Impaired proteasome function rescues thermosensitivity of yeast cells lacking the coatomer subunit epsilon-COP

Formation of COPI-coated transport vesicles requires a cytosolic protein co mplex consisting of seven subunits: alpha-, beta-, beta'-, gamma-, delta-, epsilon-, and zeta-COP, collectively designated coatomer. The yeast Sacchar omyces cerevisiae gene encoding the E-COP subunit is known as SEC28/ ANU2. anu2 null mutant cells (anu2 Delta) are temperature-sensitive, and LU-COP i s 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 anu-2 Delta cells. Genetic analysis revealed th at one of the suppressors is allelic to PRE8 (PRS4), which encodes a 20 S p roteasome subunit. In the presence of a proteasome inhibitor, MG132, anu2 D elta cells did not cease growth even at the restrictive temperature. Furthe rmore, MG132 inhibited the rapid decrease of cu-COP levels in anu2 Delta ce lls shifted to the restrictive temperature. However, secretion of certain p roteins by these cells was impaired even in the presence of MG132. In concl usion, impairment of proteasome-dependent proteolysis rescued some, but not all, temperature-sensitive defects of anu2 Delta cells. These results are discussed in terms of evidence that E-COP plays a critical role in maintain ing the structural integrity of alpha-COP.