INTERPRETING A MEDIUM-RESOLUTION MODEL OF TUBULIN - COMPARISON OF ZINC-SHEET AND MICROTUBULE STRUCTURE

We previously used electron crystallography of zinc-induced two-dimens ional crystalline sheets of tubulin to construct a medium-resolution t hree dimensional (3-D) reconstruction (at 6.5 Angstrom) of this protei n. Here we present an improved model, and extend the interpretation to correlate it to microtubule structure. Secondary sequence predictions and projection density maps of subtilisin-cleaved tubulin provide inf ormation on the location of the C-terminal portion, which has been sug gested to be involved in the binding of microtubule-associated protein s. The zinc-sheet tubulin model is compared to microtubules in two way s; comparison of electron diffraction from the zinc-sheets to electron diffraction from microtubules, and by docking the zinc-sheet protofil ament 3-D model into a helical reconstruction from ice-embedded microt ubules. By correlating the zinc-sheet protofilament to a reconstructio n of axonemal protofilaments, we assigned polarity to the protofilamen t in our model. The polarity assignment, together with our model for d imer boundaries and the assignment of alpha- and beta-monomers in our reconstruction, provides a microtubule model where the alpha-monomer c rowns the plus- (or fast-growing) end of the microtubule and contact i s made in the centrosome with gamma-tubulin via the beta-monomer. (C) 1996 Academic Press Limited