Originally termed VP8 because it was identified as the eighth largest virion protein, and subsequently the upper matrix protein, the product of the UL82 gene of HCMV is most commonly referred to as pp71. As its name implies, pp71 is a 71-kilodalton phosphoprotein whose gene is located next to and shares homology with the UL83 gene encoding for pp65. The UL82 gene is expressed with early-late kinetics, and the pp71 protein localizes to the nucleus in UL82-transfected cells. pp71 is incorporated into the tegument layer of infectious virions and is thus delivered to newly infected cells upon viral entry. The sub-cellular localization of tegument-delivered pp71 varies depending upon the differentiation status of the cell. Tegument-delivered pp71 localizes to the nucleus of terminally differentiated cells, but remains in the cytoplasm of incompletely differentiated cell types. The sub cellular localization of tegument-delivered pp71 is one factor that helps control whether upon entry into a cell, HCMV initiates a lytic, or establishes a latent infection.
The ability of HCMV virion proteins to activate expression from the viral major immediate early promoter (MIEP) was previously established when a candidate approach identified pp71 as the first known HCMV virion transactivator. Subsequent work showed that pp71 increased the infectivity of transfected viral genomic DNA. Although pp71 is not absolutely essential, it is required for efficient viral replication. The null-mutant virus has a severe growth defect. Cells infected with pp71-null viruses show decreased expression of viral immediate early genes from at least four different loci: UL36-UL38, UL106-UL109, UL115-119, and the major immediate early (MIE) locus UL122-UL123.
At least one mechanism through which pp71 activates viral gene expression is to counteract the repressive effects of the cellular Daxx protein. Daxx binds to histone deacetylases and is recruited to promoters by DNA-binding transcription factors resulting in transcriptional repression. In addition, Daxx localizes to PML-nuclear bodies (PML-NBs), sites where other cellular transcriptional repressors also accumulate. pp71 binds to Daxx through two Daxx-Interaction-Domains termed DIDs, and through this interaction partially localizes to PML-NBs. After binding to Daxx, pp71 disrupts its binding to the ATRX protein, induces Daxx SUMOylation, and ultimately Daxx degradation. Daxx degradation induced by pp71 occurs through a rare proteasome-dependent, ubiquitin-independent mechanism, and allows for viral IE gene expression at the start of a lytic infection.
When latent HCMV infections are established, expression of the viral IE genes that initiate lytic replication is inhibited. Daxx-mediated repression contributes in part to this repression. Daxx is not degraded in latently infected CD34+ hematopoietic progenitor cells because tegumant-delivered pp71 fails to reach the nucleus.
In addition to regulating viral IE gene expression, pp71 has additional activities. pp71 targets the hypophosphorylated forms of the Rb family of tumor suppressors for proteasome-dependent, ubiquitin-independent degradation. By degrading Rb proteins, pp71 stimulates cell cycle progression by driving quiescent (G0) cells into the S phase. Interestingly, pp71 also appears to have an Rb-independent ability to accelerate cells through the G1 phase of the cell cycle. It is presently unclear what function these activities of pp71 play during HCMV infection, but a mutant HCMV that expresses only a pp71 protein unable to degrade Rb replicates as well as wild type virus. Thus, although pp71 does mediate Rb degradation in HCMV infected cells, such degradation is not essential for lytic viral replication in vitro. This is likely because in addition to pp71-mediated Rb degradation, HCMV has other ways to impact the Rb-E2F pathway. Multiple redundant mechanisms attest to the importance of modulating this cellular pathway during HCMV infection.
pp71 expression late during infection of semi-permissive glioblastoma cells decreases the cell surface expression of MHC class I proteins by slowing their intracellular transport. This activity of pp71 may be cell type dependent as it was not observed in fully permissive fibroblasts. Much like the case for Rb/E2F described above, pp71 is only one of many HCMV proteins to modulate MHC class I proteins.