Solution structure, backbone dynamics and chitin binding of the anti-fungal protein from Streptomyces tendae TU901

AFP1 is a recently discovered anti-fungal, chitin-binding protein from Streptomyces tendae Tu901. Mature AFP1 comprises 86 residues and exhibits limited sequence similarity to the cellulose-binding domains of bacterial cellulases and xylanases. No similarity to the Cys and Gly-rich domains of plant chitin-binding proteins (e.g. agglutinins, lectins, hevein) is observed. AFP1 is the first chitin-binding protein from a bacterium for which anti-fungal activity was shown. Here, we report the three-dimensional solution structure of AFP1, determined by nuclear magnetic resonance spectroscopy. The protein contains two antiparallel P-sheets (five and four P-strands each), that pack against each other in a parallel P-sandwich. This type of architecture is conserved in the functionally related family IT of cellulose-binding domains, albeit with different connectivity. A similar fold is also observed in other unrelated proteins (spore coat protein from Myxococcus xanthus, beta -B-2 and gamma -B crystallins from Bos taurus, canavalin from Jack bean). AFP1 is therefore classified as a new member of the py-crystallin superfamily. The dynamics of the protein was characterized by NMR using amide N-15 relaxation and solvent exchange data. We demonstrate that the protein exhibits an axially sym metric (oblate-like) rotational diffusion tenser whose principal axis coincides to within 15 degrees with that of the inertial tenser. After completion of the present structure of AFP1, an identical fold was reported for a Streptomyces killer toxin-like protein. Based on sequence comparisons and clustering of conserved residues on the protein surface for different cellulose and chitin-binding proteins, we postulate a putative sugar-binding site for AFP1. The inability of the protein to bind short chitin fragments suggests that certain particular architectural features of the solid chitin surface are crucial for the interaction. (C) 2001 Academic Press.