Hexagonal lattices, protofilament, and tropomodulin-TM complex of the erythrocyte membrane skeletal network.

(A) The top view of the hexagonal network based on the electron micrographs.1-3 As indicated by an arrow, 6 spectrin tetramers are associated with 1 junctional complex. The junctional complexes are approximately 33-37 nm in length. Each spectrin tetramer is approximately 200 nm long and comprises 2 αβ spectrin dimers associated with a head-to-head fashion. The 2 tail ends of the spectrin tetramer join the junctional complexes, and the 2 head ends meet with each other in the middle. The pair of smaller complexes in the mid regions of the spectrin tetramers are protein 4.2/band 3/ankyrin complexes, which hang the membrane skeletal network to the lipid bilayer. (B) The molecular model of an actin protofilament in the erythrocyte membrane skeleton. The filament's length is approximately 6-7 G-actin and is associated with only 2 TM molecules, 1 in each groove of the actin filament, and 1 tropomodulin molecule at the pointed end. The barbed end may either be uncapped or capped by adducin,7 gelsolin,8 or another barbed-end capping protein. The number of G-actin limits the number of spectrin binding to the protofilament and defines the hexagonal geometry of the membrane network. (C) The model of the tropomodulin-TM complex that stabilizes the actin protofilament. Tropomodulin binds near the N-terminal of TM5 or TM5b (homodimer or heterodimer approximately 33-35 nm in length, 6 actin-binding sites), at the pointed end of the actin filament. The complex functions as a measuring device, determining the number of G-actin to be protected. N and C stand for the N-terminal and C-terminal of the TM molecule, respectively.