Structural changes within mixed populations of microtubules and protofilament ribbons caused by dilution and cold incubation.

By means of electron microscopy we have investigated the influence of dilution and cold incubation (0 degree C) on mixed populations of tubulin assemblies consisting of microtubules (MTs) and protofilament ribbons with C- and S-shaped profiles formed in the presence of glycerol. Dilution results in a partial disappearance of ribbons, whereas cold incubation causes a decrease of the percentage of MTs in favour of C-ribbons, probably produced by splitting of MT ends. In the case of dilution, we assume a lower dynamic stability of ribbons compared with MTs, which was already observed during long-time incubation of mixed population (Böhm et al., Biochem. Biophys. Acta 929, 154, 1987), whereas the splitting effect in the cold should be caused by the deficiency of MT-bound microtubule-associated proteins (MAPs), which is observed in the presence of glycerol.