Structure of the tubulin dimer in zinc-induced sheets.

The structure of tubulin has been studied in projection by minimum beam electron microscopy and image processing of negatively stained zinc-induoed sheets. The reconstructed images include data to 15 A resolution. We report here a clear and reproducible 82 if repeat arising from the arrangement of heterodimers in sheet aggregates of tubulin. This repeat is only observed in diffraction patterns from images recorded by minimum beam methods (10 to 20 e/AZ) and arises from small, but consistent, structural differences between two similar subunits believed to represent the two chemical species of tubulin monomer (M,, 55,000). At higher electron doses (100 to 200 e/AZ), the additional information is lost or very much reduced, and only a repeat of 41 A is observed, owing to the loss of distinction between monomers in the tubulin heterodimer. The sheets are composed of 49 A wide, polar protolllaments, similar to those observed in microtubules; however, the interprotofilament packing is completely different in the two structures. In these sheets, adjacent protofilaments point and face in opposite directions; i.e. they are related by dyad-screw axes normal to the protofilament axes and in the plane of the sheet. Thus, the zinoinduced sheets are orystals of space group P2,, with cell dimensions of about 97 A x 82 4, containing one tubulin heterodimer per asymmetric unit. Reconstructed images of four individual sheets, and their average, show the arrangement and shapes of the two heterodimers contained in each unit cell. The structure and packing of heterodimers in sheets are compared to those in opened out microtubules where all protofilaments point and face in the same direction.