A Golgi retention signal in a membrane-spanning domain of coronavirus E1 protein

Since El is a viral protein, our results need to be confirmed for endogenous Golgi proteins. Unfortunately, cDNAs are not yet available for endogenous cis-Golgi proteins, although cDNAs encoding trans-Golgi glycosyltransferases have been isolated (reviewed in 33) . All Golgi glycosyltransferases that have been sequenced have a "type 11" membrane topology, with the amino terminus in the cytoplasm, the carboxy terminus in the lumen, and an uncleaved signal sequence which also serves as the membrane-spanning domain (33) . There is no obvious sequence homology between the ml domain of El and the membrane-spanning domains of these proteins . However, there are several observations which support the idea that sequences associated with the lipid bilayer might be involved in retention of proteins in the trans-Golgi complex . Colley et al . (4) have shown that ca2,6 sialyl transferase is efficiently secreted from transfected cells if a cleavable signal sequence is engineered in place of the normal signal an- Figure 9 Two different mutations in the membrane-spanning domain of Gml release retention from the Golgi region . The conserved Gln (Gln 37 in El) was changed to Ile (GmIQI), and the 2 Ile residue insert was introduced into Gml (Gmlins) . Double-label indirect immunofluorescence microscopy was performed as described in Fig. 2 . Both GmIQI and Gmlins were readily detected on the plasma membrane. Bar, 10 uM .