title: Structural dynamics within mixed populations of microtubules and protofilament ribbons.
abstract: In the presence of glycerol, microtubule proteins reassemble into both microtubules and protofilament ribbons with C- and S-shaped cross-section profiles. By means of electron micrographs of cross-sectioned assemblies, we have demonstrated that, during the steady state, the percentage of ribbons, especially of C-shaped ones, decreases in favour of the formation of microtubules. The following conversion modes are discussed: A, closure of the protofilament wall by increasing its curvature; B, lateral association of C-ribbons; C, completion of C-ribbons to microtubules by lateral association of tubulin; D, disassembly of ribbons and elongation of microtubules. We conclude that ribbon disassembly proceeding in an end-wise fashion and microtubule elongation is the favoured mode of conversion. Microtubule-associated proteins were found to be required for the steady-state conversions of ribbons into microtubules. In the absence of microtubule-associated proteins, C-ribbons associate laterally, forming S-ribbons. It is shown that the protofilaments of the counter-curved parts of S-ribbons have the same polarity.
mesh_terms: D000470:Alkaloids / Q000494:pharmacology; D000818:Animals; D005990:Glycerol; D008854:Microscopy, Electron; D008870:Microtubules / Q000502:physiology* / Q000648:ultrastructure; D009024:Morphogenesis; D017239:Paclitaxel; D011485:Protein Binding; D013552:Swine; D014404:Tubulin / Q000502:physiology*
chemical_list: D000470:Alkaloids; D014404:Tubulin; D017239:Paclitaxel; D005990:Glycerol