The small myelin-associated glycoprotein binds to tubulin and microtubules

The myelin-associated glycoprotein (MAG) exists as two isoforms, differing only by their respective cytoplasmic domains, that have been suggested to f unction in the formation and maintenance of myelin. In the present study, a 50 kDa protein binding directly to the small MAG (S-MAG) cytoplasmic domai n was detected and identified as tubulin, the core component of: the microt ubular cytoskeleton. In vitro, the S-MAG cytoplasmic domain slowed the poly merization rate of tubulin and co-purified with assembled microtubules. A s ignificant sequence homology was found between the tau family tubulin-bindi ng repeats and the carboxy-terminus of S-MAG. Our results indicate that S-M AG is the first member of the Ig superfamily that can be classified as a mi crotubule-associated protein, and place S-MAG in a dynamic structural compl ex that could participate in linking the axonal surface and the myelinating Schwann cell cytoskeleton. (C) 2001 Elsevier Science B.V. All rights reser ved.