Solution structure of the module X2_1 of unknown function of the cellulosomal scaffolding protein CipC of Clostridium cellulolyticum

Abstract

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 3 .10 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.