In response to viral infections, host cells initiate a cell suicide program called apoptosis to eliminate the infected cells, and key regulatory proteins of this process are the pro-apoptotic and anti-apoptotic Bcl-2 proteins. To counter this host defense mechanism, poxviruses utilize many different strategies, including the mimicry of anti-apoptotic Bcl-2 proteins, to ensure their own survival.
Here, we report that a Fowlpox virus anti-apoptotic viral Bcl-2 (vBcl-2) protein, FPV039, is a promiscuous binder of the cellular pro-apoptotic Bcl-2 proteins, and engages all of the apoptosis initiating BH3-only proteins and the apoptosis executor protein Bak. Furthermore, we found that unlike most vBcl-2, FPV039 engages with cellular Bcl-2 with high affinities comparable to the affinities of cellular anti apoptotic Bcl-2. These findings may suggest that FPV039 inhibits apoptosis of the host cell primarily through the inhibition of BH3-only proteins.
Structural studies reveal that FPV039 adopts the conserved Bcl-2 fold that consists of eight α-helices, and is most similar to the cellular anti-apoptotic Mcl-1 protein. The structures of FPV039-Bmf BH3 domain and FPV039-Bik BH3 domain complexes reveal that FPV039 possesses hydrophobic pockets that can accommodate hydrophobic residues from BH3 domain of Bmf and Bik. Several hydrogen bonds and ionic interactions can be observed forming at the FPV039-Bmf and FPV-Bik interfaces.
Collectively, these findings reveal that FPV039 is a monomeric pan BH3-only protein binding virulence factor that engages the BH3 domain of pro-apoptotic proteins with high affinity unlike most other vBcl-2 proteins.