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The cylindrical chaperonin GroEL of  E. coli  and its ring-shaped cofactor GroES cooperate in mediating the ATP-dependent folding of a wide range of polypeptides  in vivo  and  in vitro . By binding to the ends of the GroEL cylinder, GroES displaces GroEL-bound polypeptide into an enclosed folding cage, thereby preventing protein aggregation during folding. The dynamic interaction of GroEL and GroES is regulated by the GroEL ATPase and involves the formation of asymmetrical GroEL:GroES 1  and symmetrical GroEL: GroES 2  complexes. The proposed role of the symmetrical complex as a catalytic intermediate of the chaperonin mechanism has been controversial. It has also been suggested that the formation of GroEL:GroES 2  complexes allows the folding of two polypeptide molecules per GroEL reaction cycle, one in each ring of GroEL. By making use of a procedure to stabilize chaperonin complexes by rapid crosslinking for subsequent analysis by native PAGE, we have quantified the occurrence of GroEL:GroES 1  and GroEL:Groesr 2  complexes in active refolding reactions under a variety of conditions using mitochondrial malate dehydrogenase (mMDH) as a substrate. Our results show that the symmetrical complexes are neither required for chaperonin function nor does their presence significantly increase the rate of mMDH refolding. In contrast, chaperonin-assisted folding is strictly dependent on the formation of asymmetrical GroEL:Groes 1  complexes. These findings support the view that GroEL:Groes 2  complexes have no essential role in the chaperonin mechanism.

On the Role of Symmetrical and Asymmetrical Chaperonin Complexes in Assisted Protein Folding