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 contrast, the effect of the NH2-terminal cytoplasmic domain is quite different. It does not require the presence of Rerlp and appears to be less efficient than the TMD for ER localization. The analysis on mannosyl modification on SDD, a construct localized to the ER by the cytoplasmic domain, indicates that this molecule is largely detained in the ER. The rate of al--->6 modification is as slow as that of the authentic Sec12p. This strongly suggests that the ability of the cytoplasmic domain to localize the protein in the ER is due to the static retention mechanism. Interestingly, the SDD molecules that have escaped the retention and acquired the etl--->6 linkage appear to be further modified by the etl---~3 mannose in time. This may be because the retrieval system does not work effectively on SDD. This is also consistent with the fact that the ER localization of SDD is not affected by the deletion of RERI.