A 26-S PROTEASE SUBUNIT THAT BINDS UBIQUITIN CONJUGATES

Ubiquitin-mediated proteolysis provides an important mechanism for reg ulating a variety of cellular processes. Ubiquitin-conjugated proteins are degraded by a 26 S protease that contains more than 30 different subunits. Of these, a single 50-kDa polypeptide, subunit 5, specifical ly binds ubiquitin-lysozyme conjugates. Binding is inhibited by short polymeric chains of ubiquitin but not by ubiquitin monomers or by lyso zyme. In addition, subunit 5 binds free ubiquitin chains with efficien t association requiring at least four ubiquitins. Thus, proteins conju gated to polymers of ubiquitin may be selected for degradation by a si ngle subunit of the 26 S protease complex.