The accessibility of fluorescein-5-maleimide to sulfhydryl groups in the molecular chaperone GroEL was used to follow structural rearrangements in the protein triggered by binding Mg2+ and/or adenine nucleotides. Three peptides, each containing one of the cysteines of GroEL (C138, C458 and C519) were identified. GroEL labeled in 50mM TrisHCl, pH 7.8, incorporated ˜0.3 labels each on C138 and C458. With 10mM MgCl2, the labeling increased to ˜0.8 labels each on C138 and C458. The increase was partially due to a conformational change which occurred upon Mg2+ binding as well as to an increase in ionic strength. When ADP, ATP, or AMP-PNP were added to a solution of GroEL and Mg2+, C138 incorporated ˜0.8 labels, while C458 incorporated ˜0.1 labels. These results suggest that the binding of adenine nucleotides changed the conformation of GroEL and made a previously highly exposed sulfhydryl group inaccessible. GroEL slowly dissociated into monomers when it was extensively labeled at C458. GroEL labeled with fluorescein-5-maleimide, under any of the conditions examined, was able to bind but not release active rhodanese. The observed variations in sulfhydryl accessibility are consistent with mechanisms that suggest binding of GroES to GroEL differs from the binding of substrate protein to GroEL, and that the binding of Mg2+ or Mg-adenine nucleotides results in conformational changes in GroEL.