Different tubulin polymers are produced by microtubule-associated proteins MAP2 and tau in the presence of guanosine 5'-(alpha, beta-methylene)triphosphate.

Tubulin and the two microtubule-associated proteins MAP2 and tau were purified from pig brain. The ability of MAP2 and tau to induce tubulin polymerization in the presence of guanyl-5'-yl methylene diphosphonate was compared in parallel experiments. MAP2 at 0.2 mol/mol of tubulin induced the polymerization of tubulin exclusively into ribbons 0.45 +/- 0.05 micrometer long with an average of six protofilaments. At the same molar ratio, tau promoted the polymerizaton of tubulin exclusively into microtubules. These results strongly suggest that MAP2 and tau are functionally different proteins.