Together with the chaperonin GroEL, plays an essential role in assisting protein folding. The GroEL-GroES system forms a nano-cage that allows encapsulation of the non-native substrate proteins and provides a physical environment optimized to promote and accelerate protein folding. GroES binds to the apical surface of the GroEL ring, thereby capping the opening of the GroEL channel. Heptamer of 7 subunits arranged in a ring. Interacts with the chaperonin GroEL. By NifA. Belongs to the GroES chaperonin family.