[Structure and function of mammalian brain microtubule-associated proteins]

Cytoplasmic microtubules are fibrous intracellular organelles found in almost all eukaryotic cells and play an important role in maintenance of cell shape, cell division, axonal transport, secretion and receptor activity. Besides tubulin dimers, microtubule proteins consist of several other components called MAPs which promote microtubule assembly and form long filamentous projection on the surface of the polymer. In mammalian brain, two classes of MAPs have been characterized; one is structural MAPs including MAP1 (1A and 1B), MAP2 (2A, 2B and 2C) and tau which function in the morphogenesis and maintenance of neural tissues and cells, and the other contains motor MAPs (kinesin and MAP1C) which are related to translocation of vesicles along microtubules in axon and to mitosis. The primary sequences of MAPs have been determined from their cDNAs. The functions of structural MAPs are modulated by their binding to other intracellular components, different expressions of isoforms during brain development and phosphorylation-dephosphorylation by various protein kinases and phosphatases. Biochemical characterization of MAP2 and tau have been well investigated. However, little is known about the function of MAP1 under the biochemical level, because MAP1 is unstable and high sensitive to proteases. We have developed a simple and rapid purification procedure for MAP1 using poly (L-aspartic acid) and taxol, and observed MAP1-F-actin interaction as well as MAP1-microtubules interaction. Recently, we have found that three specific kinases which can phosphorylate MAP1A and 1B are associated with MAP1 preparation and called it MAP1 kinase. Some evidence suggest that one of them is an unknown kinase and others are casein kinase I- and II-like kinases. Further studies to examine MAP1 kinase and phosphorylation of MAP1 provide a valuable insight for understanding thoroughly the microtubule-mediated functions.