Protofilament and hexagon: A three-dimensional mechanical model for the junctional complex in the erythrocyte membrane skeleton
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Figure 7. A 3-D presentation of a JC and a top view of six plus one JC in the membrane. (a) shows a protofilament that may function as the mechanical axis for 3 pairs of Sp. Each of the 6 Sp may connect to a SC in the lipid bilayer, forming a small hexagon. Both Sp pairs (top, middle, and bottom) and G actin pairs to which Sp pairs are attached are color-coded. Lines separate same colored G actin pairs into two, one for each Sp to attach. The hexagon may be defined by the position of SC, which has no physical edges, meaning no proteins serve to connect SC1-6. Note the relative positions of Sp1 to Sp6 on the protofilament, their dihedral angles and the order of SC in the hexagon. In (b) 6 peripheral JC and one central JC may rotate in various orientations to connect and form a large hexagon. All six edges and the lattices connecting six corners to the center of the hexagon are made out of spectrin tetramers. Note all Sp heads are presumably near SC and the plane of lipid bilayer, facilitating the collision among the heads.