Filament structure and interactions based on electron microscopy. (a) Diagram of the cross-section of a doublet or triplet microtubule, with the tubulin protofilaments numbered as in . Attached to the complete A-tubule are rows of outer dynein arms (ODA), inner dynein arms (IDA), radial spokes (RS), dynein regulatory complexes (DRC) and the incomplete B-tubule. The outer A-B junction is a direct interaction between two tubulin protofilaments but, at the inner junction, the so-called 11th protofilament of the B-tubule  turned out to be a row of non-tubulin crosslinks (d). In the case of a triplet microtubule, the C-tubule is probably attached in a similar way to the outside of the B-tubule. Green material on either surface of the shared partition between A- and B-tubules in (a) represents that seen in (c-f). (b) Electron microscope (EM) images of disintegrating doublet microtubules isolated from sea urchin sperm tails and contrasted with uranyl acetate negative stain (reproduced with permission from ). The A-tubule and B-tubule  can be distinguished even after the loss of accessory structures. An arrowhead indicates the loss of the B-tubule, an arrow shows where most of the A-tubule ends, leaving just the partition. After continued extraction, SDS gels of the remaining ribbons showed that the main proteins present, in addition to some tubulin, were three tektins plus two or three other bands [1, 2]. The scale bar represents 100 nm. (c-f) Images obtained by EM tomography of frozen doublet microtubules (reproduced with permission from ). Tubulin has been colored purple and all other material green. (e, f) End-on views, with the tubulin protofilaments cut through, of the side view of the A-tubule shown in (c) and the junction between the A-tubule and B-tubule shown in (d), respectively. Magenta and black circles in (e, f) denote the groups of A-tubule and B-tubule protofilaments viewed in (c, d), respectively, and the black arrows indicate the directions in which they are viewed. At the tip of the black arrow in (e) is a small hole representing the core of an axially continuous thin filament whose outer surface is seen running down the middle of (c). Projections from this filament extend across the protofilaments on either side of the thin filament. To improve the signal-to noise ratio, the 3D image was averaged in the axial direction at 16 nm intervals, so any longer periodicities have been lost. The blue arrow in (e) indicates material between protofilaments of the A-tubule that may be involved in the attachment and organization of the radial spokes and sets of inner dynein arms. (c, d) Scale bar = 10 nm. (g, h) Models of tektin dimers proposed in  (reproduced with permission from ). (g) 32 nm long tektin AB heterodimer with amino-terminal segment of tektin A that may form a sideways projection from a filament composed of heterodimers. S S indicates the position of disulfide bonds. (h) 40-48 nm long tektin C homodimer. Colored asterisks in (g, h) show the predicted positions of the nonapeptide loops that may bind strongly to tubulin. (h) Model of a 2 nm tektin AB 'core' filament, consisting of heterodimers joined end-to-end to form two strands (coloured red or cyan; they may differ slightly, as there are two isoforms of tektin A ). Colored asterisks show the predicted positions of the nonapeptide loops. Heterodimers in the two strands are shown half-staggered to explain the prominent 16 nm periodicity seen in (b). The red and cyan projections represent the amino-terminal headers of tektin A monomers (see g) in each strand. A strand made up of tektin C homodimers (yellow) is drawn alongside, although the exact relationship between tektin C dimers/tetramers and tektin AB filaments is not clear at present. A pair of 48 nm long tektin C molecules might organize a group of radial spokes (RS1, RS2 and RS3) to give an overall longitudinal repeat distance of 96 nm. The 32 nm spacing between RS1 and RS2 and the 24 nm spacing between RS2 and RS3 are indicated by double-headed arrows. (j) The same 2 nm filament as in (i) shown in cross-section at four successive positions to indicate how four individual α-helical strands (two AB dimers) might twist smoothly around each other. In this model, tektin C C homodimers (yellow circles) are shown associated with, but not integrated into, the filament (unlike the model in ), as it is hard to account for crosslinking evidence that tektin C forms tetramers but not filaments . (k) Cross-section through a possible model of an intermediate filament in which pairs of 2 nm filaments are twisted to form 4 nm filaments and four of these are bundled to form a 10 nm filament; each light-brown or dark-brown circle represents a 2 nm filament; thus, each circle here corresponds to the larger circles in (j). IFs have been proposed to be tubes built from eight 2 nm filaments  or supercoils of four 4 nm filaments, each with a pitch of 96 nm ; a cross-section through the latter at some levels might appear to be a ring of eight smaller filaments (dark brown), while slices at other levels would show 4 nm filaments arranged as a cross (light brown). Each subfilament of an IF is thought to be bipolar, whereas tektin filaments are most probably polar to match the polar tubulin protofilaments.