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

Previous studies from this laboratory showed that the first membrane span (ml) of the M glycoprotein of 1BV (previously called the E1 glycoprotein) causes the chimeric protein Gml to be retained in the Golgi. The sequence of ml is shown in Fig. 1 A. Using site directed mutagenesis, four key residues in the ml domain were shown to be critical for Golgi retention of Gml: asparagine (N,65), two threonines (T~ and T,76), and glutamine (Q~). Some substitutions at Q~0 are tolerated (e.g., GmlQI'hs0) whereas other changes (e.g., GmlQI~o) result in transport of the protein to the cell surface (Machamer et al., 1993) . Another mutation in which two isoleucine residues were inserted near the center of the transmembrane domain (Gmli,~) also disrupts retention in the Golgi complex. These three chimeric proteins (GmL~, GmlQLso, and GmlQH_~o) were expressed in transiently transfected COS-'/ cells and localized by indirect immunofluorescence ( Fig. 1 B) . VSV G and GrnlQI~o were readily detected at the plasma membrane, while Gml and GmlQI-hso were detected only in the Golgi region and largely colocalized with lens culinaris lectin staining, even after treatment with nocodazole or brefeldin A (Machamer et al., 1993) . Gmlin~ staining was less prominent at the plasma membrane than GmlQI~0, and a considerable amount of fluorescence was detected in the Golgi complex. The localizations of these proteins were confirmed in HeLa cells and BHK cells transfected using a vaccinia-virusmediated expression system (not shown).