Protofilament and hexagon: a three-dimensional mechanical model for the junctional complex in the erythrocyte membrane skeleton.

It is a long-standing mystery why erythrocyte actin filaments in the junctional complex (JC) are uniformly approximately 37 nm and the membrane skeleton consists of hexagons. We have previously proposed that a "molecular ruler" formed by E-tropomodulin and tropomyosin 5 or 5b functions to generate protofilaments of 12 G actin under mechanical stress. Here, we illustrate that intrinsic properties of actin filaments, e.g., turns, chemical bonds, and dimensions of the helix, also favor fragmentation into protofilaments under mechanical stress. We further construct a mechanical model in that a pair of G actin is wrapped around by a split alpha and beta spectrin, which may spin to two potential positions, and stabilize to one when the tail end is restricted. A reinforced protofilament may function as a mechanical axis to anchor three (top, middle, and bottom) pairs of Sp. Each Sp pair may wrap around the protofilament with a wide dihedral angle (approximately 166.2 degrees) and a minimal axial distance (2.75 nm). Such three Sp pairs may spiral down (right handed) the protofilament from the pointed end with a dihedral angle of approximately 55.4 degrees in between the Sp pairs. This first three-dimensional model of JC may explain the hexagonal geometry of the erythrocyte membrane skeleton.