Comparison between human cytomegalovirus pUL97 and murine cytomegalovirus (MCMV) pM97 expressed by MCMV and vaccinia virus: pM97 does not confer ganciclovir sensitivity

The UL97 protein (pUL97) of human cytomegalovirus (HCMV) is a protein kinas e that also phosphorylates ganciclovir (GCV), but its biological function i s not yet clear. The M97 protein (pM97) of mouse cytomegalovirus (MCMV) is the homolog of pUL97. First, we studied the consequences of genetic replace ment of M97 by UL97. Using the infectious bacterial plasmid clone of the fu ll-length MCMV genome (M. Wagner, S. Jonjic, U. H. Koszinowski, and M. Mess erle, J. Virol. 73:7056-7060, 1999), we replaced the M97 gene with the UL97 gene and constructed an MCMV M97 deletion mutant and a revertant virus. In addition, pUL97 and pM97 were expressed by recombinant vaccinia virus to c ompare both for known functions. Remarkably, pM97 proved not to be the reas on for the GCV sensitivity of MCMV. When expressed by the recombinant MCMV, however, pUL97 was phosphorylated and endowed MCMV with the capacity to ph osphorylate GCV, thereby rendering MCMV more susceptible to GCV. We found t hat deletion of pM97, although it is not essential for MCMV replication, se verely affected virus growth. This growth deficit was only partially amende d by pUL97 expression. When expressed by recombinant vaccinia viruses, both proteins were phosphorylated and supported phosphorylation of GCV, but pUL 97 was about 10 times more effective than pM97. One hint of the functional differences between the proteins was provided by the finding that pUL97 acc umulates in the nucleus, whereas pM97 is predominantly located in the cytop lasm of infected cells. In vivo testing revealed that the UL97-MCMV recombi nant should allow evaluation of novel antiviral drugs targeted to the UL97 protein of;HCMV in mice.