Interactions Among Murine Cytomegalovirus US22 Family Gene Products That Influence Viral Pathogenesis

Cytomegalovirus (CMV) is a complex, ubiquitous herpesvirus that is characterized by acute, chronic, and latent infections. Monocytes-macrophages are the key target cell type involved in pathogenesis, which is most effectively studied using the murine model of CMV infection. Previously three murine CMV (MCMV) genes (M139, M140, and M141) were identified to regulate viral expression in cultured macrophages and in mice. These genes are members of the US22 gene family with respect to HCMV homology. There is no function assigned to the proteins encoded by these genes. However, deletion of M139, M140, and M141 significantly curtails growth of MCMV in macrophages in vitro and in macrophage-dense target organs in vivo (Hanson et al. 1999, J.Virol. 73(7): 5970–80). Therefore, M139, M140, and/or M141 gene products likely affect tissue specific viral infectivity. The purpose of this study was to characterize these proteins (pM139, pM140, and pM141) and interaction among them. The M139, gene encodes two protein of 75 and 61 kD; M140 encodes a single protein of 56 kD, and M141 encodes a 52 kD protein. Most interestingly, when infected cell lysates were immunoprecipitated with anti-M139 antibody under non-denaturing (but not denaturing) conditions, five bands of 98-, 75-, 61-, 56-, and 52-kD proteins were co-precipitated. Likewise, anti-M140 antisera co-precipitated two bands of 56- and 52-kD, and anti-M141 antibody precipitated a less abundant 56- and an abundant 52-kD band. The co-precipitating bands were identified as products of M139, M140, and M141 genes in experiments employing mutant viruses deleted of each gene. Complex formation between the M140 and M141 proteins (PM140 and pM141) was confirmed by sequential immunoprecipitations and combined immunoprecipitation and western blotting. These two proteins also formed a complex in the absence of other viral proteins. At least one function of the pM140/pM141 complex is to stabilize expression of pM141, which is unstable in the absence of pM140. Given the complexity of viral pathogenesis and the fact that pM139, pM140, and pM141 proteins are dispensable for viral replication in tissue culture, it is possible that each single protein as well as the complex(s) they form may have a distinct function which influences tissue specific infectivity