Posttranslational quality control: Folding, refolding, and degrading proteins

Polypeptides emerging from the ribosome must fold into stable three-dimensi onal structures and maintain that structure throughout their functional Lif etimes. Maintaining quality control over protein structure and function dep ends on molecular chaperones and proteases, both of which can recognize hyd rophobic regions exposed on unfolded polypeptides. Molecular chaperones pro mote proper protein folding and prevent aggregation, and energy-dependent p roteases eliminate irreversibly damaged proteins. The kinetics of partition ing between chaperones and proteases determines whether a protein will be d estroyed before it folds properly. When both quality control options fail, damaged proteins accumulate as aggregates, a process associated with amyloi d diseases.