INTERACTIONS OF WILD-TYPE AND MUTANT M-PROTEIN OF VESICULAR STOMATITIS-VIRUS WITH VIRAL NUCLEOCAPSID AND ENVELOPE IN INTACT VIRIONS - EVIDENCE FROM [IODONAPHTHYL-I-125 AZIDE LABELING AND SPECIFIC CROSS-LINKING

Four different temperature-sensitive M protein mutants (tsM) of vesicular stomatitis virus (VSV) were characterized with regard to the association of the mutated M protein either with nucleocapsids or with membranes in the intact virions. Virions were labeled with the photoreactive hydrophobic probe [125I]iodonaphthyl azide (INA) to assess interactions between viral proteins and the lipid envelope. In wild type (wt) virions, the 3 major structural proteins, G, M, and N, were labeled in the ratio .apprx. 1.0:0.4:0.2. INA labeled only the membrane-associated peptide of G protein, both in the intact virion and in reconstituted G protein-viral lipid vesicles, demonstrating the specificity of INA for lipid bilayer regions. Labeling of tsM virions with INA resulted in a 2- to 3-fold greater incorporation into M protein than was found for wt virions, suggesting increased M-membrane associations in the mutant virions. Temperature-stable revertants from tsM possessed wt labeling characteristics. Interaction of the M protein with nucleocapsids was assessed from the abundance of disulfide-linked M-N complexes found after disruption of the virions by sodium dodecyl sulfate solution under nonreducing conditions. The abundance of such complexes was 30-80% less from tsM virions than from wt virions, suggesting decreased M-nucleocapsid interactions in tsM virions. Temperature-stable revertants from tsM resembled wt in the abundance of M-N complex formed. Thus, the mutations alter M protein in such a way as simultaneously to increase its association with membrane and to decrease its affinity for nucleocapsids in the intact virion.