The influence of microtubule associated proteins on the production of polymorphic products from tubulin and microtubules.

Crude preparations of microtubule-associated proteins (MAPs), as well as purified MAP 2, influence the structure of products assembled from purified tubulin at low pH values. At pH 6.2, only 12% of the assembled products were microtubules (MTs) when assembly was conducted in 10% DMSO; 88% were large sheets of protofilaments. In the absence of DMSO, 28% of the structures were MTs. As the content of MAPs in the assembly reaction was increased, the proportion of MTs increased to 87% at a MAP/tubulin (w/w) ratio of 0.67 in the presence of DMSO and to 98% at a MAP/tubulin (w/w) ratio of 0.33 in the absence of DMSO. Purified MAP 2 was as effective as crude MAP preparations in promoting MT formation at pH 6.2. MTs formed from purified tubulin and MAP 2 were transformed into spirals of protofilaments upon the addition of Vinblastine (VLB). Spirals were also formed when VLB was added to a mixture of tubulin and MAP 2 at 4 degrees C. It thus appears that MAP 2 is a causative factor in initiating spiral formation in the presence of VLB.