The Mechanism of Equilibrium Binding of Microtubule - associated Protein 2 to Microtubules BINDING IS A MULTI - PHASIC PROCESS AND EXHIBITS POSITIVE

The mechanism of binding of microtubule-associated protein 2 (MAP2) to taxol-stabilized microtubules (MTs) was examined through Scatchard analysis of equilibrium binding and by immunoelectron microscopy. We demonstrate the following. 1) Binding is a cooperative process as indicated by sigmoidal binding curves, prominent humps in Scatchard plots, and an all-or-none response in binding during ligand titrations. At high tubulinIMAP2 ratios, the Kd for noncontiguous binding (5-25 PM) is estimated to be 100-1500 times greater than that predicted for contiguous binding, suggesting a high degree of co perativity. 2) Cooperativity is indicated independently by a highly clustered or patchy distribution of MAP2 on MTs as revealed by immunoelectron microscopy. 3) The binding of truncated constructs of mouse MAP2 protein suggests that a domain of MAP2 conferring cooperativity is located in or near the MT binding site near the carboxyl terminus. We speculate that in the cell, cooperativity may generate MTs with uniform biochemical properties and contribute to the segregation of MAPs in neuronal cell processes.