Mechanisms of human cytomegalovirus capsid maturation

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Infection with human cytomegalovirus (HCMV) can lead to life-threatening disease in immunocompromised patients and immunologically immature newborns. The currently available medication against CMV mainly targets the viral DNA polymerase and viral genome replication, and is associated with considerable harmful adverse effects. Accordingly, there is a need for the development of new antiviral compounds that target other virus-specific mechanisms. HCMV capsid maturation requires the interaction of several essential viral proteins and thus represents a promising target for intervention with antiviral drugs. However, the molecular mechanisms underlying the maturation of HCMV capsids are not well understood yet. In particular, the role of four sparsely characterized HCMV proteins (pUL51, pUL52, pUL77 and pUL93), which were suggested to be involved in genome encapsidation and subsequent capsid maturation steps, remains to be defined. In the current funding period we have shown that pUL51 is essential for HCMV genome cleavage and packaging, and that it interacts with the terminase subunits pUL56 and pUL89, possibly promoting the formation of the terminase complex or its translocation to viral replication compartments. Using an HCMV mutant expressing a tagged UL52 protein and tandem affinity purification we could enrich a putative interaction partner of pUL52. Preliminary characterization of a UL77 deletion mutant provides evidence of a contribution of pUL77 to HCMV genome encapsidation. By constructing and analyzing suitable HCMV mutants we will further investigate the phenotypic consequences on the viral infection cycle upon disruption of each of the respective open reading frames, especially of those for pUL77 and its suggested viral partner protein pUL93. Moreover, we will evaluate the interactions of the HCMV encapsidation proteins with each other, and with other viral proteins as well as putative cellular interaction partners in the context of viral infection, with a focus on pUL51 and pUL52. We expect that the project will yield comprehensive knowledge of the mechanisms mediating HCMV capsid maturation and of the interactions of the proteins contributing to this process. Definition of the protein domains mediating these protein-protein interactions will lay the basis for the development of screening assays for compounds disrupting these interactions.