Role of Cysteine-291 and Cysteine-322 in the polymerization of human tau into Alzheimer-like filaments

Filamentous tau pathology is central to a large number of dementing disorde rs, including Alzheimer's disease in which polymerized tau is hyperphosphor ylated. Previous studies on heparin-dependent tau polymerization, using rec ombinant tan isoforms lacking Cys-291, suggest that tau dimerization via Cy s-322 is critical for initiation of assembly of soluble tau into filaments, We report heparin-dependent in vitro polymerization of human recombinant t au (1-383 isoform), containing both Cys-291 and Cys-322, into paired helica l filaments as characterized by electron microscopy, Tau polymerization, un der physiological tau concentrations in the presence of dithiothreitol (DTT ), was followed by a Thioflavine S fluorescence assay. To understand the mo lecular basis for heparin-induced tau polymerization, we expressed and puri fied C291A, C322A, and C291A/C322A tau mutants. The DTT requirement for tau polymerization was abolished using either the C291A or C322A tau mutant an d polymerization was not observed with the C291A/C322A tau double mutant. A nalysis by sodium dodecyl sulfate gel electrophoresis showed that, unlike w ild type tau, a significant amount of the C291A mutant and the C322A mutant is present as a disulfide bonded dimer, Taken together these results sugge st that, in isoforms containing both Cys-291 and Cys-322, a dimeric tau wit h an intermolecular disulfide bond through either Cys291 or Cys-322 is pres umably acting as a seed for initiation of tau polymerization, (C) 2001 Acad emic Press.