In the presence of nucleotide, a complex is formed between GroEL and GroES (Chandrasekhar et al, 1986). GroES binds tightly to one ring of GroEL after ATP hydrolysis, trapping ADP in the seven sites on that ring (Todd et al, 1994). Adenosine triphosphate binding by GroEL is cooperative, and this positive cooperativity is increased by GroES, suggesting that all seven sites in the opposite ring of GroEL act together. With each round of ATP hydrolysis, the GroES is released and rebinds. The biggest structural change in GroEL is seen in its complex with GroES. From negative stain EM studies of the complex it was immediately apparent that GroES binding was asymmetric and that it appeared to cause a significant expansion in GroEL (Saibil et al, 1991; Ishii et al, 1992; Langer et al, 1992). At that time, the asymmetry was surprising, since the two rings of GroEL are originally identical. It is now known that the nucleotides required for GroES binding create asymmetry between the two rings of GroEL (Bochkareva and Girshovich, 1994; Burston et al., 1995). It implies strong negative cooperativity of GroES binding, and accords well with the observation that the rate of chaperonin-assisted refolding of Rubisco in vitro as a function of GroES/GroEL molar ratio saturates as the GroES 7-mer concentration approaches that of the GroEL 14-mer (Azem et al, 1994), although not with the interpretation of those authors. However, it was also shown that double-ended GroES binding could take place (Harris et al, 1994; Azem et al, 1994; Schmidt et al, 1994; Todd et al, 1994), and that the "football" form was favored under certain ionic conditions (high pH and Mg2+ seem to be important). The second GroES binds more weakly (Todd et al, 1994).
Averaged side views and sections through the GroEL-GroES complex show large displacements of the apical domains, consistent with a large rotation of the apical domains in contact with GroES about the upper hinge region (Figs. 7a and 7c). This dramatic rotation occurs at both ends of GroEL in the case of the football structures (Fig. 7b).
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