Solution structure of a type I dockerin domain, a novel prokaryotic, extracellular calcium-binding domain

The type I dockerin domain is responsible for incorporating its associated glycosyl hydrolase into the bacterial cellulosome, a multienzyme cellulolyt ic complex, via its interaction with a receptor domain (cohesin domain) of the cellulosomal scaffolding subunit. The highly conserved dockerin domain is characterized by two Ca2+-binding sites with sequence similarity to the EF-hand motif. Here, we present the three-dimensional solution structure of the 69 residue dockerin domain of Clostridium thermocellum cellobiohydrola se CelS, Torsion angle dynamics calculations utilizing a total of 728 NOE-d erived distance constraints and 79 torsion angle restraints yielded an ense mble of 20 structures with an average backbone r.m.s.d. for residues 5 to 2 9 and 32 to 66 of 0.54 Angstrom from the mean structure. The structure cons ists of two Ca2+-binding loop-helix motifs connected by a linker; the E hel ices entering each loop of the classical EF-hand motif are absent from the dockerin domain. Each dockerin Ca2+-binding subdomain is stabilized by a cl uster of buried hydrophobic side-chains. Structural comparisons reveal that , in its non-complexed state, the dockerin fold displays a dramatic departu re from that of Ca2+-bound EF-hand domains. A putative cohesin-binding surf ace, comprised of conserved hydrophobic and basic residues, is proposed, pr oviding new insight into cellulosome assembly. (C) 2001 Academic Press.