COMPLEMENTATION OF M-GENE MUTANTS OF VESICULAR STOMATITIS-VIRUS BY PLASMID-DERIVED M-PROTEIN CONVERTS SPHERICAL EXTRACELLULAR PARTICLES INTO NATIVE BULLET SHAPES

The matrix (M) protein of vesicular stomatitis virus (VSV) binds the n ucleocapsid to the cytoplasmic surface of the host plasma membrane dur ing virus assembly by budding. It also condenses the nucleocapsid into a tightly coiled nucleocapsid-M protein complex that appears to give the virion its bullet-like shape. As described here, temperature-sensi tive (ts) M mutants produced two classes of membrane-containing extrac ellular particles al the nonpermissive temperature. These could be dis tinguished by sedimentation in sucrose gradients and by electron micro scopy. One class contained nucleocapsids and envelope glycoprotein, bu t very little M protein. The other class was devoid of nucleocapsids. Most of these particles were spherical or pleiomorphic in shape as det ermined by electron microscopy. Expression of wild-type (wt) M protein from plasmid DNA using the vaccinia/T7 virus system did not enhance t he incorporation of nucleocapsids into extracellular particles from ce lls coinfected with the ts M mutants but did enhance the incorporation of M protein into these particles. Electron microscopy showed that wt M protein served to impart the bullet-like shape typical of VSV virio ns to what would otherwise be spherical or pleiomorphic virus-like par ticles. These data suggest that there are two distinct processes in VS V envelope biogenesis. One process involves envelopment of the nucleoc apsid and can be accomplished by the ts M mutants at the nonpermissive temperature, albeit at a low level compared to wt VSV. The other proc ess involves conversion of virion components into the bullet-like shap e and requires a function provided by wt M protein. (C) 1996 Academic Press, Inc.