Protein degradation in cell cultures: general considerations on mechanisms and regulation

The general characteristics of proteolysis of endogenous proteins and its regulation are outlined, with particular reference to studies with cultured cells. The distinction between 'basal' (in nutritionally complete conditions) and 'accelerated' (in conditions deficient in certain nutrients or hormones) degradation is discussed. Evidence substantiating lysosomal participation in accelerated proteolysis is summarized. It is argued that basal proteolysis may also involve lysosomes, and new evidence on basal degradation in mouse peritoneal macrophages is presented. This work has utilized a new group specific inhibitor of thiol proteinases, synthesized by Dr. Elliot Shaw, carbobenzoxycarbonyl-L-phenylalanyl-L-alanine-D-dizaomethane (Z-Phe-Ala-CHN2). The inhibitor produces a correlated dose-dependent inhibition of cellular cathepsin B and of ongoing proteolysis. There is a lag before it is effective that has been shown to be due to its slow entry (probably by pinocytosis) into the cells. Possible mechanisms of selectivity of degradation, and the relative roles of nonlysosomal and lysosomal degradative routes, are discussed briefly.