The three-dimensional structure of porcine brain tubulin in planar sheets formed in the presence of zinc has been determined to a resolution of approximately 20 Å by electron microscopy and image reconstruction on negatively stained samples. The samples were prepared with a mica floatation technique, which yields tubulin sheets with 36 reciprocal space maxima on lattice lines at 21, 28, 42 and 84 Å-1in Fourier transforms of digitized images. In order to obtain three-dimensional data, sheets were tilted with the goniometer stage of the electron microscope to provide images at various angles between 0 ° and ± 60 °. Transforms of 33 tilted images plus the transform of untilted sheets based on an average of nine untilted images were combined to give the third dimension of reciprocal space (z*). These data, were expressed in terms of the phases and amplitudes along the z*lattice line for each of the 36 maxima observed in untilted samples, as well as five additional lattice lines which have zero-amplitudes in the non-tilted central section of the three-dimensional transform. Home of these zero-amplitudes arise from systematic absences which are due to a 2-fold screw axis relating adjacent protofilaments of tubulin in the zinc-induced sheets. Thus in the three-dimensional reconstructions of the sheets a polarity of the protofilaments is apparent, with adjacent protofilaments aligned in opposite directions to give an antiparallel pattern, in contrast to normal microtubules composed of protofilaments in parallel alignment. Two classes of morphological units, each with a mass corresponding to a molecular weight of about 55,000, are found to alternate along the protofilaments. These distinct morphological units are identified as the α and β subunits of tubulin, confirming the representation of tubulin as an αβ heterodimer. Furthermore, the extensive internal contact between subunits within a dimer can readily be distinguished from the less extensive contact between dimer units. Such differences in contacts were not apparent in the earlier two-dimensional reconstructions. In addition, areas of excluded stain joining one class of subunits to the subunits of the other class in adjacent protofilaments have been resolved for tubulin polymerized in zinc-induced sheets. Of the two classes of subunits one is distinguished by a prominent cleft. Identification of which class of subunits is α and which is β is not yet possible. © 1979.
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