APPROACHING MICROTUBULE STRUCTURE WITH THE SCANNING TUNNELING MICROSCOPE (STM)

We demonstrate that the scanning tunneling microscope can be used to o btain information about arrangement of tubulin subunits in the microtu bule wall. Long rows of subunits with a periodicity of 3.8+/-0.4 nm we re clearly visible in the images of microtubules, The separation betwe en the rows of subunits was 4.8+/-0.4 nm. Close inspection of two imag es revealed another periodicity of 7.8+/-0.4 nm in the contour levels of the protofilaments. This indicates that alpha and beta tubulin mono mers can be resolved, In these areas the monomers were arranged accord ing to a 'B-type' lattice. Scanning tunneling microscope images confir m that the lateral contacts between tubulin monomers in adjacent proto filaments are compatible with a three-start, left-handed helix model. This study demonstrates that scanning tunneling microscopy can give di rect information on the structure and organization of macromolecular a ssemblies and can complement the classical methods of electron microsc opy and X-ray scattering.