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

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 phi 11 infects S. aureus SA113. It was shown previously that phi 11 requires alpha- or beta-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 phi 11 receptor. Here we report the crystal structure of the RBP of phi 11, 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 phi 11 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 phi 11 tail. This assembly is conserved among most glycan-recognizing Siphoviridae, and the RBP orientation would allow host adhesion and infection without an activation step.