PHOTOLABELING IDENTIFIES A PUTATIVE FUSION DOMAIN IN THE ENVELOPE GLYCOPROTEIN OF RABIES AND VESICULAR STOMATITIS VIRUSES

Vesicular stomatitis and rabies viruses enter cells through receptor-m ediated endocytosis, followed by fusion of the viral with the endosoma l membrane. The latter step is catalyzed by the viral envelope glycopr otein, which, in the low pH environment of the endosome, undergoes a c onformational transition to a fusion com petent state. To investigate whether fusion competence involves the low pH exposure of a hydrophobi c fusion region(s), we have applied hydrophobic photolabeling using th e recently developed phospholipid analogue oxy]carbonyl]nonanoyl]-sn-g lycero-3-phosphocholine ([I-125]TID-PC/16) (Weber, T., and Brunner, J. (1995) J. Am. Chem. Soc. 117, 3084-3095). Rosettes of rabies virus gl ycoprotein, whole rabies virus, or vesicular stomatitis virus were inc ubated with large unilamellar vesicles containing [I-125]TID-PC/16. Fo llowing reagent activation, the labeled glycoprotein was isolated and analyzed. In all cases, labeling of the glycoprotein strongly increase d as the pH was lowered from 7.0 to 6.0, suggesting the exposure at ac idic 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 la beled within fragment VCr1 (residues 59-221). These results demonstrat e that rhabdovirus glycoprotein contains a domain that at low pH is ca pable of interacting with a target membrane in a hydrophobic manner. T his domain may play a role similar to that of the fusion peptide found in many other viral fusion proteins.