Differential association of the different brain microtubule proteins in different in vitro assembly conditions.

Microtubule protein was assembled in vitro for different time periods and at different protein concentrations near and far from the critical concentration needed for assembly. When the isolated polymers were characterized by electrophoresis, a higher association of high-molecular-weight microtubule-associated proteins, particularly of the larger microtubule-associated protein, was found in the polymers assembled close to the critical concentration, while tau factor polypeptides are predominantly present in those polymers assembled at higher protein concentration. Also, a higher proportion of high-molecular-weight microtubule associated proteins MAP1 and MAP2 was observed in microtubules assembled for short time periods, compared with those obtained after the monomer-polymer equilibrium was reached. When the assembled protein was further characterized by isoelectric focusing, no differences were found in the proportion of the different isotubulins present in the polymers assembled under different conditions tested.