A new model for microtubule-associated protein (MAP)-induced microtubule assembly - The Pro-rich region of MAP4 promotes nucleation of microtubule assembly in vitro

The microtubule-binding domains of microtubule-associated protein (MAP) 2, tau, and MAP4 are divided into three distinctive regions: the Pro-rich regi on, the AP sequence region and the tail region (Aizawa, H., Emori, Y., Muro fushi, H., Kawasaki, H., Sakai., H., and Suzuki, K. (1990) J. Biol. Chem. 2 65, 13849-13855). Electron microscopic observation showed that the taxol-st abilized microtubules alone and those mixed with the A4T fragment (containi ng the AP sequence region and the tail region) had a long, wavy appearance, while those mixed with the PA4T fragment (containing the Pro-rich region, the AP sequence region, and the tail region) or the PA4 fragment (containin g the Pro-rich region and the AP sequence region) were shorter and straight er. Stoichiometries of the binding between the fragments and the tubulin di mers were approximately between 1 and 2, suggesting that not all of the AP sequences in the AP sequence region bound to tubulin. Binding affinity of t he PA4T fragment is only four times higher than that of the A4T fragment, w hile the microtubule nucleating activity of the PA4T fragment is far greate r. Based on these results, we propose that the nucleation of microtubule as sembly is promoted by the bridging activity of the Pro-rich region in the M APs.