The exchangeable GTP-binding site on beta-tubulin has been extensively studied, but the primary sequence elements which form the binding site on beta-tubulin remain unknown. We have used site-directed mutagenesis of the single beta-tubulin gene of Saccharomyces cerevisiae to test a model for the GTP-binding site on beta-tubulin, which was based on sequence comparisons with members of the GTPase superfamily [Sternlicht, H., Yaffe, M.B., & Farr, G. W. (1987) FEBS Lett. 214, 226-235]. We analyzed the effects of D295N, N298K, and N298Q mutations in a proposed base-binding motif, 295DAKN298, on tubulin-GTP binding and on nucleotide-binding specificity. We also examined the effects of a D203S mutation in a putative phosphate-binding region, 203DNEA206, on nucleotide binding affinity, on the assembly-dependent tubulin GTPase activity in vitro, and on the dynamic properties of individual "mutant" microtubules in vitro. The effects of the mutations on cell phenotype and on microtubule polymerization in cells were also measured. The results do not support the proposal that the 203DNEA206 and 295DAKN298 [corrected] motifs are cognate to motifs found in GTPase superfamily members. Instead, the data argue that the primary sequence elements of beta-tubulins that interact with bound nucleotide, and presumably also those of the alpha- and gamma-tubulin family members, are different from those of "typical" GTPase superfamily members, such as p21ras. The GTPase superfamily should thus be broadened to include not just the typical GTPases that show strong conservation of primary sequence consensus motifs (GxxxxGK, T, DxxG, NKxD) [corrected] but also "atypical" GTPases, exemplified by the tubulins and other recently identified GTPases, that do not show the consensus motifs of typical GTPases and which also show no obvious primary sequence relationships between themselves. The tubulins and other atypical GTPases thus appear to represent convergent solutions to the GTP-binding and hydrolysis problem.