Association of Herpes Simplex Virus pU L 31 with Capsid Vertices and Components of the Capsid Vertex-Specific Complex

Abstract

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 31, 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.