Physical properties of a soluble form of the glycoprotein of vesicular stomatitis virus at neutral and acidic pH.

We have analyzed a soluble form of the glycoprotein (G) obtained from vesicular stomatitis virus (VSV) by treatment of intact virions with cathepsin D. This form lacks the carboxy-terminal and membrane-spanning domains and thus is analogous to the previously described secreted form of G, Gs. The molecular weight of the cathepsin D produced G, G(Cath D), measured by sedimentation equilibrium in the analytical ultracentrifuge is 57 600, indicating that it is a monomer. Intact G protein extracted from virions by octyl beta-D-glucoside also is monomeric, based on sedimentation equilibrium analysis. These results suggest that G may be monomeric in virions. The Stokes radii (Rs) of the two forms of G were obtained from their migration in nondenaturing polyacrylamide gradient gels. The Rs of G(Cath D) in the absence of nonionic detergent was 37 A; in the presence of nonionic detergent, it increased to 55 A. The Rs of detergent-extracted intact G was 63 A in nonionic detergent. From the molecular weight and Rs of G(Cath D), we calculated a sedimentation coefficient of 3.8 S; the value determined by centrifugation in a sucrose gradient was 3.7 S. Viruses such as VSV fuse with cell membranes at low pH [White, J., Matlin, K., & Helenius, A. (1981) J. Cell Biol. 89, 674-679]. We have used the fluorescent probe cis,trans,trans,cis-9,11,13,15-parinaric acid (cis-PnA) to detect a reversible conformational change in G(Cath D) when the protein was exposed to an acidic environment close to pH 5. cis-PnA binds to hydrophobic regions of protein, causing a quenching of the intrinsic tryptophan fluorescence and an increase in the fluorescence of the probe.