AcrIIA22 is an oligomeric PC4-like protein.
(A) AcrIIA22’s crystal structure reveals a homodimer of 2 four-stranded β-sheets. (B) AcrIIA22 elutes as an oligomer that is approximately 4 times the predicted molecular mass of its monomer, which is 7 kDa. The gray, dashed trace depicts protein standards of the indicated molecular weight, in kDa. The orange trace depicts the elution profile of a 2-amino acid C-terminal AcrIIA22 truncation mutant that is predicted to disrupt oligomerization. (C) Ribbon diagram of a proposed AcrIIA22 tetramer, which requires binding between antiparallel β-strands at the C-termini of AcrIIA22 monomers to form extended, concave β-sheets. The putative oligomerization interface is indicated by the regions highlighted in yellow and the dashed box and is detailed further in panel F. Each monomer in the proposed tetramer is labeled with lowercase Roman numerals (i-iv). (D) β-sheet topology and orientation in AcrIIA22 (blue) resemble that of PC4-like family proteins (in gray, PDB:4BG7 from phage T5). (E) A monomer of AcrIIA22 (in blue, PDB:7JTA) is structurally similar to a PC4-like single-stranded DNA binding protein, which is proposed to promote recombination in phage T5 (in gray, PDB:4BG7, Z-score = 6.2, matched residues 15%), except for a missing C-terminal alpha helix. (F) A putative oligomerization interface between the C-termini of 2 AcrIIA22 monomers from panel (C) is shown in more detail. Dashed lines indicate potential hydrogen bonds between the polypeptide backbones. This interface occurs twice in the putative tetramer, between red-hued and blue-hued monomers in panel C.