Sec12p requires Rer1p for sorting to coatomer (COPI)-coated vesicles and retrieval to the ER

In Saccharomyces cerevisiae cells lacking the Rer1 protein (Rer1p), the type II transmembrane protein Sec12p fails to be retained in the ER. The transmembrane domain of Sec12p is sufficient to confer Rer1p-dependent ER retention to other membrane proteins. In rer1 mutants a large part of the Sec12-derived proteins can escape to the late Golgi. In contrast, rer3 mutants accumulate Sec12-derived hybrid proteins carrying early Golgi modifications. We found that rer3 mutants harbour unique alleles of the alpha-COP-encoding RET1 gene. ret1 mutants, along with other coatomer mutants, fail to retrieve KKXX-tagged type I transmembrane proteins from the Golgi back to the ER. Surprisingly rer3-11(=ret1-12) mutants do not affect this kind of ER recycling. Pulse-chase experiments using these mutants show that alpha-COP and Rer1p function together in a very early Golgi compartment to initiate the recycling of Sec12p-derived hybrid proteins. Rer1p protein may be directly involved in the retrieval process since it also recycles between the early Golgi and ER in a coatomer (COPI)-dependent manner. Rer1p may act as an adapter coupling the recycling of non-KKXX transmembrane proteins like Sec12p to the coatomer (COPI)-mediated backward traffic.