Oligomerization of a membrane protein correlates with its retention in the Golgi complex

The kinetics of Gml oligomer formation were consistent with the time at which newly synthesized proteins arrive at the cis-Golgi. To determine if oligomerization occurred in a post-ER compartment, we asked whether the SDSresistant oligomer would form when ER to Golgi traffic was blocked by incubation at 16°C. At this temperature, newly synthesized proteins accumulate in the intermediate compartment (Saraste and Kuismanen, 1984) . HeLa cells were transfected with constructs encoding Gml or VSV G, metabolically radiolabeled for 5 min, and then chased at 16 or 37°C as indicated for up to 90 min (Fig. 4) . No oligomer formed when cells expressing Gml were chased at 160C; however, if cells chased at 16°C were subsequently transferred to 370C, oligomer formation occurred. In separate experiments, we found that the kinetics of oligomer formation at 37°C were unaffected by the inclusion of a 160C pretreatment step (data not shown). In addition, treatment with 0.3 mM CCCP during the chase period also inhibited SDS-resistant oligomer formation (not shown). This treatment blocks transport of newly synthesized proteins from the ER. These experiments suggested that Gml oligomer formation normally occurred in a post-ER compartment. However, brefeldin A pretreatment (or treatment during chase) did not block oligomer formation (not shown), although it did cause redistribution of Gml to an ER-like pattern (Machamer et al., 1993) .