Microtubule-associated protein 1B: identification of a major component of the neuronal cytoskeleton.

The major nontubulin proteins in purified brain microtubules are high molecular weight species traditionally classified into two groups known as microtubule-associated proteins 1 and 2 (MAP 1 and MAP 2). In an earlier study, we found that MAP 1 consisted of a complex of polypeptides and we characterized the highest molecular weight species--MAP 1A--with the use of a monoclonal antibody. In the current report, we describe four monoclonal antibodies raised against electrophoretically purified MAP 1B. All of the antibodies reacted exclusively with this protein. Together with peptide mapping, these results indicated that MAP 1B was structurally distinct from the other MAPs. Another distinctive property of MAP 1B was that most of it remained soluble during microtubule polymerization, resulting in an extreme underestimate of its abundance in the brain. Immunofluorescence microscopy of rat brain sections and cultured rat brain cells indicated that compared to MAP 1A and MAP 2, MAP 1B was particularly prominent in axonal as well as dendritic processes. Together, these data indicate that MAP 1B is a major, previously undescribed component of the neuronal cytoskeleton.