A comparative study of the in vitro polymerization of tubulin in the presence of the microtubule-associated proteins MAP2 and tau.

At concentrations of microtubule-associated proteins MAP2 and tau inducing maximal microtubule formation, MAP2 promotes higher rates and higher levels of microtubule nucleation that tau. Microtubules polymerized to steady state by tau show a 2-fold higher rate of tubulin loss compared to microtubules polymerized by MAP2. Microtubules polymerized in the presence of both MAP2 and tau contain lower levels of MAP2 (0.06 mol/mol of tubulin) and tau (0.07 mol/mol of tubulin) than microtubules polymerized in the exclusive presence of MAP2 (0.2 mol/mol of tubulin) or tau (0.25 mol/mol of tubulin). Addition of tau to microtubules polymerized by MAP2 results in incorporation of tau into microtubules (0.047 mol/mol of tubulin) and loss of MAP2 (0.1 mol/mol of tubulin). Similarly, incubation of microtubules polymerized by tau in the presence of MAP2 results in microtubule incorporation of MAP2 (0.1 mol/mol of tubulin) and loss of tau (0.08 mol/mol of tubulin). Microtubules and their ribbon precursor contain comparable levels of MAP2. Cross-sectional views of microtubules show no difference in the number of protofilaments (13 to 15) forming the MAP2 and tau microtubules. Fingerprints of cysteine-labeled tryptic peptides of two tau polypeptides (tau 4 and tau 6) are very similar and differ totally from similar fingerprints of MAP2.