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1.VESICLES PROTEIN-COATED
vesicles are similarly involved in the transport of materials taken up at the cell surface. Vesicular transport is thus a major cellular activity, responsible for molecular traffic between a variety of specific membrane-enclosed compartments. The selectivity of such transport is therefore key to maintaining the functional organization of the cell. These proteins are transported within vesicles, so the specificity of transport is based on the selective packaging of the intended cargo into vesicles that recognize and fuse only with the appropriate target membrane.
2.ER-RESIDENT PROTEINS MAINTAINED
To exit from the ER, proteins must be properly folded and, if they are subunits of multimeric protein complexes, they may need to be completely assembled. Those that are misfolded or incompletely assembled are retained in the ER, where they are bound to chaperone proteins , such as BiP or calnexin.
The chaperones may cover up the exit signals or somehow anchor the proteins in the ER . Such failed proteins are eventually transported back into the cytosol where they are degraded by proteasomes. This quality-control step is important, as misfolded or misassembled proteins could potentially interfere with the functions of normal proteins if they were transported onward. The amount of corrective action is surprisingly large.
More than 90% of the newly synthesized subunits of the T cell receptor and of the acetylcholine receptor, for example, are normally degraded in the cell without ever reaching the cell surface, where they function. Thus, cells must make a large excess of many protein molecules from which to select the few that fold and assemble properly.
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