Deletions into an NH2-terminal hydrophobic domain result in secretion of rotavirus VP7, a resident endoplasmic reticulum membrane glycoprotein

Other glycoproteins of the ER have been cloned and some sequenced. In the cases of HMG-CoA reductase (27) and coronavirus E 1 (4), there was no obvious homology between the NH2-terminal hydrophobic domains in these molecules and VP7, nor with the dual NH2-terminal hydrophobic domains in nonstructural rotavirus glycoprotein NCVP5 (8) . Cytochrome P-450 (31), another ER protein, did not display any obvious homologies in its multiple hydrophobic domains. It should be noted, however, that 3-hydroxy-3-methylglutaryl-CoA reductase, coronavirus El, and cytochrome P-450 probably interact with the lipid bilayer much more extensively than does VP7, via multiple membrane spanning domains. Since VP7 is not normally secreted, presumably there could be no specific ER receptor (14, 28) mediating its secretion, and therefore its movement along the secretory pathway in these cells is constitutive, rather than specific. This is the first demonstration of an alteration in the primary sequence which allows a naturally targeted ER molecule to be secreted, and shows unequivocally that glycoproteins can be secreted without the intervention of a specific receptor. Also of significance is that an unglycosylated, mutant VP7 is efficiently secreted. The precise reason(s) for the secretion of VP7 in the mutants 42-61, 43-61, and 47-61 is not yet clear. It could be that an anchor region has been shortened so that it no longer functions, that a positive signal for ER location has been disrupted, or that a peptidase cleavage site, e.g., the Ala-Tyr-Ala sequence at residues 66-68 (43), has been brought into proximity of the aminopeptidase used for signal peptide cleavage. These possibilities are being investigated. Our results become particularly interesting when juxtaposed with several other observations. Firstly, some normally soluble molecules of the ER (2, 5) remain in the ER, whereas soluble VP7 is secreted. Secondly, influenza neuraminidase is anchored in the membrane by an NH2-terminal hydrophobic domain, but this molecule, unlike VP7, is exported to the plasma membrane (6) . From the above, we conclude that the region of the second hydrophobic domain of VP7 not only serves to anchor the protein in the membrane but may also contain the positive and specific signal for maintaining the protein in the ER.