Vaccinia Virus Protein F1L Is a Caspase-9 Inhibitor

Dayong Zhai,Eric Yu,Chaofang Jin,Kate Welsh,Chung-Wei Shiau,Lili Chen,Guy S Salvesen,Robert Liddington,John C Reed

doi:10.1074/jbc.m109.078113

Abstract

Apoptosis plays important roles in host defense, including the elimination of virus-infected cells. The executioners of apoptosis are caspase family proteases. We report that vaccinia virus-encoded F1L protein, previously recognized as anti-apoptotic viral Bcl-2 family protein, is a caspase-9 inhibitor. F1L binds to and specifically inhibits caspase-9, the apical protease in the mitochondrial cell death pathway while failing to inhibit other caspases. In cells, F1L inhibits apoptosis and proteolytic processing of caspases induced by overexpression of caspase-9 but not caspase-8. An N-terminal region of F1L preceding the Bcl-2-like fold accounts for caspase-9 inhibition and significantly contributes to the anti-apoptotic activity of F1L. Viral F1L thus provides the first example of caspase inhibition by a Bcl-2 family member; it functions both as a suppressor of proapoptotic Bcl-2 family proteins and as an inhibitor of caspase-9, thereby neutralizing two sequential steps in the mitochondrial cell death pathway.

Highlights

Viruses have either evolved independently or usurped from host cells a diversity of anti-apoptotic strategies
Viruses encode homologs of cellular anti-apoptotic Bcl-2 proteins to protect infected host cells by keeping the mitochondria intact, and several viral Bcl-2 protein homologs have been reported to date [1, 2]
To interfere with this pathway, viruses encode various antagonists including 1) vFLIPs, viral homologs of cellular FLIP, which bind procaspase-8 and-10 and inhibit their activation [1, 2, 6]; 2) CrmA, a viral serpin that binds irreversibly to and inhibits caspase-8 and -10 [7, 8]; and 3) soluble decoy receptors that compete with host TNF family receptors to bind cognate ligands

Summary

Introduction

Viruses have either evolved independently or usurped from host cells a diversity of anti-apoptotic strategies. We noticed that recombinant F1L protein, unlike Bcl-XL, inhibits caspase activation induced in cell extracts by cytochrome c (Fig. 1A). We found that F1L⌬TM recombinant protein (amino acids 1–206; produced without its C-terminal transmembrane domain to aid solubility) at 1 ␮M potently suppressed cytochrome c-induced caspase activation in cytosolic extracts, whereas another anti-apoptotic vaccinia protein, N1L, did not.

Results

Conclusion