The effects of dimethyl sulfoxide on the kinetics of tubulin assembly.

The kinetics of tubulin assembly were examined in the absence and presence of dimethyl sulfoxide at 37 degrees C. Inclusion of 1.4 M (10%) dimethyl sulfoxide lowered the critical protein concentration about 8-10-fold, from 9.4 microM in the absence of the organic solvent to 1.1 microM in its presence. This decrease was due solely to an effect on k-, the off rate constant. The on rate constant k+, was essentially unaffected. Another effect of dimethyl sulfoxide was in the nucleation process. The pseudo-first-order rate constant of elongation, kapp (k+[m]), was greatly increased by inclusion of dimethyl sulfoxide. This was due to an increase in the microtubule number concentration, [m]. The microtubules formed in the presence of dimethyl sulfoxide were much shorter than those formed in its absence, accounting for the higher number concentration. The nucleation number, n, was calculated by plots of ln kapp vs. ln c0 or ln t10% vs. ln c0, and the value appeared to be about 4 to 5, although some variability was found. It was shown that a plot of kapp vs. c0 to determine n, is not appropriate because of the inability to distinguish between linear and curved plots in the range of tubulin concentration used in assembly studies.