Model of the unit cell of the membrane skeleton. (A) The membrane skeleton is a quasihexagonal lattice centered around ∼40 000 F-actin protofilaments. Each 37-nm, double-helical protofilament is capped by adducin (A) and tropomodulin (T) and contains ∼14 G-actin subunits. The red cell surface area is ∼140 μm2, so each hexagonal unit cell is 3500 nm2 and the average length of a spectrin dimer is ∼32 nm, which is one-third its length when fully stretched. Available data suggest that the spectrin chains are coiled about each other and expand and contract by changing their pitch and diameter28 (Figure 5E). They are shown here as straight, although they are sometimes kinked.31 Spectrin oligomers are not shown in this model but exist in vivo. In addition, the skeleton is not as regularly arrayed in vivo as in this averaged model. (B) A recent model of the ankyrin complex (Ank Cmplx; green ovals) assembled from known structures of the proteins by Burton and Bruce7 has a cross-sectional area of 17.5 nm × 14 nm, assuming the proteins are closely packed. The cross-sectional area of the actin junctional complex is estimated to be ∼20 nm × 17 nm if it contains only a single band 3 dimer7 (Actin Cmplx 1-Bd3; small tan ovals). Such a complex would presumably lie toward the end of the actin protofilament where adducin resides. This position is shown in the 2 unit cells on the right. Alternatively, if the actin junctional complex contains 6 band 3 dimers (Actin Cmplx 6-Bd3) interacting with all 6 proteins 4.1R, it would probably be more centered and approximate the area of the large tan oval in the unit cell on the left. Recent data favor the larger arrangement,96 but in either case, it is likely that the ankyrin and actin junctional complexes would be close to each other and probably often touch, particularly during red cell deformation. Band 3 dimers that are not associated with either the ankyrin or actin junctional complexes (small blue ovals) diffuse in the lipid bilayer within the spectrin corrals. There are 3 to 8 of these per unit cell, depending on the number of band 3 dimers in the actin junctional complex. Reprinted from Lux6 with permission.
