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

protein in the ER, which is essential for the formation of transport vesicles. From biochemical and morphological lines of evidence, we have proposed that Secl2p is localized to the ER by two mechanisms: static retention in the ER and dynamic retrieval from the early Golgi compartment. We have also shown that Rerlp, a membrane protein in the Golgi, is required for correct localization of Secl2p. In the present study, we have performed a systematic analysis to determine the ER localization signals in Sec12p corresponding to these two mechanisms. Both the transmembrane domain (TMD) and the NH2-terminal cytoplasmic domain of Sec12p show the ability to localize the protein to the ER. The effect of the TMD is potent and sufficient by itself for the ER localization and is strongly dependent on Rerlp. On the other hand, the cytoplasmic domain shows a moderate ER-localization capability which is independent of Rerlp. The rate of mannosyl modification has been measured to distinguish between retention and retrieval. The cytoplasmic domain significantly delays the transport from the ER to the cis-Golgi. In contrast, the TMD shows only a subtle retardation in the transport from the ER to the cis-Golgi but strictly prevents the transport beyond there. From these observations, we conclude that the TMD mainly acts as the retrieval signal and the cytoplasmic domain contains the retention signal. This study not only supports the two-mechanisms hypothesis but also provides powerful tools to dissect the two.