Location of α-tubulin mutations in this study. (A) Two α-β dimers longitudinally aligned in a protofilament show the location of yeast α-tubulin mutations relative to the interdimer interface. The structure shown is bovine tubulin (Nogales et al., 1998b) with amino acid side chains drawn that correspond to those identical residues mutated in the following S. cerevisiae alleles: TUB1-820 (E156A, E157A) and tub1-827 (R244A, D246A) shown in orange, and TUB1-828 (D252A, E255A) (Richards et al., 2000), TUB1-D252A, TUB1-E255A, and TUB3-E255A shown in red. Structure is oriented with the plus end up and the outside face of the microtubule to the right. Guanine nucleotides are shown in yellow. Structure drawn with Molscript (Kraulis, 1991). (B) Alignment of α-tubulin, β-tubulin, and FtsZ sequences in the region of longitudinal interaction. Residues shared by the tubulins and FtsZ are shaded. Residues mutated in the yeast tubulin alleles described in A are orange or red. The residue mutated in the GTPase-defective allele ftsZ2 (D212G) is green (Dai et al., 1994). Amino acid sequences (Swiss-Prot ID: TUBA1_YEAST, TUBA3_YEAST, TBA_PIG, TBB_YEAST, TBB_PIG, FTSZ_ECOLI) were aligned with Clustal W (Thompson et al., 1994). Because bovine tubulin sequence is unknown, porcine tubulin sequence is applied to the bovine crystal structure (Nogales et al., 1998b).