On the structure of microtubules, tau, and paired helical filaments.

Microtubules and their associated proteins form the basis of axonal transport; they are degraded during the neuronal degeneration in Alzheimer's disease. This article surveys recent results on the structure of microtubules, tau protein, and PHFs. Microtubules have been investigated by electron microscopy and image processing after labeling them with the head domain of the motor protein kinesin. This reveals the arrangement of tubulin subunits in microtubules and the shape of the tubulin-motor complex. Tau protein was studied by electron microscopy, solution X-ray scattering, and spectroscopic methods. It appears as an elongated molecule (about 35 nm) without recognizable secondary structure. Alzheimer PHFs were examined by FTIR and X-ray diffraction; they, too, show evidence for secondary structure such as beta sheets.

Structure, microtubule interactions, and phosphorylation of tau protein.

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.

Structural studies of tau protein and Alzheimer paired helical filaments show no evidence for beta-structure.

We have studied the conformation of tau protein and Alzheimer paired helical filaments (PHF) by several spectroscopic, scattering, and imaging methods revealing the overall shape and the conformation of the polypeptide backbone. Tau protein behaves in solution as if it were denatured; no evidence for compact folding was detected. The protein is highly extended, there is no defined radius of gyration, and the scattering is best described by that of a random ("Gaussian") polymer. CD and Fourier transform infrared spectroscopy show only a minimal content of ordered secondary structure (alpha-helix or beta-sheet). Similarly, PHFs from Alzheimer brain tissue show no detectable secondary structure by x-ray diffraction or spectroscopy. It is thus unlikely that the aggregation of tau into Alzheimer PHFs is based on interactions between strands of beta-sheets (a model currently favored for other disease-related polymers such as beta-amyloid fibers of Alzheimer's disease).

Oxidation of cysteine-322 in the repeat domain of microtubule-associated protein tau controls the in vitro assembly of paired helical filaments.

One of the hallmarks of Alzheimer disease is the pathological aggregation of tau protein into paired helical filaments (PHFs) and neurofibrillary tangles. Here we describe the in vitro assembly of recombinant tau protein and constructs derived from it into PHFs. Though whole tau assembled poorly, constructs containing three internal repeats (corresponding to the fetal tau isoform) formed PHFs reproducibly. This ability depended on intermolecular disulfide bridges formed by the single Cys-322. Blocking the SH group, mutating Cys for Ala, or keeping tau in a reducing environment all inhibited assembly. With constructs derived from four-repeat tau (having the additional repeat no. 2 and a second Cys-291), PHF assembly was blocked because Cys-291 and Cys-322 interact within the molecule. PHF assembly was enabled again by mutating Cys-291 for Ala. The synthetic PHFs bound the dye thioflavin S used in Alzheimer disease diagnostics. The data imply that the redox potential in the neuron is crucial for PHF assembly, independently or in addition to pathological phosphorylation reactions.

Biophysical characterization of the c-Myb DNA-binding domain.

We have examined proteins containing the DNA-binding domain of c-Myb with biophysical methods. This DNA-binding domain consists of three imperfect repeats (R1, R2, and R3) conserved among many species. Our results indicate that the DNA-binding domain forms unspecific and specific complexes with oligodeoxynucleotides. In the presence of R1, DNA sequences related to a canonical c-Myb-binding site are better discriminated. Furthermore, although R2 and R3 are sufficient for sequence-specific DNA binding, a structural change of the DNA-binding domain upon specific complex formation is induced only when R1 is present. Therefore, R1 might serve as an important element required for secondary structure alteration upon binding and its stabilization as well as for better discrimination between specific and related DNA sequences.