Human cytomegalovirus tegument protein pp150 acts as a cyclin A2–CDK-dependent sensor of the host cell cycle and differentiation state

Abstract

Upon cell entry, herpesviruses deliver a multitude of premade virion proteins to their hosts. The interplay between these incoming proteins and cell-specific regulatory factors dictates the outcome of infections at the cellular level. Here, we report a unique type of virion-host cell interaction that is essential for the cell cycle and differentiation state-dependent onset of human cytomegalovirus (HCMV) lytic gene expression. The major tegument 150-kDa phosphoprotein (pp150) of HCMV binds to cyclin A2 via a functional RXL/Cy motif resulting in its cyclin A2-dependent phosphorylation. Alanine substitution of the RXL/Cy motif prevents this interaction and allows the virus to fully escape the cyclin-dependent kinase (CDK)-mediated block of immediate early (IE) gene expression in S/G2 phase that normally restricts the onset of the HCMV replication cycle to G0/G1. Furthermore, the cyclin A2-CDKpp150 axis is also involved in the establishment of HCMV quiescence in NTera2 cells, showing the importance of this molecular switch for differentiation state-dependent regulation of IE gene expression. Consistent with the known nucleocapsid-binding function of pp150, its RXL/Cy-dependent phosphorylation affects gene expression of the parental virion only, suggesting a cis-acting, virus particle-associated mechanism of control. The pp150 homologs of other primate and mammalian CMVs lack an RXL/Cy motif and accordingly even the nearest relative of HCMV, chimpanzee CMV, starts its lytic cycle in a cell cycle-independent manner. Thus, HCMV has evolved a molecular sensor for cyclin A2-CDK activity to restrict its IE gene expression program as a unique level of selflimitation and adaptation to its human host.H erpesviruses are highly adapted to their hosts. This is due to a long coevolutionary process and based on an intricate molecular virus-host interaction network. Interestingly, a significant fraction of herpesviral gene products is already present in the very first phase of infection, after virus entry but before the onset of de novo viral gene expression. These factors originate from the tegument, a characteristic, protein-rich layer within herpesvirus particles, occupying the space between nucleocapsid and virus envelope. Tegument proteins serve a number of important functions. They are involved in the cytoplasmic transport of nucleocapsids, nuclear delivery of viral genomes, immune evasion, and transactivation of immediate early (IE) gene transcription (1). Importantly, they also provide a unique opportunity for the virus to flexibly respond to different cell types and states immediately after entry and set the course for a productive or nonproductive infection at the earliest possible stage.A compelling example for a tegument-derived sensor of the host cell milieu is the 71-kDa phosphoprotein (pp71, also referred to as the UL82 gene product "pUL82") of human cytomegalovirus (HCMV, human herpesvirus 5), a widespread human pathogen. Permissiveness for HCMV depends on the cellular differentiation state (2) and is intimately l...

Studies on the Contribution of Human Cytomegalovirus UL21a and UL97 to Viral Growth and Inactivation of the Anaphase-Promoting Complex/Cyclosome (APC/C) E3 Ubiquitin Ligase Reveal a Unique Cellular Mechanism for Downmodulation of the APC/C Subunits APC1, APC4, and APC5

“…It is only when cells are infected in S phase, where there are already high levels of cyclin A2, that it might be necessary for UL21a to degrade the cyclin A2 in order for the infection to initiate. The situation is likely more complicated, as it has been reported that the HCMV tegument protein pp150 also binds to cyclin A2, and this binding between pp150 on the incoming particle and cyclin A2 is responsible for the block in initiation of HCMV IE gene expression when cells are infected in S phase (50,51). We cannot exclude that increases in cyclin A2 contribute to the growth deficiency of our double mutant.…”

MicroRNA miR-21 Attenuates Human Cytomegalovirus Replication in Neural Cells by Targeting Cdc25a

“…Infection also alters expression of cellular gene products involved in cell cycle regulation, DNA damage-repair, and intrinsic defense mechanisms (25)(26)(27)(28)(29)(30). For example, cellular cyclin A2-CDK inhibits HCMV IE gene expression (31,32) and promotes quiescent infection (33), while p53, a cell cycle regulator, is required for efficient HCMV replication (28,34).…”

Antagonistic Relationship between Human Cytomegalovirus pUL27 and pUL97 Activities during Infection

“…Cells in G 0 or G 1 are permissive to initiation of viral immediate early gene expression during lytic infection. In contrast, expression is inhibited during S, G 2 , and M and involves an interaction between cyclin A and the viral protein pp150 (33,34). Lytic infection with HCMV results in a pseudocell cycle phase characterized by inhibition of cellular DNA synthesis along with processes that normally occur during the G 1 /S transition or mitosis (35)(36)(37)(38)(39)(40)(41)(42).…”