GroEL is a bacterial chaperonin of 14 identical subunits required to help fold newly synthesized proteins. The crystal structure of GroEL with ATPγS bound to each subunit shows that ATP binds to a novel pocket, whose primary sequence is highly conserved among chaperonins. Interaction of Mg²⁺ and ATP involves phosphate oxygens of the α-, β- and γ-phosphates, which is unique for known structures of nucleotide-binding proteins. Although bound ATP induces modest conformational shifts in the equatorial domain, the stereochemistry that functionally coordinates GroEL's affinity for nucleotides, polypeptide, and GroES remains uncertain.