Originally published as Genetics Published Articles Ahead of Print on August 22, 2005.

Genetics, Vol. 171, 1477-1484, December 2005, Copyright © 2005
doi:10.1534/genetics.105.046888

Saccharomyces cerevisiae Ub-Conjugating Enzyme Ubc4 Binds the Proteasome in the Presence of Translationally Damaged Proteins

Department of Biochemistry, Robert Wood Johnson Medical School, Piscataway, New Jersey 08854

1 Corresponding author: Department of Biochemistry, SPH/Research Bldg., Room 383, 683 Hoes Lane, Piscataway, NJ 08854.
E-mail: maduraki{at}umdnj.edu

Surveillance mechanisms that monitor protein synthesis can promote rapid elimination of misfolded nascent proteins. We showed that the translation elongation factor eEF1A and the proteasome subunit Rpt1 play a central role in the translocation of nascent-damaged proteins to the proteasome. We show here that multiubiquitinated proteins, and the ubiquitin-conjugating (E2) enzyme Ubc4, are rapidly detected in the proteasome following translational damage. However, Ubc4 levels in the proteasome were reduced significantly in a strain that expressed a mutant Rpt1 subunit. Ubc4 and Ubc5 are functionally redundant E2 enzymes that represent ideal candidates for ubiquitinating damaged nascent proteins because they lack significant substrate specificity, are required for the degradation of bulk, damaged proteins, and contribute to cellular stress-tolerance mechanisms. In agreement with this hypothesis, we determined that ubc4{Delta} ubc5{Delta} is exceedingly sensitive to protein translation inhibitors. Collectively, these studies suggest a specific role for Ubc4 and Ubc5 in the degradation of cotranslationally damaged proteins that are targeted to the proteasome.




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