MAP-2 Polymerization into Alzheimer-like Filaments

While the microtubule-associated protein Tau is probably tile main component of paired helical filaments (PHFs) comprising Alzheimer neurofibrillary tangles (NFT), recent work indicates similar filament structures can assemble directly from MAP-2 polypeptides, starting with microtubule-binding region (MTBR) fragments or full length MAP-2c [DeTure, M. A., Zhang, E. Y., Bubb, M. R., and Purich, D. L., (1996) J. Biol. (:hem., 271, 32702-32706]. As with Tau polymerization, dimerization preceded filament assembly, and MAP2 polymers bind thioflavin-S, a NFT-staining histochemical dye used to confirm Alzheimer's Disease (AD). Because these unprecedented findings raise fundamental questions about MAP-2's role in AD pathobiology, we were motivated to characterize MAP-2 polymerization in greater detail. We now report (a) that solution conditions have been optimized such that MAP-2 polymerization can now occur at protein concentrations typically used for Tan polymerization; (b) that the required concentration for MAP-2 polymerization falls in a physiologically relevant range; (c) that MAP-2 MTBR fragments can elongate from the ends of intact PHFs isolated from AD autopsy brain; and (d) that the roles of the thiol cross-linking as well as acidic/basic residues in the socalled second repeated sequence can be evaluated by sitedirected mutagenesis. In particular, the seeded assembly findings indicate that MAP-2 may play a role in modulating the polymerization of Tau into PttFs. especially when one considers that NFT formation occurs in the neuronal (:ell bodies and dendrites where the concentration of MAP-2 greatly exceeds that of tau.