The R1 subunit of herpes simplex virus ribonucleotide reductase protects cells against apoptosis at, or upstream of, caspase-8 activation

Centre de recherche du Centre hospitalier de l'Université de Montréal, Hôpital Notre-Dame, 1560 Sherbrooke Est, Montréal, Québec, CanadaH2L 4M1 1 Département de microbiologie et immunologie de l’Université de Montréal, Montréal, Québec, Canada 2 Institut de recherche en biotechnologie, 6100 ave Royalmount, Montréal, CanadaH4P 2R2 3 INRS-IAF Université du Québec, Laval, Québec, CanadaH7N 4Z3 4 Author for correspondence: Yves Langelier (at Centre de recherche du Centre hospitalier de l’Université de Montréal). Fax +1 514 412 7590. e-mail yves.langelier{at}umontreal.ca The R1 subunit of herpes simplex virus (HSV) ribonucleotide reductase, which in addition to its C-terminal reductase domain possesses a unique N-terminal domain of about 400 amino acids, is thought to have an additional, as yet unknown, function. Here, we report that the full-length HSV-2 R1 has an anti-apoptotic function able to protect cells against death triggered by expression of R1( 2–357), an HSV-2 R1 subunit with its first 357 amino acids deleted. We further substantiate the R1 anti-apoptotic activity by showing that its accumulation at low level could completely block apoptosis induced by TNF-receptor family triggering. Activation of caspase-8 induced either by TNF or by Fas ligand expression was prevented by the R1 protein. As HSV R1 did not inhibit cell death mediated by several agents acting via the mitochondrial pathway (Bax overexpression, etoposide, staurosporine and menadione), it is proposed that it functions to interrupt specifically death receptor-mediated signalling at, or upstream of, caspase-8 activation. The N-terminal domain on its own did not exhibit anti-apoptotic activity, suggesting that both domains of R1 or part(s) of them are necessary for this new function. Evidence for the importance of HSV R1 in protecting HSV-infected cells against cytokine-induced apoptosis was obtained with the HSV-1 R1 deletion mutants ICP6 and hr R3. These results show that, in addition to its ribonucleotide reductase function, which is essential for virus reactivation, HSV R1 could contribute to virus propagation by preventing apoptosis induced by the immune system.