Journal
VIRUSES-BASEL
Volume 13, Issue 2, Pages -Publisher
MDPI
DOI: 10.3390/v13020165
Keywords
human cytomegalovirus; core nuclear egress complex; ORF-UL50 deletion; pUL50 phosphosite mutants; phenotypical changes; differential functional relevance
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Funding
- Deutsche Forschungsgemeinschaft (DFG) [SFB796/C3, MA1289/11-1, MI2143/2-1]
- Deutscher Akademischer Austauschdienst [DAAD-Go8]
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Research has shown that pUL50 plays a crucial role in nuclear egress of human cytomegalovirus (HCMV), but mutations in phosphorylation sites of pUL50 have minimal impact on the virus's replication capabilities.
Nuclear egress is a common herpesviral process regulating nucleocytoplasmic capsid release. For human cytomegalovirus (HCMV), the nuclear egress complex (NEC) is determined by the pUL50-pUL53 core that regulates multicomponent assembly with NEC-associated proteins and capsids. Recently, NEC crystal structures were resolved for alpha-, beta- and gamma-herpesviruses, revealing profound structural conservation, which was not mirrored, however, by primary sequence and binding properties. The NEC binding principle is based on hook-into-groove interaction through an N-terminal hook-like pUL53 protrusion that embraces an alpha-helical pUL50 binding groove. So far, pUL50 has been considered as the major kinase-interacting determinant and massive phosphorylation of pUL50-pUL53 was assigned to NEC formation and functionality. Here, we addressed the question of phenotypical changes of ORF-UL50-mutated HCMVs. Surprisingly, our analyses did not detect a predominant replication defect for most of these viral mutants, concerning parameters of replication kinetics (qPCR), viral protein production (Western blot/CoIP) and capsid egress (confocal imaging/EM). Specifically, only the ORF-UL50 deletion rescue virus showed a block of genome synthesis during late stages of infection, whereas all phosphosite mutants exhibited marginal differences compared to wild-type or revertants. These results (i) emphasize a rate-limiting function of pUL50 for nuclear egress, and (ii) demonstrate that mutations in all mapped pUL50 phosphosites may be largely compensated. A refined mechanistic concept points to a multifaceted nuclear egress regulation, for which the dependence on the expression and phosphorylation of pUL50 is discussed.
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