Photolabeling identifies a putative fusion domain in the envelope glycoprotein of rabies and vesicular stomatitis viruses

Abstract

Vesicular stomatitis and rabies viruses enter cells through receptor-mediated endocytosis, followed by fusion of the viral with the endosomal membrane. The latter step is catalyzed by the viral envelope glycoprotein, which, in the low pH environment of the endosome, undergoes a conformational transition to a fusion-competent state. To investigate whether fusion competence involves the low pH exposure of a hydrophobic fusion region(s), we have applied hydrophobic photolabeling using the recently developed phospholipid analogue 1-O-hexadecanoyl-2-O-[9-[[[2-[125I]iodo-4-(trifluoromethyl-3H- diazirin-3-yl)benzyl]oxy]carbonyl] nonanoyl]-sn-glycero-3-phosphocholine ([125I]TID-PC/16) (Weber, T., and Brunner, J. (1995) J. Am. Chem. Soc. 117, 3084-3095). Rosettes of rabies virus glycoprotein, whole rabies virus, or vesicular stomatitis virus were incubated with large unilamellar vesicles containing [125I]TID-PC/16. Following reagent activation, the labeled glycoprotein was isolated and analyzed. In all cases, labeling of the glycoprotein strongly increased as the pH was lowered from 7.0 to 6.0, suggesting the exposure at acidic pH of a domain capable of interacting with membranes. To identify the labeled region(s), CNBr fragments were generated and analyzed by SDS-polyacrylamide followed by autoradiography. In rabies glycoprotein, the labeled segment was found to be contained within fragment RCr5 (residues 103-179). Glycoprotein from vesicular stomatitis virus was labeled within fragment VCr1 (residues 59-221). These results demonstrate that rhabdovirus glycoprotein contains a domain that at low pH is capable of interacting with a target membrane in a hydrophobic manner. This domain may play a role similar to that of the fusion peptide found in many other viral fusion proteins.