Structure of the host-recognition device of Staphylococcus aureus phage ϕ11

Abstract

Phages play key roles in the pathogenicity and adaptation of the human pathogen Staphylococcus aureus. However, little is known about the molecular recognition events that mediate phage adsorption to the surface of S. aureus. The lysogenic siphophage ϕ11 infects S. aureus SA113. It was shown previously that ϕ11 requires α- or β-N-acetylglucosamine (GlcNAc) moieties on cell wall teichoic acid (WTA) for adsorption. Gp45 was identified as the receptor binding protein (RBP) involved in this process and GlcNAc residues on WTA were found to be the key component of the ϕ11 receptor. Here we report the crystal structure of the RBP of ϕ11, which assembles into a large, multidomain homotrimer. Each monomer contains a five-bladed propeller domain with a cavity that could accommodate a GlcNAc moiety. An electron microscopy reconstruction of the ϕ11 host adhesion component, the baseplate, reveals that six RBP trimers are assembled around the baseplate core. The Gp45 and baseplate structures provide insights into the overall organization and molecular recognition process of the phage ϕ11 tail. This assembly is conserved among most glycan-recognizing Siphoviridae, and the RBP orientation would allow host adhesion and infection without an activation step.

Structure of the host cell recognition and penetration machinery of a Staphylococcus aureus bacteriophage

“…Six such trimers surround the baseplate core, making up the bulk of the observed peripheral structures ( Fig 2D). As expected, the 80α RBP is very similar to ϕ11 gp45 [24] (Fig 5B, S4J Fig; S1 Table). The greatest difference between the 80α and ϕ11 RBPs is in the orientation of the stem domains relative to the platform and tower domains ( Fig 5B).…”

“…80α gp61 is 97% identical in amino acid sequence to gp45 of the closely related phage ϕ11, for which the crystal structure was previously determined [24]. Gp45 was previously identified as the primary receptor binding protein for ϕ11, with binding affinity for WTA, and was thus denoted RBP [21].…”

“…The N-terminal part of ϕ11 gp54, corresponding to 80α gp62, was shown to have some similarity with upper baseplate protein (BppU) of lactococcal phage TP901-1 [21,23]. The crystal structure of ϕ11 RBP (gp45), which is 97% identical to the 80α RBP, was recently determined [24]. Crystal structures have also been determined for Bacillus phage SPP1 Dit [25] and the baseplates of lactococcal phages p2 and TP901-1, which include Dit, Tal, BppU, and receptor binding proteins that are not similar to the ϕ11 RBP [23,26].…”

“…Gp45 was previously identified as the primary receptor binding protein for ϕ11, with binding affinity for WTA, and was thus denoted RBP [21]. RBP is a trimeric protein consisting of four domains: an N-terminal α-helical coiled-coil "stem" domain (residues 1-142) that is folded by 155˚at a central hinge; a fivebladed β-propeller "platform" domain (residues 143-442) that was previously suggested to bind to the GlcNAc residues of WTA on the host cell surface [21,24]; and two highly similar C-terminal "tower" domains (residues 443-636; S1 Table), each consisting of a bent β-sheet superposed by an α-helix ( Fig 5A). Six such trimers surround the baseplate core, making up the bulk of the observed peripheral structures ( Fig 2D).…”

“…The initial model for RBP was adapted from the ϕ11 RBP (PDB ID: 5EFV) [24]. Initial models for the remaining proteins were generated using I-TASSER [29].…”