Glycosylation membrane proteins

The a2/8 subunit is a glycosylated membrane protein of 125,018 Da, which is processed post-translationally by proteolysis, resulting in an U2 pro-... 

Four different types of lipid-anchoring motifs have been found to date. These are amide-linked myristoyl anchors, thioester-linked fatty acyl anchors, thioether-linked prenyl anchors, and amide-linked glycosyl phosphatidylinosi-tol anchors. Each of these anchoring motifs is used by a variety of membrane proteins, but each nonetheless exhibits a characteristic pattern of structural requirements. 

Most ABC-transporters, especially those located in the plasma membrane, are phosphorylated and glycosylated transmembrane proteins of different molecular weights (e.g., P-gp 170 kDa MRP2 190 kDa BCRP 72 kDa). Topologically, most ABC-transporter show a similar structure they are organized in two transmembrane domains (TMD), each consisting of six... 

Low, M. G. The glycosyl-phosphatidyl anchor of membrane proteins. Biochim. Biophys. Acta. 988 4217-454,1989. 

Processing of proteins in the Golgi complex includes sorting and glycosylation of membrane proteins and secretory proteins 148 Proteins and lipids move through Golgi cisternae from the cis to the trans direction 148... 

C-termini and a large glycosylated extracellular loop between transmembrane domains 3 and 4. The proteins show the most homology in their transmembrane spanning domains, particularly domains 1, 2, and 4-8, which may be involved in moving the transmitter across the membrane. The transporters are substrates for PKC-dependent phosphorylation, which reduces their activity. The dopamine transporter is phosphorylated on the extreme end of the N-terminal tail, and consensus phosphorylation sites for various other kinases are present in the intracellular loops and domains [20-22] (Fig. 12-4). The dopamine and norepinephrine transporters form functional homo-oligomers, although it is not known if this is required for transport activity, and the transporters also interact with many other membrane proteins that may control their cell-surface expression or other properties. 

However, certain limitations do exist that need to be considered. Although enzymes necessary for post-translational modifications can be added, in principle there is currently no productive system available for the preparation of glycosylated proteins, although some interesting results have already been obtained [161]. Also, the expression of functional membrane proteins in quantities necessary for structural analysis will be a challenging task for the future. 

A subsequent study in 2002 of 27 families with a condition known as multiminicore disease (MmD) also linked mutations in SEPNl to disease pathology. Multiple mutations were identified in exons 1, 5, 7, 8, 10, and 11, and the authors also mentioned that this region (RSMD) had been previously linked to MmD. Minicores are lesions by histochemistry of mitochondrial depletion within muscle tissue. The first biochemical study of selenoprotein N aimed to identify the protein localization by immunohistochemistry and found that the primary protein product of several identified mRNAs (splice variants) was a 70 kDa protein present in the endoplasmic reticulum. Two potential ER targeting domains were shown to be present and the peptide expressed from the first exon was shown to be required for localization into the ER. This study also revealed that selenoprotein N was an integral membrane protein that is N-glycosylated. Expression analysis showed pronounced levels in embryonic tissue with a reduction after development and differentiation. 

Figure 4-4. The domain organization of an integral, transmembrane protein as well as the mechanisms for interaction of proteins with membranes. The numbers illustrate the various ways by which proteins can associate with membranes I, multiple transmembrane domains formed of a-helices 2, a pore-forming structure composed of multiple transmembrane domains 3, a transmembrane protein with a single a-helical membrane-spanning domain 4, a protein bound to the membrane by insertion into the bilayer of a covalently attached fatty acid (from the inside) or 5, a glycosyl phosphatidylinositol anchor (from the outside) 6, a protein composed only of an extracellular domain and a membrane-embedded nonpolar tail 7, a peripheral membrane protein noncova-lently bound to an integral membrane protein.

C. Most membrane proteins undergo post-translational glycosylation to improve their interactions with the aqueous environment and to protect them from degradation by proteases. 

Thus, the effects of glycosylation inhibitors on intact cells may also be studied best with virus-infected cells. Before release of virus, the glycoproteins are detected in the water-insoluble, membranous fraction. Furthermore, the lipid-linked oligosaccharides may be rather specifically extracted from whole cells, and monosaccharide-lipids may also be determined.3-116 It is thus seen that the various tools of virology and of lipid and carbohydrate biochemistry have proved productive in establishing the mode of action of inhibitors of lipid-depen-dent glycosylation of proteins. 

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