Analysis of the protein interaction networks that orchestrate tegument assembly and envelopment of human cytomegalovirus virions and dense bodies

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The morphogenesis of human cytomegalovirus (HCMV) particles is a complicated process that requires multiple interactions of viral proteins to assemble capsids, to attach the tegument layer, and to perform final envelopment in the cytoplasm. Although a lot has been learned about capsid assembly in the nucleus, the molecular mechanisms that drive tegumentation and envelopment are only poorly defined. The proteins located in the inner tegument are thought to provide the interface between capsids and the outer tegument. The outer tegument structure appears to be important for particle stability. In addition, the outer tegument layer faces the inner lamina of the envelope. Thus the outer tegument is likely involved in final virion envelopment and in maintaining the integrity of the particles. Finally, several proteins, located in the outer tegument layer of the virion target innate restriction systems that are designed to sense incoming virions and to block viral gene expression and replication at a very early stage. Thus the proteins in the outer tegument serve several important functions in the virus life cycle. The knowledge about the protein interaction networks that are required to set up and maintain the outer tegument structure is underdeveloped. We have shown in proteomic analyses that the composition of the HCMV tegument is remarkably conserved between strains, indicating that tegument assembly is highly regulated. This suggests that there is selective pressure on the virus to package certain proteins at a given molarity to achieve optimal conditions for particle envelopment and for the initiation of its replication cycle. The work programme devised here aims at investigating the mechanisms that drive outer HCMV tegument assembly and its role in secondary envelopment. The project focuses on the hypotheses that (i) a tegument protein network is formed in the nucleus of HCMV infected cells, that (ii) these proteins are targeted to cytoplasmic viral assembly sites by virtue of the shuttling function of the major matrix protein pp65, that (iii) a cytoplasmic pre-tegument complex is subsequently formed as a hub for outer tegument assembly, and that (iv) the outer tegument then drives secondary envelopment by interaction with components of autophagy.