Mechanisms Of SMC5/6 Complex Inhibiting KSHV Replication

Kaposi’s sarcoma-associated herpesvirus(KSHV),also known as human herpesvirus 8,belongs to the Gammaherpesvirinae subfamily,the Rhadinovirus genus.KSHV is a lymphotropic and oncogenic virus that can infect B lymphocytes,T lymphocytes,monocytes,endothelial cells and keratinocytes,etc.Unlike other human herpesviruses,KSHV does not cause ubiquitous infection in most population,but it can establish life-long latent infection in infected person.KSHV is the major causative agent of several human malignancies,including Kaposi sarcoma(KS),primary effusion lymphoma(PEL),and multicentric Castleman’s diseases(MCDs),and so on.KSHV employs two replication programs,the default latent replication and lytic replication.During latency,multiple copies of the viral genome are maintained as circular episomes in the nucleus of infected cells,and the viral genome remains a state of transcriptional repression.Latent infection is characterized by the expression of only a limited number of viral genes.The virus can be activated in response to several pathophysiological conditions,such as hypoxia,oxidative stress,immunocompromised,and viral coinfection.During the lytic phase,the viral genome is in an open state,and all viral genes are expressed in a cascading fashion,and infectious virus particles are generated.However,the regulatory mechanisms of the viral genome from transcriptional repression to transcriptional activation remain to be further elucidated.In this study,we found that the cellular protein structural maintenance of chromosome(SMC)complex SMC5/6 can inhibit KSHV replication.We utilized Brd U to label the newly synthesized KSHV genome and found that SMC5/6 colocalized with viral genomic DNA in the cell nuclei.The results of chromatin immunoprecipitation and luciferase reporter gene assay further confirmed that the SMC5/6 complex can bind to the viral genome and inhibit the transcription of viral genes,and this transcriptional repression depends on the activities of DNA binding and ATPase of the complex.Furthermore,we found that SMC5/6 can induce the viral chromatin in a closed state and reduce H3K27 ac modification on the KSHV genome to form an inhibitory chromatin structure.Replication and transcription activator(RTA),an immediate-early gene encoded by ORF50,is the switch protein for reactivation of KSHV from latency,and is essential for the activation and regulation of KSHV lytic replication.RTA promotes KSHV reactivation to lytic replication through multiple mechanisms.In this study,we found that RTA protein interacts with the SMC5/6 complex and hijack the ubiquitin-proteasome system to degrade the SMC5/6 complex,thereby antagonizing the inhibition caused by the SMC5/6 complex.Importantly,other herpesvirus-encoded RTA homologs can also degrade the SMC5/6 complex.In summary,our study demonstrated that the host protein SMC5/6 complex associates with the KSHV genome and results in direct transcriptional inhibition by condensing the viral chromatin and creating a repressive chromatin structure.In contrast,the viral protein RTA exerts its E3 ligase function to hijack the ubiquitin-proteasome system and degrade the SMC5/6 complex,thereby antagonizing its inhibitory effect on viral gene transcription.Exploring the interaction between KSHV and the host restriction factor SMC5/6 complex would help us better understand the molecular mechanisms of the KSHV life cycle.