PROTEIN ANATOMY - C-TAIL REGION OF HUMAN TAU-PROTEIN AS A CRUCIAL STRUCTURAL ELEMENT IN ALZHEIMERS PAIRED HELICAL FILAMENT FORMATION IN-VITRO

Tau is a microtubule-associated protein in mammalian brain. In Alzheim er's disease, this protein is present in the somatodendritic compartme nt of certain nerve cells, where it forms a portion of paired helical filament, the major constituent of the neurofibrillary tangle. For cla rification of the mechanism of this formation, recombinant human tau a nd its fragments (N-terminal half, C-terminal half, and 4-repeats) exp ressed in Escherichia coli were prepared, eight peptide fragments (C-t ails 1-8) of the C-tail region were synthesized, and the conformation and capacity for aggregation essential for filamentous structure forma tion in vitro were examined. Recombinant full-length tau, the N-termin al half, 4-repeats, and the C-terminal half did not form filamentous s tructures in aqueous solution after standing at 20 degrees C. Peptides corresponding to the C-tail region of tau, C-tail 5, C-tail 7, and C- tail 8, produced the paired filament or single straight filament in ac idic solution. The rate of filament formation by each peptide was foll owed by circular dichroism, which showed the C-tails to have predomina ntly random coil structures immediately following dissolution in aqueo us solution and be gradually converted to the beta-sheet structure. Th e kinetics of aggregation were characterized by a delay period during which the solution remained clear, followed by a nucleation event whic h led to a growth phase, whose negative peak intensity at 218 nm in ci rcular dichroism increased due to filamentous structure formation. Thi s delay was eliminated by seeding supersaturated solution of preformed filaments. C-tails interacted with recombinant full-length tau to for m definite single straight filament. The C-tail region of tau is thus shown indispensable to the formation of paired helical filament and nu cleation to reduce the rate of paired helical filament formation in am yloidogenesis in vitro. These findings may provide some clarification of the pathogenesis of Alzheimer's disease.