Interaction between S-100 proteins and steady-state and taxol-stabilized microtubules in vitro

S-100 proteins are a group of three 21-kilodalton, acidic, Ca2+-binding proteins of the "E-F hand" type shown to regulate several cell activities, including microtubule (MT) assembly-disassembly. We show here that S-100 proteins interact with MTs assembled from either whole microtubule protein or purified tubulin, both in the absence and in the presence of the MT-stabilizing drug taxol. Evidence for the binding of S-100 to MTs comes from both kinetic (turbidimetric) and binding studies. Kinetically, S-100 enhances the disassembly of steady-state MTs in the presence of high concentrations of colchicine or vinblastine at 10 microM free Ca2+ and disassembles taxol-stabilized MTs at high Ca2+ concentrations. Experiments performed using 125I-labeled S-100 show that S-100 binds Ca2+ independently to a single set of sites on taxol-stabilized MTs assembled from pure tubulin with an affinity of 6 x 10(-5) M and a stoichiometry of 0.15 mol of S-100/mol of polymerized tubulin. Under certain conditions, S-100 proteins also cosediment with MTs prepared by coassembly of S-100 with MTs, probably in the form of an S-100-tubulin complex. Because S-100 binds to MTs under conditions where this protein fraction does not produce observable effects on the kinetics of assembly-disassembly, e.g., in the absence of Ca2+ at pH 6.7, we conclude that the S-100 binding to MTs does not affect the stability of MTs per se, but rather creates conditions for increased sensitivity of MTs to Ca2+.