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

The SDS-resistant oligomerization did not depend on the type of cells used for expression, since the same results were found using transiently transfected BHK cells and stably transfected CHO cells (not shown). Because the SDSresistant material was easiest to detect in HeLa cells, we used these cells for subsequent experiments. The material at the top of the stacking gel could be detected after transfer to nitrocellulose and probing with anti-VSV G antibodies by immunoblotting, suggesting that it contained Gml (or the related constructs; data not shown). Furthermore, this large species was efficiently precipitated by several conformationsensitive anti-VSV G monoclonal antibodies that do not recognize grossly misfolded proteins (Doms et ai., 1988) . We tried to solubilize the Gml oligomer using a wide variety of lysis conditions and sample treatments. None of the modifications we tested, including changes in salt concentration, reducing agents, detergents, chaotropic agents (e.g., urea, guanidinium HC1), or temperature of solubilization or sample elution disrupted the SDS-resistant oligomer (data not shown). A very small amount of SDS-sensitive Gml could be produced when the top of the stacking gel (containing the SDS-resistant oligomer) was isolated after electrophoresis, placed in the well of a fresh gel, and reelectrophoresed (not shown). Together, these observations suggest that SDS-resistant oligomer formation is an intrinsic property of Gml and mutant Gml proteins that are retained in the Golgi complex, but not of mutants that efficiently reach the plasma membrane.