Divide-and-conquer crystallographic approach towards an atomic structure of intermediate filaments.

Intermediate filaments (IFs) represent an essential component of the cytoskeleton in higher eukaryotic cells. The elementary building block of the IF architecture is an elongated dimer with its dominant central part being a parallel double-stranded alpha-helical coiled coil. Filament formation proceeds via a specific multi-step association of the dimers into the unit-length filaments, which subsequently anneal longitudinally and finally radially compact into mature filaments. To tackle the challenge of a crystallographic structure determination, we have produced and characterised 17 overlapping soluble fragments of human IF protein vimentin. For six fragments ranging in length between 39 and 84 amino acid residues, conditions yielding macroscopic crystals could be established and X-ray diffraction data were collected to the highest resolution limit between 1.4 and 3 A. We expect that solving the crystal structures of these and further fragments will eventually allow us to patch together a molecular model for the full-length vimentin dimer. This divide-and-conquer approach will be subsequently extended to determining the crystal structures of a number of complexes formed by appropriate vimentin fragments, and will eventually allow us to establish the three- dimensional architecture of complete filaments at atomic resolution.