Human cytomegalovirus UL97 kinase and nonkinase functions mediate viral cytoplasmic secondary envelopment

Previous studies have revealed critical roles for the human cytomegalovirus (HCMV) UL97 kinase in viral nuclear maturation events. We have shown recently that UL97 affects the morphology of the viral cytoplasmic assembly compartment (AC) (M. Azzeh, A. Honigman, A. Taraboulos, A. Rouvinski, and D. G. Wolf, Virology 354:69-79, 2006). Here, we employed a comprehensive ultrastructural analysis to dissect the impact of UL97 on cytoplasmic steps of HCMV assembly. Using UL97 deletion (ΔUL97) and kinase-null (K355M) mutants, as well as the UL97 kinase inhibitor NGIC-I, we demonstrated that the loss of UL97 kinase activity resulted in a unique combination of cytoplasmic features: (i) the formation of pp65-rich aberrant cytoplasmic tegument aggregates, (ii) distorted intracytoplasmic membranes, which replaced the normal architecture of the AC, and (iv) a paucity of cytoplasmic tegumented capsids and dense bodies (DBs). We further showed that these abnormal assembly intermediates did not result from impaired nuclear capsid maturation and egress per se by using 2-bromo-5,6-dichloro-1-(β-d-ribofuranosyl) benzimidizole (BDCRB) to induce the artificial inhibition of nuclear maturation and the nucleocytoplasmic translocation of capsids. The specific abrogation of UL97 kinase activity under low-multiplicity-of-infection conditions resulted in the improved release of extracellular virus compared to that of ΔUL97, despite similar rates of viral DNA accumulation and similar effects on nuclear capsid maturation and egress. The only ultrastructural correlate of the growth difference was a higher number of cytoplasmic DBs, tegumented capsids, and clustered viral particles observed upon the specific abrogation of UL97 kinase activity compared to that of ΔUL97. These combined findings reveal a novel role for UL97 in HCMV cytoplasmic secondary envelopment steps, with a further distinction of kinase-mediated function in the formation of the virus-induced AC and a nonkinase function enhancing the efficacy of viral tegumentation and release.