Diagrammatic representation of the fluid mosaic model of the cell membrane. The basic structure of the membrane is that of a lipid bilayer in which the lipid portion (long tails) points inward and the polar portion (round head ) points outward. The membrane is penenetrated by transmembrane (or integral) proteins. Attached to the surface of the membrane are peripheral proteins (inner surface) and carbohydrates that bind to lipid and protein molecules (outer surface). (Modified from Ref. 14.)...

Peripheral proteins Associated with only leaflet—easily isolated by altering ionic strength or pH of the encasing medium... 

It is easy to see that nonpolar, lipophilic molecules will easily cross membranes. But we know that polar metabolites and ions must also gain entrance to as well as exit from cells and organelles. This is accomplished through another type of membrane component - membrane proteins. These proteins may lie on the surface of the membrane (peripheral proteins) or be located either entirely within the nonpolar interior, or be partially exterior and interior, or completely span the bilayer (integral proteins). See Figure 2c. These proteins function as channels, carriers, receptors, or signal transmission devices. 

Figure 3.2 The molecular arrangement of the cell membrane (A) integral proteins (B) glycoprotein (C) pore formed from integral protein (D) various phospholipids with saturated fatty acid chains (E) phospholipids with unsaturated fatty acid chains (F) network proteins (G) cholesterol (H) glycolipid (I) peripheral protein. There are four different phospholipids phosphatidyl serine phosphatidyl choline phosphatidyl ethanolamine and sphingomyelin represented as , o. The stippled area of the protein represents the hydrophobic portion. Source From Ref. 1.

Peripheral proteins are loosely associated with the membrane through electrostatic interactions and hydrogen bonds or by covalently attached lipid anchors. Integral proteins associate firmly with membranes by hydrophobic interactions between the lipid bilayer and their nonpolar amino acid side chains, which are oriented toward the outside of the protein molecule. 

After the cream has been washed sufficiently to be essentially free of peripheral proteins and contains only integral membrane proteins, the emulsion is broken by freeze thawing or churning by mechanical... 

Spectrin also binds to one domain of another large 215-kDa peripheral protein called ankyrin (band 2.1, in Fig. 8-14) which anchors the spectrin network to the membrane.258 259 Ankyrin is actually a multigene family of related proteins that are present in many metazoan tissues.258 260 These are modular proteins... 

Peripheral proteins of the outer membrane surface. Many integral membrane glycoproteins have their sugar-bearing portions exposed on the outer surface of the plasma membrane. Among these are receptors, ion pumps, and biochemical markers of individuality. In addition to these proteins, which are actually embedded in the bilayer, there are external peripheral proteins. One of the best known of these is... 

Besides the so-called integral membrane proteins (which are embedded in the hydrophobic part of the bilayer) there are peripheral proteins adsorbed on the hydrophilic surface of the membrane. Some of these peripheral proteins act as support, because they are associated with several integral proteins. A well known example is spectrin, situated at the inside of the erythrocyte membrane Z). 

Grassi, J., Frobert, Y., Lamourette, P, andLagoutte, B. (1988) Screening ofmono-clonal antibodies using antigens labeled with acetylcholinesterase application to the peripheral proteins of photosystem 1. Anal Biochem 168, 436-450... 

Membranes Have Both Integral and Peripheral Proteins... 

The fluid-mosaic model for biological membranes as envisioned by Singer and Nicolson. Integral membrane proteins are embedded in the lipid bilayer peripheral proteins are attached more loosely to protruding regions of the integral proteins. The proteins are free to diffuse laterally or to rotate about an axis perpendicular to the plane of the membrane. For further information, see S. J. Singer and G. L. Nicolson, The fluid mosaic model of the structure of cell membranes, Science 175 720, 1972. 

The cytoskeletons of other eukaryotic cells typically include both microtubules and microfilaments, which consist of long, chainlike oligomers of the proteins tubulin and actin, respectively. Bundles of microfilaments often lie just underneath the plasma membrane (fig. 17.22). They participate in processes that require changes in the shape of the cell, such as locomotion and phagocytosis. In some cells, cytoskeletal microfilaments appear to be linked indirectly through the plasma membrane to peripheral proteins on the outer surface of the cell (fig. 17.23). Among the cell surface proteins connected to this network is fibronectin, a glycoprotein believed to play a role in cell-cell interactions. The lateral diffusion of fibronectin is at least 5,000 times slower than that of freely diffusible membrane proteins. 

Membranes contain proteins that merely bind to their surface (peripheral proteins) and those that are embedded in the lipid matrix (integral proteins). Integral membrane proteins contain transmembrane a-helices. 

What physical properties are conferred on biological membranes by phospholipids How can the charge characteristics of the phospholipids affect binding of peripheral proteins to the membrane What role... 

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