Structures of two repeats of spectrin suggest models of flexibility

Spectrin is a vital component of the cytoskeleton, conferring flexibility o n 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 re solution of two connected repeats of chicken brain a-spectrin. In all of th e structures, the linker region between adjacent units is or-helical withou t breaks, kinks, or obvious boundaries. Two features observed in the struct ures are (1) conformational rearrangement in one repeat, resulting in movem ent of the position of a loop, and (2) varying degrees of bending at the li nker region. These features form the basis of two different models of flexi bility: a conformational rearrangement and a bending model. These models pr ovide novel atomic details of spectrin flexibility.