The assembly of infectious virus particles is a complex event. For human cytomegalovirus (HCMV) this process requires the coordinated expression and localization of at least 60 viral proteins that comprise the infectious virion. To gain insight into the mechanisms controlling this process, we identified protein binding partners for two viral proteins, pUL99 (also termed pp28) and pUL32 (pp150), which are essential for HCMV virion assembly. We utilized HCMV strains expressing pUL99 or pUL32 carboxyl-terminal green fluorescent protein fusion proteins from their native location in the HCMV genome. Based on the presence of ubiquitin in the pUL99 immunoisolation, we discovered that this viral protein colocalizes with components of the cellular endosomal sorting complex required for transport (ESCRT) pathway during the initial stages of virion assembly. We identified the nucleocapsid and a large number of tegument proteins as pUL32 binding partners, suggesting that events controlling trafficking of this viral protein in the cytoplasm regulate nucleocapsid/ tegument maturation. The finding that pUL32, but not pUL99, associates with clathrin led to the discovery that the two viral proteins traffic via distinct pathways during the early stages of virion assembly. Additional investigation revealed that the majority of the major viral glycoprotein gB initially resides in a third compartment. Analysis of the trafficking of these three viral proteins throughout a time course of virion assembly allowed us to visualize their merger into a single large cytoplasmic structure during the late stages of viral assembly. We propose a model of HCMV virion maturation in which multiple components of the virion traffic independently of one another before merging.
