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

A similar pulse-chase experiment was performed on DSD and SDD using anti-Dap2p and anti-Secl2p anti- bodies, respectively (Fig. 9) . DSD was detected as triplet bands at around 110 kD (Fig. 9 A) . This heterogeneity is due to the N-linked oligosaccharides because they coalesced to a single band by endo H treatment. Apparently, the rate of ctl--->6 modification of DSD was much faster than that of Secl2p. 40% of DSD became precipitable by the od--->6 mannose antibody at 60 min. Even at 0 min, a population (N6%) that was modified with cd--->6 linkage was detectable after endo H treatment. DSD was little modified with cd--->3 mannosyl linkage, indicating that this chimera efficiently arrives at the early Golgi but does not go beyond there. Considering the fact that DSD is exclusively localized to the ER as observed by immunofluorescence, this result strongly suggests that it is the retrieval from the Golgi to the ER that operates predominantly to localize DSD to the ER.