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

Increasing numbers of reports suggest that some membrane proteins have multiple signals for localization. For example, Graham and Krasnov (1995) showed that the COOH-terminal lumenal domain and the TMD of yeast Mnnlp are important for the Golgi localization; Itin et al. (1995) reported that both the lumenal and cytoplasmic domains of human ERGIC-53 are involved in ER-ERGICc/s-Golgi recycling; and Szczesna-Skorupa et al. (1995) claimed that the TMD and the COOH-terminal cytoplasmic domain of rabbit cytochrome P450 2C1 and 2C2 contain signals for the ER localization, However, the functional differentiation of these signals remains unclear. In this paper, we have shown for the first time that yeast Sec12p contains two distinct signals that correpond to two different mechanisms, retention and retrieval. Previously, Boehm et al. (1994) implied the importance of the Secl2p TMD by comparing the secretion of invertase from Sec22et-factor-invertase and Sec22/12-et-factor-invertase fusions. They showed that secretion of the invertase activity was suppressed by introduction of the Secp TMD and that this the ER, whereas the TMD of Sec12p is required for the efficient Rerlp-dependent retrieval from the Golgi to the ER. The parts of Secl2p that act as signals of retention and retrieval are filled in black.