Human cytomegalovirus pp28 (UL99) localizes to a cytoplasmic compartment which overlaps the endoplasmic reticulum-Golgi-intermediate compartment

Although the assembly of herpesviruses has remained an active area of inves tigation, considerable controversy continues to surround the cellular locat ion of tegument and envelope acquisition. This controversy is particularly evident when the proposed pathways for alpha- and beta-herpesvirus assembly are compared. We have approached this aspect of human cytomegalovirus (HCM V) assembly, specifically, envelopment, by investigating the intracellular trafficking of viral tegument proteins which localize in the cytoplasms of infected cells. In this study we have demonstrated that the virion tegument protein pp28 (UL99), a true late protein, was membrane associated as a res ult of myristoylation, A mutation in this protein which prevented incorpora tion of [H-3]myristic acid also altered the detergent solubility and intrac ellular distribution of the protein when it was expressed in transfected ce lls. Using a panel of markers for intracellular compartments, we could loca lize the expression of wild-type pp28 to an intracellular compartment which colocalized with the endoplasmic reticulum-Golgi-intermediate compartment (ERGIC), a dynamic compartment of the secretory pathway which interfaces wi th both the ER and Golgi apparatus. The localization of this viral tegument protein within an early secretory compartment of the cell provided further evidence that the assembly of the HCMV tegument likely includes a cytoplas mic phase. Because pp28 has been shown to be localized to a cytoplasmic ass embly compartment in HCMV-infected cells, our findings also suggested that viral tegument protein interactions within the secretory pathway may have a n important role in the assembly of the virion.