Viral membrane fusion: is glycoprotein G of rhabdoviruses a representative of a new class of viral fusion proteins?

Enveloped viruses always gain entry into the cytoplasm by fusion of their lipid envelope with a cell membrane. Some enveloped viruses fuse directly with the host cell plasma membrane after virus binding to the cell receptor. Other enveloped viruses enter the cells by the endocytic pathway, and fusion depends on the acidification of the endosomal compartment. In both cases, virus-induced membrane fusion is triggered by conformational changes in viral envelope glycoproteins. Two different classes of viral fusion proteins have been described on the basis of their molecular architecture. Several structural data permitted the elucidation of the mechanisms of membrane fusion mediated by class I and class II fusion proteins. In this article, we review a number of results obtained by our laboratory and by others that suggest that the mechanisms involved in rhabdovirus fusion are different from those used by the two well-studied classes of viral glycoproteins. We focus our discussion on the electrostatic nature of virus binding and interaction with membranes, especially through phosphatidylserine, and on the reversibility of the conformational changes of the rhabdovirus glycoprotein involved in fusion. Taken together, these data suggest the existence of a third class of fusion proteins and support the idea that new insights should emerge from studies of membrane fusion mediated by the G protein of rhabdoviruses. In particular, the elucidation of the three-dimensional structure of the G protein or even of the fusion peptide at different pH's might provide valuable information for understanding the fusion mechanism of this new class of fusion proteins.

Inactivation of a beta-glucosidase from Arthrobotrys conoides by diethyl pyrocarbonate: evidence of histidine at the active site.

Modification of A. conoides beta-glucosidase by diethylpyrocarbonate caused rapid inactivation of the enzyme. The kinetic analyses showed that the inactivation by diethylpyrocarbonate resulted from the modification of an average of one histidine residue per mole of enzyme. The modified enzyme showed an increase in absorbance at 240 nm. Sulphydryl, lysine and tyrosine residues were not modified by diethylpyrocarbonate treatment. The substrate offered significant protection against diethylpyrocarbonates modification. The results indicate that diethylpyrocarbonate was interacting with the enzyme at or near the active site.

Inactivation of leucine aminotransferase with diethylpyrocarbonate and rose bengal: evidence for an active site histidine residue.

Modification of leucine aminotransferase by diethylpyrocarbonate or rose bengal-sensitized photo-oxidation caused rapid inactivation of the enzyme. The inactivation of leucine aminotransferase depended on the concentration of the reagent, the time of incubation and exhibited pseudo-first order kinetics. Rose bengal-sensitized photo-oxidation was maximum at pH 6.5 and 9. Substrates leucine and alpha-ketoglutarate protected the enzyme against inactivation by these reagents, thus suggesting participation of histidine residue at the substrate binding site.