HUMAN CYTOMEGALOVIRUS CAPSID ASSEMBLY PROTEIN-PRECURSOR (PUL80.5) INTERACTS WITH ITSELF AND WITH THE MAJOR CAPSID PROTEIN (PU86) THROUGH 2 DIFFERENT DOMAINS

We have used the yeast GAL4 two-hybrid system to examine interactions between the human cytomegalovirus (HCMV) major capsid protein (MCP, en coded by UL86) and the precursor assembly protein (pAP, encoded by UL8 0.5 and cleaved at its carboxyl end to yield AP) and found that (i) th e pAP interacts with the MCP through residues located within the carbo xy-terminal 21 amino acids of the pAP, called the carboxyl conserved d omain (CCD); (ii) the pAP interacts with itself through a separate reg ion, called the amino conserved domain (ACD), located between amino ac ids His34 and Arg52 near the amino end of the molecule; (iii) the simi an CMV (SCMV) pAP and AP can interact with or replace their HCMV count erparts in these interactions, whereas the herpes simplex virus pAP an d AP homologs cannot; and (iv) the HCMV and SCMV maturational proteina se precursors (AC(pra), encoded by UL80a and APNG1, respectively) can interact with the pAP and MCP. The ACD and CCD amino acid sequences ar e highly conserved among members of the betaherpesvirus group and appe ar to have counterparts in the alpha- and gammaherpesvirus pAP homolog s. Deleting the ACD from the HCMV pAP, or substituting Ala for a conse rved Leu in the ACD, eliminated detectable pAP self-interaction and al so substantially reduced MCP binding in the two-hybrid assay. This fin ding indicates that the pAP self-interaction influences the pAP-MCP in teraction. Immunofluorescence studies corroborated the pAP-MCP interac tion detected in the GAL4 two-hybrid experiments ana showed that nucle ar transport of the MCP was mediated by pAP but not AP. We conclude th at the pAP interacts with the MCP, that this interaction is mediated b y the CCD and is influenced by pAP self-interaction, and that one func tion of the pAP-MCP interaction may be to provide a controlled mechani sm for transporting the MCP into the nucleus.