The low molecular weight form of microtubule-associated protein 2 is transported into both axons and dendrites

In the developing brain microtubule-associated protein MAP2 occurs as both a high molecular weight form, MAP2b, which is present only in dendrites, and a low molecular weight form, MAP2c, which is also present in axons. Because the MAP2c amino acid sequence is entirely contained within that of MAP2b it is not possible to raise a MAP2c-specific antibody, so that it has been impossible to determine whether MAP2c is present in dendrites along with MAP2b. To answer this question we have generated a MAP2c cDNA clone tagged with a 10 amino acid epitope from human c-myc. This additional sequence does not alter either the binding of MAP2c to microtubules or its effects on microtubules in non-neuronal cells. When expressed in cultured primary neurons by transfection, the myc tag allowed the distribution of MAP2c to be determined independently of endogenous MAP2 protein by immunostaining with an anti-myc antibody. This showed that MAP2c is present in all processes, indicating that it can enter all kinds of processes and is stable in their cytoplasm. The results further suggest that the selective association of high molecular weight MAP2 with dendrites depends on a mechanism that prevents either its entrance or survival in the axonal compartment.