Effect of Divalent Cations on the Molecular Structure of the GroEL Oligomer

Structural analysis, by chemical cross-linking with glutardialdehyde (GA), and by urea denaturation, was carried out for the chaperonin oligomer GroELu from Escherichia coli. The cross linking reaction of GroEL₁₄ presents two phases: a rapid intralayer cross-linking reaction, which first occurs between the monomers of individual GroEL₇ heptameric rings, and a slow interlayer cross-linking reaction, which later occurs between the two stacked heptameric rings of the GroEL₁₄ oligomer. The biphasic behavior of the cross-linking reaction indicates that the surfaces of contact between GroEL monomers within individual heptameric rings are more extensive than the surfaces of contact between the two GroEL₇ rings of the oligomer. Millimolar amounts of the divalent cations Mg²⁺, Mn²⁺, Ca²⁺, or Zn²⁺, but not of monovalent ions, increase the velocity of both intra- and interlayer cross-linking. Divalent cations increase the stability of the native GroEL₁₄ oligomer in urea. In contrast, Mg²⁺ activates ATP hydrolysis by GroEL₁₄, with an activation constant in the micromolar range, while Ca²⁺ does not significantly assist ATP hydrolysis. It is concluded that divalent cations affect the structure of GroEL₁₄, in particular the contacts between monomers within the GroEL₇ heptameric layers. The effect of divalent cations on the structure of the chaperonin molecule is quantitatively and qualitatively distinct from that of magnesium ions on the chaperonin ATPase activity.