Endoplasmic reticulum localization of Sec12p is achieved by two mechanisms: Rer1p-dependent retrieval that requires the transmembrane domain and Rer1p-independent retention that involves the cytoplasmic domain

In order to understand molecular mechanisms underlying these sorting events, we designed a screening to isolate mutants that mislocalize Secl2p beyond the early Golgi. A fusion protein of Secl2p and the precursor of a-mating factor (Secl2-Mfctlp) was used as a marker protein. If this fusion protein is mislocalized to the late Golgi, the Mfetl moiety is processed to mature a-factor, which is then secreted to the medium. Using this trick, we have isolated two rer mutants (Nishikawa and Nakano, 1993) . The RER1 gene encodes a protein of 188 amino acid residues (Rerlp) containing four putative transmembrane domains. The null mutant of RER1 is viable, and even in this mutant, a significant portion of Secl2p is still located in the ER. Immunofluorescence microscopy and subcellular fractionation experiments demonstrated that Rerlp is mainly localized to the Golgi complex. This suggests that Rerlp functions at the retrieval step (Sato et al., 1995) .