Association of herpes simplex virus pUL31 with capsid vertices and components of the capsid vertex-specific complex.

UNLABELLED: pU(L)34 and pU(L)31 of herpes simplex virus (HSV) comprise the nuclear egress complex (NEC) and are required for budding at the inner nuclear membrane. pU(L)31 also associates with capsids, suggesting it bridges the capsid and pU(L)34 in the nuclear membrane to initiate budding. Previous studies showed that capsid association of pU(L)31 was precluded in the absence of the C terminus of pU(L)25, which along with pU(L)17 comprises the capsid vertex-specific complex, or CVSC. The present studies show that the final 20 amino acids of pU(L)25 are required for pU(L)31 capsid association. Unexpectedly, in the complete absence of pU(L)25, or when pU(L)25 capsid binding was precluded by deletion of its first 50 amino acids, pU(L)31 still associated with capsids. Under these conditions, pU(L)31 was shown to coimmunoprecipitate weakly with pU(L)17. Based on these data, we hypothesize that the final 20 amino acids of pU(L)25 are required for pU(L)31 to associate with capsids. In the absence of pU(L)25 from the capsid, regions of capsid-associated pU(L)17 are bound by pU(L)31. Immunogold electron microscopy revealed that pU(L)31 could associate with multiple sites on a single capsid in the nucleus of infected cells. Electron tomography revealed that immunogold particles specific to pU(L)31 protein bind to densities at the vertices of the capsid, a location consistent with that of the CVSC. These data suggest that pU(L)31 loads onto CVSCs in the nucleus to eventually bind pU(L)34 located within the nuclear membrane to initiate capsid budding. IMPORTANCE: This study is important because it localizes pU(L)1, 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 (U(L)25) and U(L)17 gene products. It also shows this interaction is dependent on the C terminus of U(L)25. This information is vital for understanding how capsids bud through the inner nuclear membrane.