Association of Herpes Simplex Virus pUL31 with Capsid Vertices and Components of the Capsid Vertex-Specific Complex

pUL34 and pUL31 of herpes simplex virus (HSV) comprise the nuclear egress complex (NEC) and are required for budding at the inner nuclear membrane. pUL31 also associates with capsids, suggesting it bridges the capsid and pUL34 in the nuclear membrane to initiate budding. Previous studies showed that capsid association of pUL31 was precluded in the absence of the C terminus of pUL25, which along with pUL17 comprises the capsid vertex-specific complex, or CVSC. The present studies show that the final 20 amino acids of pUL25 are required for pUL31 capsid association. Unexpectedly, in the complete absence of pUL25, or when pUL25 capsid binding was precluded by deletion of its first 50 amino acids, pUL31 still associated with capsids. Under these conditions, pUL31 was shown to coimmunoprecipitate weakly with pUL17. Based on these data, we hypothesize that the final 20 amino acids of pUL25 are required for pUL31 to associate with capsids. In the absence of pUL25 from the capsid, regions of capsid-associated pUL17 are bound by pUL31. Immunogold electron microscopy revealed that pUL31 could associate with multiple sites on a single capsid in the nucleus of infected cells. Electron tomography revealed that immunogold particles specific to pUL31 protein bind to densities at the vertices of the capsid, a location consistent with that of the CVSC. These data suggest that pUL31 loads onto CVSCs in the nucleus to eventually bind pUL34 located within the nuclear membrane to initiate capsid budding. IMPORTANCE This study is important because it localizes pUL31, a component previously known to be required for HSV capsids to bud through the inner nuclear membrane, to the vertex-specific complex of HSV capsids, which comprises the unique long region 25 (UL25) and UL17 gene products. It also shows this interaction is dependent on the C terminus of UL25. This information is vital for understanding how capsids bud through the inner nuclear membrane.