THE CRYSTAL-STRUCTURE OF A TYPE-I COHESIN DOMAIN AT 1.7 ANGSTROM RESOLUTION

The quaternary organization of the cellulosome, a multi-enzymatic extr acellular complex produced by cellulolytic bacteria, depends on specif ic interactions between dockerin domains, double EF-hand subunits carr ied by the catalytic components, and cohesin domains, individual recep tor subunits linearly arranged within a non-catalytic scaffolding poly peptide. Cohesin-dockerin complexes with distinct specificities are al so thought to mediate the attachment of cellulosomes to the cell membr ane. We report here the crystal structure of a single cohesin domain f rom the scaffolding protein of Clostridium thermocellum. The cohesin d omain folds into a nine-stranded beta-sandwich with an overall ''jelly roll'' topology, similar to that observed in bacterial cellulose-bind ing domains. Surface-exposed patches of conserved residues promote ext ensive intermolecular contacts in the crystal, and suggest a possible binding target for the EF-hand pair of the cognate dockerin domain. Co mparative studies of cohesin domains indicate that, in spite of low se quence similarities and different functional roles, all cohesin domain s share a common nine-stranded beta-barrel fold stabilized by a conser ved hydrophobic core. The formation of stable cohesin-dockerin complex es requires the presence of Ca2+. However, the structure of the cohesi n domain reported here reveals no obvious Ca2+-binding site, and previ ous experiments have failed to detect high affinity binding of Ca2+ to the unliganded dockerin domain of endoglucanase CelD. Based on struct ural and biochemical evidence, we propose a model of the cohesin-docke rin complex in which the dockerin domain requires complexation with it s cohesin partner for protein stability and high-affinity Ca2+ binding . (C) 1997 Academic Press Limited.