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

Interestingly, when we analyzed expression of these constructs in HeLa cells and COS-7 cells by metabolic radiolabeling and immunoprecipitation, we noticed the appearance of a radiolabeled SDS-resistant species that migrated at the top of the stacking gel, in addition to products migrating at the expected molecular masses. The material at the top of the stacking gel was more prevalent in samples immunoprecipitated from HeLa cells ( Fig. 1 C) than in those from COS-7 cells ( Fig. 1 D) . This species was present only in immunoprecipitates from cells that expressed mutants with Golgi-resident populations (Gml, Gmli,, and GmlQH4s0) and it appeared regardless of the level of expression (over a >50-fold range). Although ~50% of the total radioactivity immunoprecipitated from Gml-expressing HeLa cells migrated at the top of the stacking gel, we have not quantitated this information routinely. One problem with quantitation is that this material could include radiolabeled components other than Gml; furthermore, some of the oligomer may not enter the gel at all and be lost during subsequent processing. Gmli,, was previously reported to move through the Golgi complex rapidly (with a half time of 35 rain in COS-7 cells; Swift and Machamer, 1991) . However, the kinetics of transport reflect only the population of protein that enters the separating gel. The SDS-resistant protein may therefore represent a stable Golgi-resident pool.