To obtain information on the functional domains of tubulin from a dicot plant, we investigated the interactions of tobacco tubulin with MAP2 from bovine brain supernatant. Taxol-stabilized tobacco and bovine brain microtubules had similar binding capacities for MAP2 (1 mol MAP2 per 8-9 mol tubulin). However, MAP2 dissociated from tobacco microtubules more readily than from bovine brain microtubules and induced the polymerization of tobacco tubulin into aberrant helical ribbon polymers, rather than microtubules. Ribbon assembly was partially suppressed by 50 mM KCl. Abundant tobacco microtubules formed when MAP2-induced nucleation was by-passed with microtubule seeds. Thus, deficient nucleation of tobacco tubulin assembly by MAP2 reflects distinct properties of the polymerization and regulatory domains of plant and animal tubulins.