Residues of VP26 of herpes simplex virus type 1 that are required for its interaction with capsids

VP26 is the smallest capsid protein and decorates the outer surface of the capsid shell of herpes simplex virus. It is located on the hexons at equimolar amounts with VP5. Its small size (112 amino acids) and high copy number make it an attractive molecule to use as a probe to investigate the complex pattern of capsid protein interactions. An in vitro capsid binding assay and a green fluorescent protein (GFP) localization assay were used to identify VP26 residues important for its interaction with capsids. To test for regions of VP26 that may be essential for binding to capsids, three small in-frame deletion mutations were generated in VP26, Delta18-25, Delta54-60, and Delta93-100. Their designations refer to the amino acids deleted by the mutation. The mutation at the C terminus of the molecule, which encompasses a region of highly conserved residues, abolished binding to the capsid and the localization of GFP to the nucleus in characteristic large puncta. Additional mutations revealed that a region of VP26 spanning from residue 50 to 112 was sufficient for the localization of the fused protein (VP26-GFP) to the nucleus and for it to bind to capsids. Using site-directed mutagenesis of conserved residues in VP26, two key residues for protein-protein interaction, F79 and G93, were identified as judged by the localization of GFP to nuclear puncta. When these mutations were analyzed in the capsid binding assay, they were also found to eliminate binding of VP26 to the capsid structure. Surprisingly, additional mutations that affected the ability of VP26 to bind to capsids in vitro were uncovered. Mutations at residues A58 and L64 resulted in a reduced ability of VP26 to bind to capsids. Mutation of the hydrophobic residues M78 and A80, which are adjacent to the hydrophobic residue F79, abolished VP26 capsid binding. In addition, the block of conserved amino acids in the carboxy end of the molecule had the most profound effect on the ability of VP26 to interact with capsids. Mutation of amino acid G93, L94, R95, R96, or T97 resulted in a greatly diminished ability of VP26 to bind capsids. Yet, all of these residues other than G93 were able to efficiently translocate or concentrate GFP into the nucleus, giving rise to the punctate fluorescence. Thus, the interaction of VP26 with the capsid appears to occur through at least two separate mechanisms. The initial interaction of VP26 and VP5 may occur in the cytoplasm or when VP5 is localized in the nucleus. Residues F79 and G93 are important for this bi-molecular interaction, resulting in the accumulation of VP26 in the nucleus in concentrated foci. Subsequent to this association, additional amino acids of VP26, including those in the C-terminal conserved domain, are important for interaction of VP26 with the three-dimensional capsid structure.