GroEL/S chaperonin ring complexes fold many unrelated proteins. To understand the basis and extent of the chaperonin substrate spectrum, we used rounds of selection and DNA shuffling to obtain GroEL/S variants that dramatically enhanced folding of a single substrate-green fluorescent protein (GFP). Changes in the substrate-optimized chaperonins increase the polarity of the folding cavity and alter the ATPase cycle. These findings reveal a surprising plasticity of GroEL/S, which can be exploited to aid folding of recombinant proteins. Our studies also reveal a conflict between specialization and generalization of chaperonins as increased GFP folding comes at the expense of the ability of GroEL/S to fold its natural substrates. This conflict and the nature of the ring structure may help explain the evolution of cellular chaperone systems.