Flagellar structures from the bacterium Caulobacter crescentus and implications for phage {Phi}CbK predation of multi-flagellin bacteria

Abstract

Caulobacter crescentus is a gram-negative alpha-proteobacterium that commonly lives in oligotrophic fresh and salt-water environments. C. crescentus is a host to many bacteriophages, including {phi}CbK and {phi}CbK-like bacteriophages, which first adsorb to cells by interaction with the bacterial flagellum. It is commonly thought that the six paralogs of the flagellin gene present in C. crescentus are important for bacteriophage evasion. Here, we show that deletion of specific flagellins in C. crescentus can indeed attenuate {phi}CbK adsorption efficiency, although no single deletion completely ablates {phi}CbK adsorption. Thus, bacteriophage {phi}CbK likely recognizes a common motif amongst the six known flagellins in C. crescentus with varying degrees of efficiency. Interestingly, we observe that most deletion strains still generate flagellar filaments, with the exception of a strain that contains only the most divergent flagellin, FljJ, or a strain that contains only FljN and FljO. To visualize the surface residues that are likely recognized by {phi}CbK, we determined two high-resolution structures of the FljK filament, with and without an amino acid substitution that induces straightening of the filament. We observe post-translational modifications on conserved surface threonine residues of FljK that are likely O-linked glycans. The possibility of interplay between these modifications and {phi}CbK adsorption is discussed. We also determined the structure of a filament composed of a heterogeneous mixture of FljK and FljL, the final resolution of which was limited to approximately 4.6 [A]. Altogether, this work builds a platform for future investigation of how phage {phi}CbK infects C. crescentus at the molecular level.