ε-COP is a structural component of coatomer that functions to stabilize α-COP

We isolated a novel yeast α-COP mutant, ret1-3, in which α-COP is degraded after cells are shifted to a restrictive temperature. ret1-3 cells cease growth at 28°C and accumulate the ER precursor of carboxypeptidase Y (p1 CPY). In a screen for high copy suppressors of these defects, we isolated the previously unidentified yeast ε-COP gene. ε-COP (Sec28p) overproduction suppresses the defects of ret1-3 cells up to 34°C, through stabilizing levels of α-COP. Surprisingly, cells lacking ε-COP (sec28Δ) grow well up to 34°C and display normal trafficking of carboxypeptidase Y and KKXX-tagged proteins at a permissive temperature. ε-COP is thus non-essential for yeast cell growth, but sec28Δ cells are thermosensitive. In sec28Δ cells shifted to 37°C, mild-type α-COP (Ret1p) levels diminish rapidly and cells accumulate p1 CPY; these defects can be suppressed by α-COP overproduction. Mutant coatomer from sec28Δ cells behaves as an unusually large protein complex in gel filtration experiments. The sec28Δ mutation displays allele-specific synthetic-lethal interactions with α-COP mutations: sec28Δ ret1-3 double mutants are unviable at all temperatures, whereas sec28Δ ret1-3 double mutants grow well up to 30°C. Our results suggest that a function of ε-COP is to stabilize α-COP and the coatomer complex.