Increased microtubule assembly in bovine brain tubulin lacking the type III isotype of β-tubulin

Tubulin, the major constituent protein of microtubules, is a heterodimer of α and β subunits. Both α and β exist in multiple isotypic forms. It is not clear if different isotypes perform different functions. In order to approach this question, we have made a monoclonal antibody specific for the β(III) isotype of tubulin. This particular isotype is neuron-specific and appears to be phosphorylated near the C terminus. We have used immunoaffinity depletion chromatography to prepare tubulin lacking the β(III) subunit. We find that removal of the β(III) isotype results in a tubulin mixture able to assemble much more rapidly than is unfractionated tubulin when reconstituted with either of the two microtubule-associated proteins (MAPs), tau or MAP 2. Our results suggest that the different isotypes of tubulin differ from each other in their ability to polymerize into microtubules. We have also found that the anti-β(III) antibody can stimulate microtubule assembly when reconstituted with tubulin and either tau or MAP 2. When reconstituted with tubulin lacking the β(III) isotype, the antibody causes the tubulin to polymerize into a polymer that is a microtubule in the presence of MAP 2 and a ribbon in the presence of tau.