Structural Basis of Apoptosis Inhibition by the Fowlpox Virus Protein FPV039 [Microbiology]

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.

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