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	Mol. Biol. Cell May (2007); 18(5):1953-63
A conditional yeast E1 mutant blocks the ubiquitin-proteasome pathway and reveals a role for ubiquitin conjugates in targeting Rad23 to the proteasome-
Ghaboosi N, Deshaies RJ
Howard Hughes Medical Institute, Division of Biology, California Institute of Technology, Pasadena, CA 91125, USA-
Abstract: E1 ubiquitin activating enzyme catalyzes the initial step in all ubiquitin-dependent processes- We report the isolation of uba1-204, a temperature-sensitive allele of the essential Saccharomyces cerevisiae E1 gene, UBA1- Uba1-204 cells exhibit dramatic inhibition of the ubiquitin-proteasome system, resulting in rapid depletion of cellular ubiquitin conjugates and stabilization of multiple substrates- We have employed the tight phenotype of this mutant to investigate the role ubiquitin conjugates play in the dynamic interaction of the UbL-UBA adaptor proteins Rad23 and Dsk2 with the proteasome- Although proteasomes purified from mutant cells are intact and proteolytically active, they are depleted of ubiquitin conjugates, Rad23, and Dsk2- Binding of Rad23 to these proteasomes in vitro is enhanced by addition of either free or substrate-linked ubiquitin chains- Moreover, association of Rad23 with proteasomes in mutant and wild-type cells is improved upon stabilizing ubiquitin conjugates with proteasome inhibitor- We propose that recognition of polyubiquitin chains by Rad23 promotes its shuttling to the proteasome in vivo-
[PUBMED: 17360968]