Structures of two repeats of spectrin suggest models of flexibility

Spectrin is a vital component of the cytoskeleton, conferring flexibility on cells and providing a scaffold for a variety of proteins. It is composed of tandem, antiparallel coiled-coil repeats. We report four related crystal structures at 1.45 Angstrom, 2.0 Angstrom 3.1 Angstrom and 4.0 Angstrom resolution of two connected repeats of chicken brain a-spectrin. In all of the structures, the linker region between adjacent units is or-helical without breaks, kinks, or obvious boundaries. Two features observed in the structures are (1) conformational rearrangement in one repeat, resulting in movement of the position of a loop, and (2) varying degrees of bending at the linker region. These features form the basis of two different models of flexibility: a conformational rearrangement and a bending model. These models provide novel atomic details of spectrin flexibility.