Structural and functional studies of the abundant tegument protein ORF52 from murine gammaherpesvirus 68

The tegument is a layer of proteins between the nucleocapsid and the envelope of herpesviruses. The functions of most tegument proteins are still poorly understood. In murine gammaherpesvirus 68, ORF52 is an abundant tegument protein of 135 residues that is required for the assembly and release of infectious virus particles. To help understand the molecular basis for the function of this protein, we have determined its crystal structure at 2.1 Å resolution. The structure reveals a dimeric association of this protein. Interestingly, an N-terminal α-helix that assumes different conformation in the two monomers of the dimer mediates the formation of an asymmetrical tetramer and contains many highly conserved residues. Structural and sequence analyses suggest that this helix is more likely involved in interactions with other components of the tegument or nucleocapsid of the virus and that ORF52 functions as a symmetrical dimer. The asymmetrical tetramer of ORF52 may be a "latent" form of the protein, when it is not involved in virion assembly. The self-association of ORF52 has been confirmed by co-immunoprecipitation and fluorescence resonance energy transfer experiments. Deletion of the N-terminal α-helix, as well as mutation of the conserved Arg⁹⁵ residue, abolished the function of ORF52. The results of the functional studies are fully consistent with the structural observations and indicate that the N-terminal α-helix is a crucial site of interaction for ORF52.