The Putative Leucine Zipper of the UL6-Encoded Portal Protein of Herpes Simplex Virus 1 Is Necessary for Interaction with pUL15 and pUL28 and Their Association with Capsids

Herpes simplex virus (HSV) type 1 capsids contain a single portal vertex that is composed of 12 copies of the UL6 gene product (pUL6), which forms a pore through which DNA is inserted during packaging. This unique vertex is also believed to comprise the site with which a molecular motor, termed the terminase, associates during the DNA packaging reaction. In HSV, the terminase likely comprises the UL15, UL28, and UL33 proteins (pUL15, pUL28, and pUL33, respectively). The current study was undertaken to identify portal domains required for interaction with the terminase. Both the amino and carboxyl termini, as well as amino acids 422 to 443 of pUL6 forming a putative leucine zipper motif, were critical for coimmunoprecipitation with pUL15 in the absence of other viral proteins. Amino acids 422 to 443 were also necessary for interaction with pUL28 in the absence of other viral proteins. By using an engineered recombinant virus, it was further determined that although amino acids 422 to 443 were dispensable for interaction with scaffold protein and incorporation of portal protein into capsids, they were necessary for coimmunoprecipitation of pUL6 and pUL15 from infected cell lysates, association of optimal levels of pUL15, pUL28, and pUL33 with capsids, and DNA cleavage and packaging. These data identify a portal protein domain critical for terminase association with the capsid and suggest that both the pUL15- and pUL28-bearing terminase subunits mediate docking of the terminase with the portal vertex.