We examined the association of a 34-kD light chain component to the heavy chains of MAP-1 using a monoclonal antibody that specifically binds the 34-kD component and labels neuronal microtubules in a specific and saturable manner. Immunoprecipitation of MAP-1 heavy chains together with the 34-kD component by the antibody indicates that the 34-kD polypeptide forms a complex with MAP-1 heavy chains. Both major isoforms of MAP-1 heavy chains (MAP-1A and MAP-1B) were found in the immunoprecipitate. Digestion of MAP-1 with alpha-chymotrypsin and analysis of the chymotryptic peptides reveals a 120-kD fragment of the MAP-1 heavy chain that binds to microtubules and is precipitable with the 34-kD light chain antibody, suggesting that the 34-kD light chain also binds to this domain of the molecule. Since microtubules that contain the 120-kD fragment lack the long lateral projections characteristic of microtubules with intact MAP-1, the 34-kD light chains may be localized at or near the microtubule surface.

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