Solution structure of the module X2-1 of unknown function of the cellulosomal scaffolding protein CipC of Clostridium cellulolyticum

Multidimensional, homo- and heteronuclear magnetic resonance spectroscopy combined with dynamical annealing has been used to determine the structure of a 94 residue module (X2-1) of the scaffolding protein CipC from the anaerobic bacterium Clostridium cellulolyticum. An experimental data set comprising 1647 nuclear Overhauser effect-derived restraints, 105 hydrogen bond restraints and 66 φ torsion angle restraints was used to calculate 20 converging final solutions. The calculated structures have an average rmsd about the mean structure of 0.55(±0.11) Å for backbone atoms and 1.40(±0.11) Å for all heavy atoms when fitted over the secondary structural elements. The X2-1 module has an immunoglobulin-like fold with two β-sheets packed against each other. One sheet contains three strands, the second contains four strands. An additional strand is intercalated between the β-sandwich, as well as two turns of a 310 helix. X2-1 has a surprising conformational stability and may act as a conformational linker and solubility enhancer within the scaffolding protein. The fold of X2-1 is very similar to that of telokin, titin Ig domain, hemolin D2 domain, twitchin immunoglobulin domain and the first four domains of the IgSF portion of transmembrane cell adhesion molecule. As a consequence, the X2-1 module is the first prokaryotic member assigned to the I set of the immunoglobulin superfamily even though no sequence similarity with any member of this superfamily could be detected. (C) 2000 Academic Press.