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

Arerl (SKY7) cells. Expression of each chimera was examined by immunoblotting with the anti-Mfixlp antibody. Although the cellular levels tended to be higher for the chimeras containing the NH2-terminal cytoplasmic domain of Sec12p (SSSm, SDSm, SDDm, and SSDm), no appreciable difference was observed between Rerl ÷ and zirerl cells (data not shown). The band densities were quantified by image scanning and used for further analysis (see below).