GroES and the chaperonin-assisted protein folding cycle: GroES has no affinity for nucleotides

The E. coli chaperonin proteins, GroEL and GroES, assist in folding newly synthesized proteins. GroES is necessary for GroEL-assisted folding under conditions where the substrate protein cannot spontaneously fold. On the basis of photolabelling of GroES with 8-azido-ATP, a role for nucleotide binding to GroES in chaperonin function was suggested [Martin, et al., Nature, 366 (1993) 279-282]. We confirm the photolabeling of GroES with 8-azido-ATP. However, other proteins not known to contain nucleotide binding sites also became photolabeled suggesting that labeling is non-specific. Using rigorous physical methods, isothermal calorimetry and equilibrium binding, no interaction between GroES and nucleotides could be detected. We conclude that GroES has no nucleotide binding site.