The periodic association of MAP2 with brain microtubules in vitro.

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Abstract
Several high MW polypeptides quantitatively copurify with brain tubulin during cycles of in vitro assembly-disassembly. These microtubule-associated proteins (MAP) influence the rate and extent of microtubule assembly in vitro. A heat-stable fraction highly enriched for 1 of the MAP, MAP2 (MW .apprx. 300,000 daltons), devoid of MAP1 (MW .apprx. 350,000 daltons), was purified from calf neurotubules. This MAP2 fraction stoichiometrically promotes microtubule assembly, lowering the critical concentration for tubulin assembly to 0.05 mg/ml. Microtubules saturated with MAP2 contain MAP2 and tubulin in a molar ratio of .apprx. mol of MAP2 to 9 mol of tubulin dimer. Thin sections viewed by EM of the MAP2-saturated microtubules fixed in the presence of tannic acid demonstrates a striking axial periodicity of 32 .+-. 8 nm.