MEMBRANE-BINDING AND ENDOPLASMIC-RETICULUM RETENTION SEQUENCES OF ROTAVIRUS VP7 ARE DISTINCT - ROLE OF CARBOXY-TERMINAL AND OTHER RESIDUES IN MEMBRANE-BINDING

The sequences responsible for binding rotavirus glycoprotein VP7 to th e membrane of the endoplasmic reticulum (ER) have not been identified. Here we show that the sequences which promote membrane binding in vit ro are distinct from the N-terminal sequences which promote retention of VP7 in the ER in vivo. The role of the C-terminal region in membran e binding was also examined by using truncation mutants. Membrane bind ing in vitro was reduced but not abolished by removing up to 102 resid ues from the C terminus. The data suggest that the last 36 residues of VP7 may be present in the membrane or translocation pore, possibly wi th the C terminus protruding into the cytoplasm, since these residues contribute to, but do not account for, membrane binding. Surprisingly, modified forms of VP7 which are secreted from transfected cells showe d the same membrane-binding properties in vitro as the protein retaine d in the ER membrane. Thus, secreted VP7 may not be present as a solub le polypeptide in the ER. A model to explain these results is presente d. Previously published data are consistent with the idea that the hig hly conserved C terminus of nascent VP7 could have a cytoplasmic orien tation which is important for assembly of mature virus particles.