Proteolytic control of protein topogenesis.

The data on proteolytic control of protein topogenesis and of cell organelles assembly are critically reviewed. There are grounds to suggest that the slow conformational maturation of polypeptide chains, the defects in their modification (i. e., glycosylation) and the delay in some subcellular compartments on the exocytic pathway are responsible for the enhanced proteolytic degradation of proteins. The same holds true for the miscompartmentalized proteins. Another aspect of proteolytic control is the assembly of the multisubunit protein complexes in a cell. The cases of nonstoichiometric subunits synthesis, followed by the degradation of unassembled subunits, are summarized. Stoichiometric ratios of subunits in a cell are supposed to be established by this means. Such a mechanism is especially important when these subunits are synthesized asynchronously or when they are encoded by different genomes and are produced by different protein-synthesizing systems. Most probably, proteolytic control is operative at all stages of the respiratory competent mitochondria formation: (i) when protein precursors are imported from the cytosol, (ii) when the individual polypeptides are assembled into complexes, and (iii) when functioning multienzyme ensembles are formed. From this point of view, a pleiotropic character of mutations in individual proteins of mitochondria and chloroplasts is considered. Thus proteolytic systems of a cell play an important role in protein topogenesis and in the biogenes of cell organelles.