Cytomegalovirus basic phosphoprotein (pUL32) binds to capsids in vitro through its amino one-third

The cytomegalovirus (CMV) basic phosphoprotein (BPP) is a component of the tegument, It remains with the nucleocapsid fraction under conditions that r emove most other tegument proteins from the virion, suggesting a direct and perhaps tight interaction with the capsid. As a step toward localizing thi s protein within the molecular structure of the virion and understanding it s function during infection, we have investigated the BPP-capsid interactio n. In this report we present evidence that the BPP interacts selectively, t hrough its amino one-third, with CMV capsids. Radiolabeled simian CMV (SCMV ) BPP, synthesized in vitro, bound to SCMV B-capsids, and C-capsids to a le sser extent, following incubation with either isolated capsids or lysates o f infected cells. Human CMV (HCMV) BPP (pUL32) also bound to SCMV capsids, and SCMV BPP likewise bound to HCMV capsids, indicating that the sequence(s ) involved is conserved between the two proteins. Analysis of SCMV BPP trun cation mutants localized the capsid-binding region to the amino one-third o f the molecule-the portion of BPP showing the greatest sequence conservatio n between the SCMV and HCMV homologs. This general approach may have utilit y in studying the interactions of other proteins with conformation-dependen t binding sites.