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 c ombined with dynamical annealing has been used to determine the structure o f a 94 residue module (X2_1) of the scaffolding protein CipC from the anaer obic bacterium Clostridium cellulolyticum. An exper imental data set compri sing 1647 nuclear Overhauser effect-derived restraints, 105 hydrogen bond r estraints and 66 phi torsion angle restraints was used to calculate 20 conv erging final solutions. The calculated structures have an average rmsd abou t the mean structure of 0.55(+/-0.11) Angstrom for backbone atoms and 1.40( +/-0.11) Angstrom for all heavy atoms when fitted over the secondary struct ural elements. The X2_1 module has an immunoglobulin-like fold with two bet a -sheets packed against each other. One sheet contains three strands, the second contains four strands. An additional strand is intercalated between the beta -sandwich, as well as two turns of a 3(.10) helix. X2_1 has a surp rising conformational stability and may act as a conformational linker and solubility enhancer within the scaffolding protein. The fold of X2_1 is ver y similar to that of telokin, titin Ig domain, hemolin D2 domain, twitchin immunoglobulin domain and the first four domains of the IgSF portion of tra nsmembrane cell adhesion molecule. As a consequence, the X2_1 module is the first prokaryotic member assigned to the I set of the immunoglobulin super family even though no sequence similarity with any member of this superfami ly could be detected. (C) 2000 Academic Press.