The conserved residue Lys-34 in GroES was replaced by alanine and glutamic acid using site-directed mutagenesis. This residue is near the carboxy terminus of the mobile loop in GroES (residues 17-32) which becomes immobilized upon formation of the GroEL/GroES complex [Landry et al. (1993) Nature 364, 255-258]. Both charge neutralization (Lys-34→Ala) and charge reversal (Lys-34→Glu) at this position have little effect on the binding constant of GroES to GroEL, but they increase the enhancement by GroES of cooperativity in ATP hydrolysis by GroEL. This is reflected by a change in the Hill coefficient (at 10 mM K+) from 4.10 (±0.22) in the presence of wild-type GroES to 5.17 (±0.24) and 4.46 (±0.14) in the presence of the GroES mutants Lys-34→Ala and Lys-34→Glu, respectively. The results are interpreted using the Monod-Wyman-Changeux (MWC) model for cooperativity [Monod et al. (1965) J. Mol. Biol. 12, 88—118], They suggest that Lys-34 in GroES modulates the allosteric transition in GroEL by stabilizing a relaxed (R)-like state.
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