PROTEIN-DEGRADATION - A MAJOR INTRACELLUL AR FUNCTION .1. MECHANISMS OF PROTEIN-DEGRADATION

Although often neglected, protein degradation is a major intracellular function which, on the one hand, is responsible for housekeeping of t he cell and, on the other hand, plays a major role in the regulation o f important cellular functions. Among those, one can mention the contr ol of cell homeostasis, the production of antigenic peptides and the r egulation of key steps of embryo development and of the cell cycle. Bi ological and biochemical bases of protein catabolism are not, or hardl y, elucidated. Even in the best studied cases, only sparse information is available regarding the number of catabolic pathways operating wit hin a cell, the number of these pathways operating on a single protein , the site(s) of degration of proteins shuttling between two intracell ular compartments, the motifs recognized by proteolytic systems and th e regulation of the degradation process. The first part of this review aims at summarizing our current knowledge on intracellular proteolyti c systems and their mode of action whereas the second part will addres s the regulation of protein turnover and genesis of antigenic peptides .