Phosphorylation of Eukaryotic Translation Initiation Factor 4B (EIF4B) by Open Reading Frame 45/p90 Ribosomal S6 Kinase (ORF45/RSK) Signaling Axis Facilitates Protein Translation during Kaposi Sarcoma-associated Herpesvirus (KSHV) Lytic Replication

Ersheng Kuang,Bishi Fu,Fanxiu Zhu,Jinjong Myoung,Qiming Liang

doi:10.1074/jbc.m111.280982

Abstract

Open reading frame 45 (ORF45) of Kaposi sarcoma-associated herpesvirus (KSHV) causes sustained activation of p90 ribosomal S6 kinase (RSK), which is crucial for KSHV lytic replication, but the exact functional roles remain to be determined. To characterize the biological consequence of persistent RSK activation by ORF45, we screened known cellular substrates of RSK. We found that ORF45 induced phosphorylation of eukaryotic translation initiation factor 4B (eIF4B), increased its assembly into translation initiation complex, and subsequently facilitated protein translation. The ORF45/RSK-mediated eIF4B phosphorylation was distinguishable from that caused by the canonical AKT/mammalian target of rapamycin/ribosomal S6 kinase and MEK/ERK/RSK pathways because it was resistant to both rapamycin (an mammalian target of rapamycin inhibitor) and U1026 (an MEK inhibitor). The rapamycin and U1026 doubly insensitive eIF4B phosphorylation was induced during KSHV reactivation but was abolished if either ORF45 or RSK1/2 were ablated by siRNA, a pattern that is correlated with reduced lytic gene expression as we observed previously. Ectopic expression of eIF4B but not its phosphorylation-deficient mutant form increased KSHV lytic gene expression and production of progeny viruses. Together, these results indicated that ORF45/RSK axis-induced eIF4B phosphorylation is involved in translational regulation and is required for optimal KSHV lytic replication.

Highlights

Open reading frame 45 (ORF45) of Kaposi sarcoma-associated herpesvirus (KSHV) causes sustained activation of p90 ribosomal S6 kinases (RSKs)
We further demonstrated that depletion of RSK expression or inhibition of RSK activity reduces KSHV lytic gene expression, indicating that RSK signaling is required for optimal KSHV lytic replication [17, 33], but the exact functional roles of the persistently active RSK remain to be determined
In the studies reported here, we demonstrated that ORF45mediated activation of RSK results in phosphorylation of eukaryotic translation initiation factor 4B (eIF4B) and subsequently promotes its assembly into the translation initiation complex and recruitment of mRNA to the active translation machinery

Summary

Background

ORF45 of Kaposi sarcoma-associated herpesvirus (KSHV) causes sustained activation of p90 ribosomal S6 kinases (RSKs). Results: ORF45 increases phosphorylation of eIF4B through p90 RSKs. Conclusion: The ORF45/RSK axis promotes protein translation during lytic replication. Open reading frame 45 (ORF45) of Kaposi sarcoma-associated herpesvirus (KSHV) causes sustained activation of p90 ribosomal S6 kinase (RSK), which is crucial for KSHV lytic replication, but the exact functional roles remain to be determined. We found that ORF45 induced phosphorylation of eukaryotic translation initiation factor 4B (eIF4B), increased its assembly into translation initiation complex, and subsequently facilitated protein translation. Ectopic expression of eIF4B but not its phosphorylation-deficient mutant form increased KSHV lytic gene expression and production of progeny viruses Together, these results indicated that ORF45/RSK axis-induced eIF4B phosphorylation is involved in translational regulation and is required for optimal KSHV lytic replication.

The abbreviations used are

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION