Structural basis of apoptosis inhibition by the Fowlpox virus protein FPV039

Abstract

Programmed cell death or apoptosis of infected host cells is an important defense mechanism in response to viral infections. This process is regulated by pro-apoptotic and pro-survival members of the B-cell lymphoma 2 (Bcl-2) protein family. To counter premature death of a virus-infected cell, poxviruses use a range of different molecular strategies, including the mimicry of pro-survival Bcl-2 proteins. One such viral pro-survival protein is the fowlpox virus protein FPV039, which is a potent apoptosis inhibitor, but the precise molecular mechanism by which FPV039 inhibits apoptosis is unknown. To understand how fowlpox virus inhibits apoptosis we examined FPV039 using isothermal titration calorimetry, small-angle X-ray scattering and X-ray crystallography. Here, we report that the fowlpox virus pro-survival protein FPV039 promiscuously binds to cellular pro-apoptotic Bcl-2, and engages all major pro-apoptotic Bcl-2 proteins. Unlike other identified viral Bcl-2 proteins to date, FPV039 engaged with cellular pro-apoptotic Bcl-2 with affinities comparable to those of Bcl-2's endogenous cellular counterparts. Structural studies revealed that FPV039 adopts the conserved Bcl-2 fold observed in cellular pro-survival Bcl-2 proteins, and closely mimics the structure of the pro-survival Bcl-2 family protein Mcl-1. Our findings suggest that FPV039 is a pan Bcl-2 protein inhibitor that can engage all host BH3-only proteins as well as Bcl-2 associated X, apoptosis regulator (Bax) and Bcl-2 antagonist/killer (Bak) proteins to inhibit premature apoptosis of an infected host cell. This work therefore provides a mechanistic platform to better understand FPV039-mediated apoptosis inhibition.