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.