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

We have been investigating how yeast Secl2p is localized to the ER. Secl2p is a pivotal membrane component required for vesicle budding from the ER and needs to be strictly localized to the ER membrane, but there is also evidence that it recycles between the ER and the Golgi apparatus (Nakano et al., 1998; Nishikawa and Nakano, 1993; Oka and Nakano, 1994) . We have shown in this paper that such localization of Secl2p is achieved by two mechanisms: static retention in the ER that involves the NHz-terminal cytoplasmic domain of Sec12p and dynamic retrieval from the Golgi to the ER that requires the TMD. We shall call the structural motifs contained in these domains the retention and retrieval signals, respectively. A Golgi membrane protein, Rerlp, plays an important role in the retrieval process (Sato et al., 1995) . The retrieval by the TMD signal depends on the presence of Rerlp in the Golgi, whereas the retention by the cytoplasmic signal is Rerlp-independent (Fig. 13) .