HCMV Tegument Protein PUL23Dissociates STAT1from Receptor For Activated C Kinase (RACK1)

Human cytomegalovirus (HCMV) is the prototypical member of thebetaherpesvirus family and is a ubiquitous human pathogen. HCMV, like otherherpesviruses, cannot be completely eliminated by the immune system and remainseither as a low-level persistent infection or in a quiescent latent state for the lifetimeof the infected person. Therefore HCMV infections in immunocompetent adults areusually benign and could maintain a lifelong relationship with its host, butreactivation of HCMV from latency causes life-threatening illness in immunologicallyimmature or compromised individuals. According to the global analysis of severalHCMV strains genome, it revealed HCMV contains more than200ORFS (OpenReading Frame, ORF), but only45~57ORFS of them are essential for HCMV growthin primary human fibroblasts, the rest are nonessential. Studies have revealed someHCMV proteins have involved in coordinating the relationship with the host cell bytheir interaction with host cell protein.UL23is a member of HCMV US22gene family, it encodes a tegument proteinpUL23about33kD which is located in cytoplasmic protein aggregates. But thebiological function of pUL23is controversial and has been not clear to this day. Toinvestigate the underlying function of pUL23, the yeast two-hybrid screening systemand co-immunoprecipitation (Co-IP) experiment were used to identify cellular targetsof pUL23. One of the binding partners of pUL23was RACK1, which is the foundingmember of the family of receptors for activated C kinase (PKC) collectively calledRACKs and is identified as a multifunctional scaffold protein. Furthermore pUL23and RACK1substantially co-localized in the cytoplasm when they wereco-transformed into Hela cells.RACK1and has been reported to function as an adaptor recruiting thetranscription factor STAT1to the receptor complex in the IFN system, and thispre-association of STAT1with the receptor is required for STAT1activation and IFNsignaling. Therefore it’s suggested the interaction between RACK1and STAT1plays a central role in STAT1activation and IFN signaling. Based on these clues, we exploredwhether HCMV pUL23may influence the interaction between RACK1and STAT1with RACK1as a mediator. Glutathione S-transferase pull-down experiment revealedthat the association of RACK1with HCMV protein pUL23with a higher affinity thanSTAT1. Therefore, it is suggested that the viral protein pUL23has the ability tointeract strongly with RACK1and consequently to bring about the disruption of thecomplex formed from STAT1, RACK1, and probably the IFN receptor, which maydirectly lead to the suppression of STAT1activation. While the activation of STAT1isimportant for IFN signaling pathway. Therefore, these findings raise the possibilitythat pUL23functions as a regulator in IFN signaling and provide us with manyimportant clues for the further research of pUL23.