The molecular mechanisms that control organelle size, shape,
and number remain unresolved. An approach to this issue involves
analysis of karmellae, an ER array induced by increased levels of
Hmg1p, one of two yeast HMG-CoA reductase isozymes. Previous studies
demonstrate that karmellae assembly is abnormal in many vacuole
biogenesis and secretory mutants, suggesting that maintenance of ER
composition is an essential prerequisite. In a new screen using
deletion consortium mutants, we found that cue1 and ubc7
cause cold sensitivity in cells with elevated Hmg1p. Karmellae
assembly was also abnormal in these mutants. Ubc7p is a
ubiquitin-conjugating enzyme that binds to the ER via association with
the ER membrane protein, Cue1p. Both proteins are involved in
ER-associated degradation (ERAD), a process that removes proteins from
the ER and delivers them to the proteasome. Since Hmg1p is not an ERAD
target (Hampton & Bhakta, PNAS 94:12944), the role of CUE1 and
UBC7 was puzzling. Mutations in other ERAD genes, UBC6,
DER1, HRD3, DER3, did not cause noticeable
phenotypes. Using a panel of Hmg1p mutants, we found that HMGR
activity was responsible for the cold sensitivity of cue1 and
ubc7 mutants. We hypothesize that cold-induced changes in ER
composition require resolution by Ubc7p/Cue1p. In cue1 and
ubc7 cells, persistence of such changes may prevent karmellae
assembly and make cells sensitive to sterol precursors that accumulate
as a result of unchecked HMG-CoA reductase activity.
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