Bundling of microtubules in transfected cells does not involve an autonomous dimerization site on the MAP2 molecule

We have searched for putative dimerization sites in microtubule-associated protein 2 (MAP2) that may be involved in the bundling of microtubules. An overlapping series of fragments of the embryonic form MAP2c were created and immunologically "tagged" with an 11 amino acid sequence from human c-myc. Nonneuronal cells were transfected simultaneously with one of these myc-tagged fragments and with full-length native MAP2c. Immunolabeling with site-specific antibodies allowed the two transgene products to be located independently within the cytoplasm of a single double-transfected cell. All transfected cells contained bundled microtubules to which the full-length native MAP2 was bound. The distribution of the tagged MAP2 fragment relative to these MAP2-induced bundles was determined by the anti-myc staining. None of the fragments tested, representing all of the MAP2c sequence in overlapping pieces, were associated with MAP2-induced microtubule bundles. These results suggest that MAP2-induced bundle formation in cells does not involve an autonomous dimerization site within the MAP2 sequence.