Identification of a region in the herpes simplex virus scaffolding protein required for interaction with the portal

The herpes simplex virus type 1 capsid is a protective shell that acts as a container for the genetic material of the virus. After assembly of the capsid, the viral DNA is translocated into the capsid interior through a channel formed by the portal. The portal is composed of a dodecamer of U(L)6 molecules which form a ring-like structure found at a single vertex within the icosahedron. Formation of portal-containing capsids minimally requires the four structural proteins (VP5, VT19C, VP23, and U(L)6) and a scaffolding protein (U(L)26.5). Recently, an interaction between U(L)26.5 and the portal has been identified, suggesting the scaffold functions by delivering the portal to the growing capsid shell. The aim of this study was to identify regions within U(L)26.5 required for its interaction with the portal. A specific region was identified by mutational analysis. Deletion of scaffold amino acids (aa) 143 to 151 was found to be sufficient to inhibit formation of the scaffold-portal complex as assayed in vitro. The aa 143 to 151 contain the sequence YYPGE, which is highly conserved among alphaherpesviruses. Although it did not bind to the portal, the Delta143-151 mutant was found to retain the ability to support assembly of morphologically normal capsids in vitro. Such capsids, however, did not contain the portal. The results suggest assembly of portal-containing capsids requires formation of a scaffold-portal complex in which intermolecular contact is dependent on scaffold aa 143 to 151.