Microtubule-associated proteins and the stimulation of tubulin assembly in vitro

Abstract

Microtubules [calf brain], purified by cycles of assembly and disassembly in vitro, are composed of tubulin and several microtubule-associated proteins (MAP). When the MAP were separated from the tubulin by phosphocellulose chromatography, the tubulin no longer assembled at 37.degree. C as measured by turbidity. If the MAP and tubulin were recombined and warmed to 37.degree. C, microtubules assembled. MAP stimulated tubulin assembly by affecting both the initiation and elongation processes. The effect on initiation was indicated by results showing an increase in initial rate and a decrease in average microtubule length as the MAP:tubulin ratio was increased. The effect on elongation was indicated by results showing that a greater mass of microtubules was assembled at the apparent equilibrium as the MAP:tubulin ratio was increased. The initiation and elongation activities of the MAP could be separated by storing the MAP at 4.degree. C during which time the initiating activity decreased while the ability to affect the total amount of assembly remained constant. The decrease in initiating ability was correlated with the loss of the 2 major components of the MAP fraction, MAP 1 and 2.