Abstract Description

Oncoviruses contribute to ~15 % of infection-related cancers worldwide and are a major cause of mortality and morbidity in developing countries. Of these, Kaposi’s sarcoma-associated herpesvirus (KSHV) is associated with Kaposi’s sarcoma (KS), an epidemic in sub-Saharan Africa. KSHV has a biphasic lifecycle, consisting of dormant latency and disruptive lytic replication. While the molecular chaperones, heat shock protein 70 (Hsp70) and heat shock protein 90 (Hsp90), have been previously shown to regulate the function of some KSHV viral proteins, these KSHV studies extend less to co-chaperones. The Hsp70-Hsp90 organising protein, Hop, is a co-chaperone that is upregulated in various cancers and has recently been studied in the context of KSHV. The KSHV-encoded protein, viral cyclin (v-Cyclin), with its binding partner, cyclin-dependent kinase 6 (CDK6), forms a complex that phosphorylates nucleophosmin (NPM1), promoting its binding to KSHV protein, latently associated nuclear antigen (LANA1), thus keeping the virus in the latent state of infection. Proteomics analysis of the lytic Hop interactome identified the KSHV v-Cyclin protein as significantly enriched. Therefore, to understand the role of Hop in KSHV biology, we evaluated its role in the v-Cyclin-CDK6-NPM1 complex. Our studies performed in vitro in KSHV-infected cells under conditions of latency and lytic reactivation demonstrated that Hop is in complex with v-Cyclin, CDK6, NPM1, phosphorylated NPM1 (pNPM1), and LANA1. KSHV lytic reactivation reduced Hop’s interaction with NPM1, pNPM1, and CDK6, particularly in the nucleus of cells. Despite Hop being in complex with v-Cyclin, CDK6, NPM1, and CDK6, Hop knockdown by shRNA or Hop-GFP overexpression did not affect the expression levels of these proteins. However, the increase in pNPM1 levels with KSHV lytic reactivation was enhanced with Hop knockdown, suggesting that Hop may hold CDK6 and NPM1 in complexes that inhibit v-Cyclin-mediated phosphorylation of NPM1. Taken collectively, our data demonstrates a novel link between Hop and the v-Cyclin, CDK6, and NPM1 complex, which further validates the functional role of Hop in KSHV biology and shows that Hop may be a new host target for therapeutic intervention for KSHV infection.