Structural dynamics within mixed populations of microtubules and protofilament ribbons

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.