Effect of substrate heat-denaturation on ATP- and ubiquitin-dependent proteolysis

The susceptibility of human serum albumin (HSA) to degradation by the ATP- and ubiquitin-dependent proteolytic system is enhanced by heat-denaturation. The steady-state levels of formed ubiquitin conjugates, however, are equal with denatured and native HSA. In contrast, both the hemin sensitive Pi release and proteolysis with native HSA are less than one fourth of that with denatured HSA. The rate limiting step for the degradation of HSA by the ATP- and ubiquitin-dependent proteolytic system must be beyond the ubiquitin conjugation steps. Differences concerning the effects of heat denaturation of mitochondria-containing stroma and HSA on ATP- and ubiquitin-dependent proteolysis are discussed.