EDITING OF UBIQUITIN CONJUGATES BY AN ISOPEPTIDASE IN THE 26S PROTEASOME

In eukaryotes, ubiquitin (Ub)-dependent proteolysis is essential for b ulk protein turnover as well as diverse processes including cell-cycle control, differentiation, antigen presentation, and the stress respon se(1-3). Generally, multiple ubiquitins are added onto a substrate to form an isopeptide-linked 'polyubiquitin' chain, which targets substra tes for degradation by the 26S proteasome(4-7). The specificity of Ub- dependent degradation was thought to depend primarily on the selection of targets for ubiquitination, but recently we have reported evidence (8) for a second level of specificity in which (poly)Ub-protein conjug ates are partitioned among two fates: degradation of the protein subst rate by the 26S proteasome; and disassembly by Ub isopeptidase(s) to r egenerate the protein substrate. Potentially, an isopeptidase could in fluence degradation by 'editing' (poly)Ub-protein conjugates according to the extent of ubiquitination rather than the structure of the ubiq uitination target itself. Here we describe a bovine isopeptidase that is well suited to such an editing function because of its unique local ization and specificity, This enzyme is an intrinsic subunit of PA700, the 19S regulatory complex of the 26S proteasome. By disassembling th e degradation signal from only the distal end of (poly)Ub chains, this isopeptidase can selectively rescue poorly ubiquitinated or slowly de graded Ub-protein conjugates from proteolysis.