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, crystallograp hic structural analysis of tubulin has been hampered by limited crysta l size and quality and the inability to control crystal polymorphism. We can obtain well-ordered crystals which are grown upon prolonged inc ubations (up to 24 hr). The presence of NaCl delays the degradation of the crystals, and addition of the protease inhibitor pepstatin improv es crystal quality. The crystal form (sheet or macrotube) can be contr olled with incubation conditions. The size of the crystals can reach u p to 2 mu m in width for the sheets and up to 0.5 mu m in diameter for the macrotubes. Both crystal types can reach several micrometers in l ength. Comparison of the projection maps of the two crystal structures shows that adjacent protofilaments in the macrotubes are shifted by a bout 6 Angstrom relative to their positions in the sheets. Observable changes of monomer shape appear to allow close inter-protofilament con tacts to be maintained in both crystal forms. Images of glucose-embedd ed specimens obtained under these conditions give structural informati on beyond 4 Angstrom resolution. Merging of high- and low-resolution d ata allows for unambiguous assignment of monomer boundaries to high-re solution features. (C) 1993 Academic Press, Inc.