Membrane topology of coronavirus E protein

Coronavirus small envelope protein E has two known biological functions: it plays a pivotal role in virus envelope formation, and the murine coronavir us E protein induces apoptosis in E protein-expressing cultured cells. The E protein is an integral membrane protein. Its C-terminal region extends cy toplasmically in the infected cell and in the virion toward the interior. T he N-terminal two-thirds of the E protein is hydrophobic and lies buried wi thin the membrane, but its orientation in the lipid membrane is not known. Immunofluorescent analyses of cells expressing biologically active murine c oronavirus E protein with a hydrophilic short epitope tag at the N-terminus showed that the epitope tag was exposed cytoplasmically. Immunoprecipitati on analyses of the purified microsomal membrane vesicles that contain the s ame tagged E protein revealed the N-terminal epitope tag outside the micros omal membrane vesicles. These analyses demonstrated that the epitope tag at the N-terminus of the E protein was exposed cytoplasmically. Our data were consistent with an E protein topology model, in which the N-terminal two-t hirds of the transmembrane domain spans the lipid bilayer twice, exposing t he C-terminal region to the cytoplasm or virion interior. (C) 2001 Academic Press.