Microtubule-associated protein 1B: Molecular structure, localization, and phosphorylation-dependent expression in developing neurons

Two monoclonal antibodies, SE6 and 1B6, were raised against microtubule-associated protein 1B (MAP1B), a major component of the neuronal cytoskeleton. 5E6 recognized the entire MAPIB population, while 1B6 detected only phosphorylated forms. Affinity-purified MAP1B appeared as a long, filamentous molecule (186 ± 38 nm) with a small spherical portion at one end, forming long cross-bridges between microtubules in vitro. These results, together with in vivo data from immunogold methods, demonstrate that MAP1B is a component of cross-bridges between microtubules in neurons. By immunohistochemical analysis, phosphorylated forms were shown to exist mainly in axons, whereas unphosphorylated forms were limited to cell bodies and dendrites. Phosphorylated MAP1B was quite abundant in developing axons, suggesting its essential role in axonal elongation.