Negatively stained electron micrographs of red cell membrane skeletons and spectrin. (A) A membrane skeleton that was stretched during preparation. Note the pseudohexagonal organization of the skeleton and the location of various proteins. Inset: example of a 37-nm long F-actin protofilament. Sp4, spectrin tetramer; Sp6, spectrin hexamer; 2 Sp4, double spectrin tetramers. (B) Skeleton in situ in a red cell prepared by a minimally perturbing quick-freeze, deep-etch, rotary replication procedure. The dense network of filaments averages 29 ± 9 nm between intersections. (C-D) Spectrin from partially stretched skeletons. Some of the spectrin molecules show a helical periodic substructure, as noted by the region between the arrowheads in panel C or by the single arrowhead in panel D. (E) Right-handed double-helical models of spectrin periodicity obtained from the experiments in panel D by visual filtering of the periodic regions from multiple spectrin molecules. In this model, the springlike spectrins extend and contract by varying their pitch and diameter. Native spectrin tetramer has ∼10 turns, with a pitch of ∼7 nm (70 Å) and a diameter of ∼5.9 nm (59 Å). Panel A reprinted from Liu et al²⁷ ; panel A inset from Byers and Branton⁶⁵ ; panel B from Ursitti et al⁶⁶ ; panel C from Ursitti and Wade³⁰ ; and panels D and E from McGough and Josephs,²⁸  all with permission.