Structural changes in tubulin sheets upon removal of microtubule-associated proteins

Zinc-induced tubulin sheets without microtubule-associated proteins (MAPs) were assembled from tubulin purified by phosphocellulose chromatography. Large, open sheets were obtained in five-minute incubations at pH 5.7. Electron micrographs of negatively stained sheets showed a protofilament arrangement similar to that observed for zinc-induced sheets with MAPs but with altered lattice parameters. The spacings measured from optical diffraction patterns demonstrated that the protofilaments were 2.2 A closer together in the sheets without MAPs. Each MAP-free sheet was also divided roughly in half by a discontinuity which was parallel to the protofilaments and the relationship between the two domains was deduced from computed transforms. Two-dimensional image processing was carried out by conventional Fourier techniques and by correlation analysis. The correlation analysis improved the reconstructions in this application, with the resolution limited by the inherent properties of the negative stain method to about 14 A. A prominent feature of the computed reconstructions was an alternation of light and dark protofilaments due to differential staining, as revealed by a study of folded sheets. Neighboring protofilaments are related by a 2-fold screw axis, as they are in zinc-induced sheets with MAPs, but the symmetry is masked by the differential staining. The major effect of MAP removal on the structure of the sheets is that the bilobed structure of alternate tubulin subunits is no longer observed. This observation and the closer spacing of protofilaments is consistent with the postulate that some of the MAP molecules lie in the groove between protofilaments and bind to several tubulin dimers.