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alpha and beta-Tubulin fold in a series of chaperone-assisted steps. At least five protein cofactors are involved in the post-chaperonin tubulin folding pathway and required to maintain the supply of tubulin; some of them also participate in microtubule dynamics. The first tubulin chaperone identified in the tubulin folding pathway was cofactor A (CoA). Here we describe the three-dimensional structure of human CoA at 1.7 A resolution, determined by multiwavelength anomalous diffraction (MAD). The structure is a monomer with a rod-like shape and consists of a three-alpha-helix bundle, or coiled coil, with the second helix kinked by a proline break, offering a convex surface at one face of the protein. The helices are connected by short turns, one of them, between alpha2 and alpha3, including a 3(10)-helix. Peptide mapping analysis and competition experiments with peptides show that CoA interacts with beta-tubulin via the three alpha-helical regions but not with the rod-end loops. The main interaction occurs with the middle kinked alpha2 helix, at the convex face of the rod. Strong 3D structural homology is found with the Hsp70 chaperone cofactor BAG domain, suggesting that these proteins define a family of cofactors of simple compact architecture. Further structural homology is found with alpha-spectrin/alpha-actinin repeats, all are rods of identical length of ten helical turns. We propose to call these three-helix bundles alpha ten modules.