The structure of the tubulin heterodimer in zinc-induced sheets

The structure of negatively stained zinc-induced tubulin sheets has been studied in projection by minimum beam microscopy and image processing. Figure (a) shows a typical sheet image, displaying the characteristic and prominent protofilament structure. The bar represents 500 nm. Figure (b) is an enlargement of part of (a).Optical diffraction patterns (Figure c) reveal an orthogonal unit cell lattice of about 9.7 nm X 8.2 nm. The 8.2 nm repeat arises from the arrangement of heterodimers in the protofilaments, and is the first observation of such a repeat in sheet aggregates. It is only observed consistently in diffraction patterns from images recorded by minimum beam methods (1,000-2,000 e/nm2) and arises from small, but reproducible, structural differences between two similar subunits believed to represent the two chemical species of tubulin monomer (55,000 M.W.). At higher electron doses (10,000-20,000 e/nm2), the additional information is lost or very much reduced, and only a repeat of 4.1 nm is observed, owing to the loss of distinction between adjacent monomers in the protofilaments