Receptor-binding protein of XXXX phages: identification and characterization of the saccharide receptor-binding site.

@entity7857 @entity26191 @entity1735 @entity653 @entity7878
@entity26191 , a member of the lactococcal 936 phage species, infects @entity7878 strains by binding initially to specific @entity1735 receptors using its receptor-binding protein (RBP). The structures of p2 RBP, a homotrimeric protein composed of three domains, and of its complex with a neutralizing @entity7857 VH domain (VHH5) have been determined (S. Spinelli, A. Desmyter, C. T. Verrips, H. J. de Haard, S. Moineau, and C. Cambillau, Nat. Struct. Mol. Biol. 13:85-89, 2006). Here, we show that VHH5 was able to neutralize 12 of 50 lactococcal phages belonging to the 936 species. Moreover, escape phage mutants no longer neutralized by VHH5 were isolated from 11 of these phages. All of the mutations (but one) cluster in the RBP/VHH5 interaction surface that delineates the receptor-binding area. A @entity653 molecule, observed in the 1.7-A resolution structure of RBP, was found to bind tightly (Kd= 0.26 microM) in a crevice located in this area. Other saccharides bind RBP with comparable high affinity. These data prove the saccharidic nature of the bacterial receptor recognized by @entity26191 and identify the position of its binding site in the RBP head domain.
