Abstract
R pyocins are ɸCTX-like myophage tailocins of Pseudomonas sp. Adsorption of R pyocins to target strains occurs by the interaction of tail fiber proteins with core lipopolysaccharide (LPS). Here, we demonstrate that N-terminally truncated R pyocin tail fibers corresponding to a region of variation between R-subtypes are sufficient to bind target strains according to R-subtype. We also report the crystal structures of these tail fiber proteins and show that they form an elongated helical trimer composed of three domains arranged linearly from N- to C-terminus: a baseplate proximal head, medial shaft, and distal foot. The head and shaft domains contain novel structural motifs. The foot domain, however, is composed of a conserved jellyroll fold and shares high structural similarity to the tail fiber of myophage AP22, podophage tailspike C-terminal domains (LKA-1 and ɸ297), and several eukaryotic adhesins (discoidin I/II, agglutinin, and octocoral lectin). Many of these proteins bind polysaccharides by means of their distal loop network, a series of highly variable loops at one end of the conserved jellyroll fold backbone. Our structures reveal that the majority of R-subtype specific polymorphisms cluster in patches covering a cleft formed at the oligomeric interface of the head domain and in a large patch covering much of the foot domain, including the distal loop network. Based on the structural variation in distal loops within the foot region, we propose that the foot is the primary sugar-binding domain of R pyocins and R-subtype specific structural differences in the foot domain distal loop network are responsible for binding target strains in an R-subtype dependent manner.
Highlights
Discovered in 1954 by Francois Jacob, pyocins are highly diverse peptide inhibitors of pseudomonad growth [1,2,3]
Because there is no known specific conserved function of tail fibers beyond binding host or target cells, we hypothesized that sequence differences between R-subtype tail fibers would ostensibly correspond to residues involved in R-subtype selective binding and most of the residues involved in these interactions would belong to the variable region
Propose that the head or foot domains found within this variable region contain putative LPS binding motifs
Summary
Discovered in 1954 by Francois Jacob, pyocins are highly diverse peptide inhibitors of pseudomonad growth [1,2,3]. They include S, F, L, and R-types, which are further classified into subtypes by strain sensitivity [1, 4,5,6,7]. F and R pyocins resemble phage tail assemblies in both structure and function, and are more accurately categorized as “tailocins”. Tailocins are prophages that lack head structural genes and genome packaging mechanisms, but retain conserved elements of phage tails, including the inner tube, outer sheath, baseplate, and tail fibers [1, 4].
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