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 Å, 2.0 Å, 3.1 Å, and 4.0 Å resolution of two connected repeats of chicken brain α-spectrin. In all of the structures, the linker region between adjacent units is α-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.