The Ubiquitin Ligase Hul5 Promotes Proteasomal Processivity

Abstract

The 26S proteasome is a large cytoplasmic protease that degrades polyubiquitinated proteins to short peptides in a processive manner. The proteasome 19S regulatory subcomplex tethers the target protein via its polyubiquitin adduct and unfolds the target polypeptide, which is then threaded into the proteolytic site-containing 20S subcomplex. Hul5 is a 19S subcomplex-associated ubiquitin ligase that elongates ubiquitin chains on proteasome-bound substrates. We isolated hul5Δ as a mutation with which fusions of an unstable cyclin to stable reporter proteins accumulate as partially processed products. These products appear transiently in the wild type but are strongly stabilized in 19S ATPase mutants and in the hul5Δ mutant, supporting a role for the ATPase subunits in the unfolding of proteasome substrates before insertion into the catalytic cavity and suggesting a role for Hul5 in the processive degradation of proteins that are stalled on the proteasome.

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Articles of Significant Interest Selected from This Issue by the Editors

We thank Dick Kulka, Johannes Hegemann, Bernat Crosas, and Michael Glickman for plasmids and strains, Tamar Ziv and the Smoler proteomics center for tryptic peptide analysis, Avram Hershko and Michael Glickman for helpful discussions, and Dan Finley and Sara Selig for comments on the manuscript.

This research project was supported by a grant from the Israel Science Foundation.