Intracellular degradation of newly synthesized proteins

To maintain the steady state, proteins which cannot be secreted must be degraded. On the other hand, unassembled or malfolded proteins are retained in the ER via interactions with BiP or disulfide interchange reactions, and may thus not reach the lyso-some. This apparent paradox was solved when it became clear that a proteolytic system, independent from the lysosomal one, exists in a pre-Golgi compartment. Heavy chains are degraded more rapidly when not assembled to L chains. Assembly might decrease the susceptibility to proteases, hide signals which target to degradation, or both.

The biochemistry of ER degradation is still largely unknown. Evidence has been recently obtained that ubiquitin and proteasomes are involved in the degradation of CFTR, a transmembrane protein retained in the ER in cystic fibrosis patients. As to the specificity of the process, in many cases, including T cell receptors and Igs, the signals that lead to degradation have been shown to coincide with those that mediate the assembly as well as the retention of unassembled subunits. However, there must be a lag while nascent proteins, that are bound to express many degradation targeting signals, are protected from proteolysis. A distinct subcompartment, specialized in proteolytic activity, may exist in the ER. Alternatively, nascent proteins maybe shielded by chaperones.

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