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J Biol Chem, Vol. 275, Issue 14, 10655-10660, April 7, 2000

Impaired Proteasome Function Rescues Thermosensitivity of Yeast Cells Lacking the Coatomer Subunit -COP^*

Yukio Kimata^§, Hironori Higashio, and Kenji Kohno^§¶

From the  Research and Education Center for Genetic Information, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101 and ^§ CREST, Japan Science and Technology Corporation, Tokyo 101-0062, Japan

Formation of COPI-coated transport vesicles requires a cytosolic protein complex consisting of seven subunits: -, -, '-, -, -, - and -COP, collectively designated coatomer. The yeast Saccharomyces cerevisiae gene encoding the -COP subunit is known as SEC28/ANU2. anu2 null mutant cells (anu2) are temperature-sensitive, and -COP is rapidly degraded in these cells when they are shifted to the restrictive temperature. We isolated extragenic suppressors that rescue the temperature-sensitive growth defect of anu2 cells. Genetic analysis revealed that one of the suppressors is allelic to PRE8 (PRS4), which encodes a 20 S proteasome subunit. In the presence of a proteasome inhibitor, MG132, anu2 cells did not cease growth even at the restrictive temperature. Furthermore, MG132 inhibited the rapid decrease of -COP levels in anu2 cells shifted to the restrictive temperature. However, secretion of certain proteins by these cells was impaired even in the presence of  MG132. In conclusion, impairment of proteasome-dependent proteolysis rescued some, but not all, temperature-sensitive defects of anu2 cells. These results are discussed in terms of evidence that -COP plays a critical role in maintaining the structural integrity of -COP.

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