Side Tail Fibers of Phage Lambda_2B8 Do Not Bind BamA Receptor

Shiga toxigenic Escherichia coli (STEC), carrying the Stx-converting prophages, are causative agents of severe human infections. Although bacteriophage phi24B is a typical representative of the most prevalent Stx-phages, the molecular mechanisms of its adsorption to the host cell surface are poorly understood. While the conserved outer membrane protein BamA was identified as the final receptor for phi24B, the phage receptor binding protein(s) (RBP(s)) responsible for BamA recognition remains unknown. The side tail fibers (annotated as tail spikes) formed by gp61 were previously suggested to serve for BamA recognition, although no experimental evidence for this has been published. To access the gp61 function we used another phage, λ_2B8, which carries side tail fibers with the C-terminal region nearly identical to the C-terminal part of the phi24B gp61, while the rest of the virion is highly similar to phage λ. We genetically modified the phage, obtaining virus λ_2B8Jλ by replacing the central tail fiber protein gpJ with its homologue from phage λ, ensuring high-affinity binding to the LamB receptor. Transmission electron microscopy confirmed the typical siphovirus morphology of the chimeric phage, although the flexible side tail fibers, containing collagen-like repeats, were only partially visualized. Investigation of the adsorption kinetics of λ_2B8Jλ on isogenic E. coli strains differing in the sequence of the BamA protein—which determines resistance or susceptibility to phi24B—revealed no statistically significant difference in adsorption rate constants. This finding indicates that the side tail fiber protein Stfλ_2B8 (and probably its homologue gp61phi24B) does not participate in specific binding to BamA. These data refute the existing hypothesis about gp61-BamA interaction and demonstrate that binding to this final receptor is mediated by another structural component of the virion, most probably by the central tail fiber (needle) consistently with current understanding of the podoviral tail machines action. Consequently, the potential phi24B primary receptor that may be recognized by the side tail fibers remains to be identified.