Murine gamma-herpesvirus 68 latency protein M2 binds to Vav signaling proteins and inhibits B-cell receptor-induced cell cycle arrest and apoptosis in WEHI-231 B cells

The MHV-68 latent protein, M2, does not have homology to any known viral or cellular proteins, and its function is unclear. To define the role played by M2 during MHV-68 latency as well as the molecular mechanism involved, we used M2 as bait to screen a yeast two-hybrid mouse B-cell cDNA library. Vav1 was identified as an M2-interacting protein in two independent screenings. Subsequent yeast two-hybrid interaction studies showed that M2 also binds to Vav2, but not Vav3, and that three "PXXP" motifs located at the C terminus of M2 are important for this interaction. The interactions between M2 and Vav proteins were also confirmed in vivo in 293T and WEHI-231 B-cells by co-immunoprecipitation assays. Rac1/GST-PAK "pull-down" experiments and Western blot analysis using a phospho-Vav antibody demonstrated that expression of M2 in WEHI-231 cells enhances Vav activity. We further showed in WEHI-231 cells that M2 expression promotes proliferation and survival and is associated with enhanced cyclin D2 and repressed p27(Kip1), p130, and Bim expression. Taken together, these experiments suggest that M2 might have an important role in disseminating the latent virus during the establishment and maintenance of latency by modulating B-cell receptor-mediated signaling events through Vav to promote B-cell activation, proliferation, and survival.