Analysis of correlated domain motions in IgG light chain reveals possible mechanisms of immunological signal transduction

It was shown experimentally that binding of a micelle composed of Congo red molecules to immunological complexes leads to the enhanced stability of the latter, and simultaneously prevents binding of a complement molecule (C1q). The dye binds in a cavity created by the removal of N-terminal polypeptide chain, as observed experimentally in a model system-immunoglobulin G (IgG) light chain dimer. Molecular Dynamics (MD) simulations of three forms of IgG light chain dimer, with and without the dye, were performed to investigate the role of N-terminal fragment and self-assembled ligand in coupling between V and C domains. Root-mean-square distance (RMSD) time profiles show that removal of N-terminal fragment leads to destabilization of V domain. A micelle composed of four self-assembled dye molecules stabilizes and fixes the domain. Analysis of root-mean-square fluctuation (RMSF) values and dynamic cross-correlation matrices (DCCM) reveals that removal of N-terminal fragment results in complete decoupling between V and C domains. Binding of self-assembled Congo red molecules improves the coupling, albeit slightly. The disruption of a small beta-sheet composed of N- and C-terminal fragments of the domain (NC sheet) is the most likely reason for the decoupling. Self-assembled ligand, bound in the place originally occupied by N-terminal fragment, is not able to take over the function of the beta-sheet. Lack of correlation of motions between residues in V and C domains denotes that light chain-Congo red complexes have hampered ability to transmit conformational changes between domains. This is a likely explanation of the lack of complement binding by immunological complexes, which bind Congo red, and supports the idea that the NC sheet is the key structural fragment taking part in immunological signal transduction.