Tubulin conformation in zinc-induced sheets and macrotubes.

The protein tubulin is the main constituent of microtubules. Previous studies have shown that zinc ions induce the formation of crystalline sheets and macrotubes of tubulin. Both crystal types are suitable for structural studies by electron crystallography. However, crystallographic structural analysis of tubulin has been hampered by limited crystal size and quality and the inability to control crystal polymorphism. We can obtain well-ordered crystals which are grown upon prolonged incubations (up to 24 hr). The presence of NaCl delays the degradation of the crystals, and addition of the protease inhibitor pepstatin improves crystal quality. The crystal form (sheet or macrotube) can be controlled with incubation conditions. The size of the crystals can reach up to 2 microns in width for the sheets and up to 0.5 microns in diameter for the macrotubes. Both crystal types can reach several micrometers in length. Comparison of the projection maps of the two crystal structures shows that adjacent protofilaments in the macrotubes are shifted by about 6 A relative to their positions in the sheets. Observable changes of monomer shape appear to allow close interprotofilament contacts to be maintained in both crystal forms. Images of glucose-embedded specimens obtained under these conditions give structural information beyond 4 A resolution. Merging of high- and low-resolution data allows for unambiguous assignment of monomer boundaries to high-resolution features.