One of the hallmarks of Alzheimer disease is the pathological aggregation of 7 protein into paired helical filaments (PHFs) Several diseases are characterized by the pathological aggregation of a protein into fibers. This includes Alzheimer disease (AD), an age-related dementia. Brains of AD patients contain two types of deposits, the amyloid plaques and the neurofibrillary aggregates [tangles, paired helical filaments (PHFs)]. The former consists mainly of the proteins A,B (the derivative of a membrane protein, APP); the latter consists of the microtubule-associated protein T (reviewed in refs. 1 and 2). Analyzing the disease requires an understanding of the aggregation processes of these proteins. In the case of amyloid fibers the A,B peptide self-associates by forming intermolecular interactions between ,3-pleated sheets. The fibers show a typical cross-,B structure (3, 4), and they can be stained with dyes that are thought to interact preferentially with (3-stranded proteins, such as Congo red or thioflavin S (5, 6).The second hallmark of AD is the neurofibrillary tangles (NFTs) that consist of PHFs; they also stain with thioflavin S and (to a lesser extent) Congo red, thus suggesting a 1B substructure (6-8). In a previous study it was therefore unexpected to find that neither the fibers nor the subunit protein, T, reveals secondary structure but rather behaves like a random-coil denatured protein (9). The observations suggest that the aggregation of X-is based on some principle other than ,3-strand interactions. In addition to aggregation, T protein in AD is also extensively phosphorylated, especially at Ser-Pro motifs (review, ref. 10), and it has been suspected that this modification was related to the assembly into PHFs. Thus another unexpected finding was that PHFs can be reassembled in vitro from fragments of T protein that lacked most of the phosphorylation sites and aggregated independently of phosphorylation (11). These studies also showed that the "repeat" domain of Tcan form antiparallel dimers and that synthetic PHFs assemble from dimers that are chemically crosslinked with N,N'-phenylenedimaleimide (PDM) via sulfhydryls, implying that these links can be important in generating AD PHFs.With these observations in mind, we initiated a study on the role of i-s sulfhydryl groups in PHF assembly and show here that the formation of synthetic PHFs depends on the intermolecular crossbridging of Cys-322. The results support the notion that an increase in the redox potential of aging neurons is responsible for the aggregation of PHFs, independently or in addition to the disequilibrium between protein kinases and phosphatases that leads to the concomitant hyperphosphorylation. Thus, the increase in redox potential and an excess protein oxidation found in aging brain (12) would provide a link for the aggregation of both types of AD fibers since the amyloid fibers also appear to be related to increased oxidation (13,14).

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This paper summarizes recent structural and functional studies on tau protein, its interactions with microtubules, its self-assembly into paired helical filaments (PHF)-like fibers, and its modification by phosphorylation. The structure of tau in solution resembles that of a random coil. Both tau and Alzheimer PHFs have very little secondary structure, making it improbable that the assembly of tau into PHFs is based on interacting beta sheets. Tau's binding to microtubules can be described by a "jaws" effect. The domain containing the repeats binds very weakly, while the flanking regions (jaws) bind strongly, even without the repeats. However, only the combination of flanking regions and repeats makes binding productive in terms of microtubule nucleation and assembly. Although the majority of Alzheimer-like phosphorylation sites are outside the repeats they have only a weak influence on binding, whereas the phosphorylation at Ser262 inside the repeats inhibits binding and makes microtubules dynamically unstable. This site can be phosphorylated by kinases present in brain tissue, and it is uniquely phosphorylated in Alzheimer brain.

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