Football- and bullet-shaped GroEL-GroES complexes coexist during the reaction cycle.

GroEL is an Escherichia coli chaperonin that is composed of two heptameric rings stacked back-to-back. GroEL assists protein folding with its cochaperonin GroES in an ATP-dependent manner in vitro and in vivo. However, it is still unclear whether GroES binds to both rings of GroEL simultaneously under physiological conditions. In this study, we monitored the GroEL-GroES interaction in the reaction cycle using fluorescence resonance energy transfer. We found that nearly equivalent amounts of symmetric GroEL-(GroES)(2) (football-shaped) complex and asymmetric GroEL-GroES (bullet-shaped) complex coexist during the functional reaction cycle. We also found that D398A, an ATP hydrolysis defective mutant of GroEL, forms a football-shaped complex with ATP bound to the two rings. Furthermore, we showed that ADP prevents the association of ATP to the trans-ring of GroEL, and as a consequence, the second GroES cannot bind to GroEL. Considering the concentrations of ADP and ATP in E. coli, ADP is expected to have a small effect on the inhibition of GroES binding to the trans-ring of GroEL in vivo. These results suggest that we should reconsider the chaperonin-mediated protein-folding mechanism that involves the football-shaped complex.