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

We used a coprecipitation assay to ask if otmlG formed hetero-oligomers with Gml or other related Golgi-resident proteins. HeLa cells were cotransfected to express both otmlG and either Gml, Gmlins, or VSV G. After a short metabolic labeling period (5 min), cells were solubilized either immediately or after a 60-min chase. Lysates were immunoprecipitated using anti-hCG antibody. When otmlG was expressed alone, the time-dependent accumulation of SDS-resistant oligomer at the gel interface (arrowhead) was observed as described above (Fig. 7, lane 2) . Interestingly, when otmlG was coexpressed with Gml or Gmlins, the size of the oligomer shifted dramatically, and now migated at the top of the stacking gel (Fig. 7, lanes 4 and 8) . Furthermore, some proteins appeared to be solubilized during electrophoresis, since a small amount of material migrating with authentic Gml and Gmlins w a s alSO detected in these lanes (Fig. 7, arrow) . In contrast, no change in the size of the oligomer was observed when c~mlG was coexpressed with VSV G (Fig. 7, lane 6 ) or with GmlQLs0 (data not shown), which are transported to the plasma membrane. Furthermore, when cells expressing Gml and c~m (containing the VSV G membrane-spanning domain) were immunoprecipitated using anti-hCG antibody, no Gml was recovered (data not shown).