Glycoproteins membrane surface

FIGURE 9.27 The O-Unked saccharides of glycoproteins appear in many cases to adopt extended conformations that serve to extend the functional domains of these proteins above the membrane surface. (Adaptedfrom Jentofi, N., 1990, Trends in Biochemical Sciences 15 291-294.)... 

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... 

Galactose oxidase has been used frequently to label glycoproteins of external cell membrane surfaces. Exposed terminal galactosyl or N-acetylgalactosaminyl residues are oxidized to die corresponding C-6 aldehydes and the latter are reduced under mild conditions with tritiated sodium borohydride.553... 

While one end of the dystrophin molecule binds to actin filaments, the C-terminal domain associates with several additional proteins to form a dystrophin-glycoprotein complex (see figure)/1 k Dystrophin is linked directly to the membrane-spanning protein P-dystroglycan, which in the outer membrane surfaces associates with a glycoprotein a-dystroglycan. The latter binds to laminin-2 (Fig. 8-33), a protein that binds the cell to the basal lamina. Four... 

Accumulation of potassium within the cell by a cooperative process is a transmembrane event. However, there is also good evidence that movement of potassium ions through membrane channels is modulated by the binding of calcium ions at channel sites - the "plug in the bath" model (7). Membrane surface glycoproteins with polyanionic terminal strands offer a broad and powerful substrate for these cationic interactions. Competition between hydrogen and calcium ions at these sites has been modeled as the initial transductive step in excitation (8). 

The blood-brain barrier is a biochemical as well as a physical barrier. Brain endothelial cells create an enzymatic barrier composed of secreted proteases and nucleotidases, as well as intracellular metabolizing enzymes such as cytochrome P-450. Furthermore, y-glutamyl transpeptidase, alkaline phosphatase, and aromatic acid decarboxylase are more prevalent in cerebral microvessels than in nonneuronal capillaries. The efflux transporter P-glycoprotein and other extrusion pumps are present on the membrane surface of endothelial cells, juxtaposed toward the interior of the capillary. Furthermore, CNS endothelial cells display a net negative charge at the interior of the capillaries and at the basement membrane. This provides an additional selective mechanism by impeding passage of anionic molecules across the membrane. 

The alphaviruses are a group of 26 icosahedral, positive-sense RNA viruses primarily transmitted by mosquitoes [64]. These 700-A-diameter viruses are some of the simplest of the membrane-enveloped viruses, and members of this group cause serious tropical diseases with characteristic symptoms such as myositis, fever, rash, encephalitis, and polyarthritis [65]. The structures of two different alphavirus-Fab complexes have been determined by cryo-TEM Ross River virus (RR) and Sindbis virus (SIN) [66]. The amino acid sequences of the RR and SIN virus structural and nonstructural proteins are 49 and 64% identical, respectively [67]. The viral RNA genome and 240 copies of the capsid protein form the nucleocapsid core [68-73], and the El and E2 glycoproteins form heterodimers that associate as 80 trimeric spikes on the viral surface. Native SIN and RR lack the E3 glycoprotein because it disassociates from the spike complex after its display on the plasma membrane surface [74, 75]. El has a putative fusion domain that may facilitate host membrane penetration [76, 77]. E2 contains most of the neutralizing epitopes and is also probably involved in host cell recognition [78-80]. 

Allan, Auger and Crumpton used Con A-Sepharose to isolate glycoprotein cell surface receptors for concanavalin A from pig lymphocyte membranes solubilized with sodium deoxycholate [134]. 

Bile acid release occurs at the biliary membrane surface in the canahculi either actively, facilitated by ATP-driven carrier glycoproteins such as MRP2 (also called cMRP or MOAT) and BSEP (identical to cBAT), or by exocytosis of intracellularly derived mixed vesicles, or with the help of a potential-dependent membrane carrier. 

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