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Vol. 18, Issue 1, 153-165, January 2007

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The Cytoplasmic Hsp70 Chaperone Machinery Subjects Misfolded and Endoplasmic Reticulum Import-incompetent Proteins to Degradation via the Ubiquitin–Proteasome System

Sae-Hun Park^*, Natalia Bolender^*, Frederik Eisele^*, Zlatka Kostova^*,, Junko Takeuchi, Philip Coffino, and Dieter H. Wolf^*

*Institut fuer Biochemie, Universitaet Stuttgart, 70569 Stuttgart, Germany; and Department of Microbiology and Immunology, University of California, San Francisco, CA 94143

Submitted April 21, 2006; Revised October 13, 2006; Accepted October 17, 2006
Monitoring Editor: Randy Schekman

The mechanism of protein quality control and elimination of misfolded proteins in the cytoplasm is poorly understood. We studied the involvement of cytoplasmic factors required for degradation of two endoplasmic reticulum (ER)-import–defective mutated derivatives of carboxypeptidase yscY (ssCPY* and ssCPY*-GFP) and also examined the requirements for degradation of the corresponding wild-type enzyme made ER-import incompetent by removal of its signal sequence (ssCPY). All these protein species are rapidly degraded via the ubiquitin–proteasome system. Degradation requires the ubiquitin-conjugating enzymes Ubc4p and Ubc5p, the cytoplasmic Hsp70 Ssa chaperone machinery, and the Hsp70 cochaperone Ydj1p. Neither the Hsp90 chaperones nor Hsp104 or the small heat-shock proteins Hsp26 and Hsp42 are involved in the degradation process. Elimination of a GFP fusion (GFP-cODC), containing the C-terminal 37 amino acids of ornithine decarboxylase (cODC) directing this enzyme to the proteasome, is independent of Ssa1p function. Fusion of ssCPY* to GFP-cODC to form ssCPY*-GFP-cODC reimposes a dependency on the Ssa1p chaperone for degradation. Evidently, the misfolded protein domain dictates the route of protein elimination. These data and our further results give evidence that the Ssa1p-Ydj1p machinery recognizes misfolded protein domains, keeps misfolded proteins soluble, solubilizes precipitated protein material, and escorts and delivers misfolded proteins in the ubiquitinated state to the proteasome for degradation.

Present address: Center for Cancer Research, National Cancer Institute, Frederick, MD 21702.

Address correspondence to: Dieter H. Wolf (dieter.wolf{at}ibc.uni-stuttgart.de)

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