Solution structure of γS-crystallin by molecular fragment replacement NMR

The solution structure of murine gamma S-crystallin (gamma S) has been determined by multidimensional triple resonance NMR spectroscopy, using restraints derived from two sets of dipolar couplings, recorded in different alignment media, and supplemented by a small number of NOE distance restraints. gamma S consists of two topologically similar domains, arranged with an approximate twofold symmetry, and each domain shows close structural homology to closely related (similar to 50% sequence identity) domains found in other members of the gamma-crystallin family. Each domain consists of two four-strand "Greek key" beta-sheets. Although the domains are tightly anchored to one another by the hydrophobic surfaces of the two inner Greek key motifs, the N-arm, the interdomain linker and several turn regions show unexpected flexibility and disorder in solution. This may contribute entropic stabilization to the protein in solution, but may also indicate nucleation sites for unfolding or other structural transitions. The method used for solving the gamma S structure relies on the recently introduced molecular fragment replacement method, which capitalizes on the large database of protein structures previously solved by X-ray crystallography and NMR.