Human Herpesvirus 8 Interferon Regulatory Factor-Mediated BH3-Only Protein Inhibition via Bid BH3-B Mimicry

Young Bong Choi,Gordon Sandford,John Nicholas,Shou-Jiang Gao

doi:10.1371/journal.ppat.1002748

Young Bong Choi, Gordon Sandford + Show 2 more

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https://doi.org/10.1371/journal.ppat.1002748

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Abstract

Viral replication efficiency is in large part governed by the ability of viruses to counteract pro-apoptotic signals induced by infection of host cells. For HHV-8, viral interferon regulatory factor-1 (vIRF-1) contributes to this process in part via inhibitory interactions with BH3-only protein (BOP) Bim, recently identified as an interaction partner of vIRF-1. Here we recognize that the Bim-binding domain (BBD) of vIRF-1 resembles a region (BH3-B) of Bid, another BOP, which interacts intramolecularly with the functional BH3 domain of Bid to inhibit it pro-apoptotic activity. Indeed, vIRF-1 was found to target Bid in addition to Bim and to interact, via its BBD region, with the BH3 domain of each. In functional assays, BBD could substitute for BH3-B in the context of Bid, to suppress Bid-induced apoptosis in a BH3-binding-dependent manner, and vIRF-1 was able to protect transfected cells from apoptosis induced by Bid. While vIRF-1 can mediate nuclear sequestration of Bim, this was not the case for Bid, and inhibition of Bid and Bim by vIRF-1 could occur independently of nuclear localization of the viral protein. Consistent with this finding, direct BBD-dependent inactivation by vIRF-1 of Bid-induced mitochondrial permeabilization was demonstrable in vitro and isolated BBD sequences were also active in this assay. In addition to Bim and Bid BH3 domains, BH3s of BOPs Bik, Bmf, Hrk, and Noxa also were found to bind BBD, while those of both pro- and anti-apoptotic multi-BH domain Bcl-2 proteins were not. Finally, the significance of Bid to virus replication was demonstrated via Bid-depletion in HHV-8 infected cells, which enhanced virus production. Together, our data demonstrate and characterize BH3 targeting and associated inhibition of BOP pro-apoptotic activity by vIRF-1 via Bid BH3-B mimicry, identifying a novel mechanism of viral evasion from host cell defenses.

Highlights

Human herpesvirus 8 (HHV-8) specifies a number of proteins expressed during the lytic cycle that have demonstrated or potential abilities to promote virus productive replication via inhibition of apoptotic pathways induced by infection- or replication-induced stress
The structural similarities of Bim-binding domain (BBD) and BH3-B suggested the possibility that BBD might interact with Bim via its BH3 domain and that viral interferon regulatory factor-1 (vIRF-1) might target Bid in addition to Bim
Bim and Bid BH3 targeting by vIRF-1 To test whether the BBD of vIRF-1 interacted with the BH3 domain of Bim, recombinant fusion proteins were made for coprecipitation binding assays

Summary

Introduction

Human herpesvirus 8 (HHV-8) specifies a number of proteins expressed during the lytic cycle that have demonstrated or potential abilities to promote virus productive replication via inhibition of apoptotic pathways induced by infection- or replication-induced stress. These proteins include membrane signaling receptors K1 and K15 [1,2,3], Bcl-2 and survivin homologues encoded by open reading frames 16 and K7 [4,5,6,7], viral chemokines vCCL-1 and vCCL-2 [8], and viral G proteincoupled receptor (vGPCR) [9,10]. The v-chemokines, vGPCR and vIRF-1 are the only HHV-8 proteins that have been demonstrated both to inhibit apoptosis in lytically infected cells and to promote HHV-8 productive replication, in the context of lytic reactivation in endothelial cells in the case of the vCCLs and vIRF-1 and in primary effusion lymphoma (PEL) cells for vGPCR [23,8,10]

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