Subversion of host cell apoptotic responses is a prominent feature of viral immune evasion strategies to prevent premature clearance of infected cells. Numerous poxviruses encode structural and functional homologs of the Bcl-2 family of proteins, and vaccinia virus harbors antiapoptotic F1L that potently inhibits the mitochondrial apoptotic checkpoint. Recently F1L has been assigned a caspase-9 inhibitory function attributed to an N-terminal ␣ helical region of F1L spanning residues 1-15 (1) preceding the domain-swapped Bcl-2-like domains. Using a reconstituted caspase inhibition assay in yeast we found that unlike AcP35, a well characterized caspase-9 inhibitor from the insect virus Autographa californica multiple nucleopolyhedrovirus, F1L does not prevent caspase-9-mediated yeast cell death. Furthermore, we found that deletion of the F1L N-terminal region does not impede F1L antiapoptotic activity in the context of a viral infection. Solution analysis of the F1L N-terminal regions using small angle x-ray scattering indicates that the region of F1L spanning residues 1-50 located N-terminally from the Bcl-2 fold is an intrinsically unstructured region. We conclude that the N terminus of F1L is not involved in apoptosis inhibition and may act as a regulatory element in other signaling pathways in a manner reminiscent of other unstructured regulatory elements commonly found in mammalian prosurvival Bcl-2 members including Bcl-x L and Mcl-1.

Currently, little is known about the ankyrin/F-box protein B4. Here, we report that B4R-null viruses exhibited reduced plaque size in tissue culture, and decreased ability to spread, as assessed by multiple-step growth analysis. Electron microscopy indicated that B4R-null viruses still formed mature and extracellular virions; however, there was a slight decrease of virions released into the media following deletion of B4R. Deletion of B4R did not affect the ability of the virus to rearrange actin; however, VACV811, a large vaccinia virus deletion mutant missing 55 open reading frames, had decreased ability to produce actin tails. Using ectromelia virus, a natural mouse pathogen, we demonstrated that virus devoid of EVM154, the B4R homolog, showed decreased spread to organs and was attenuated during infection. This initial characterization suggests that B4 may play a role in virus spread, and that other unidentified mediators of actin tail formation may exist in vaccinia virus.

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