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 olig omer GroEL(14) from Escherichia coli. The crosslinking reaction of Gro EL(14) presents two phases: a rapid intralayer cross-linking reaction, which first occurs between the monomers of individual GroEL(7) heptam eric rings, and a slow interlayer cross-linking reaction, which later occurs between the two stacked heptameric rings of the GroEL(14) oligo mer. The biphasic behavior of the cross-linking reaction indicates tha t the surfaces of contact between GroEL monomers within individual hep tameric rings are more extensive than the surfaces of contact between the two GroEL rings of the oligomer. Millimolar amounts of the divalen t cations Mg2+ Mn2+, Ca2+, or Zn2+, but not of monovalent ions, increa se the velocity of both intra- and interlayer cross-linking. Divalent cations increase the stability of the native GroEL(14) oligomer in ure a. In contrast, Mg2+ activates ATP hydrolysis by GroEL(14), with an ac tivation constant in the micromolar range, while Ca2+ does not signifi cantly assist ATP hydrolysis. It is concluded that divalent cations af fect the structure of GroEL(14), in particular the contacts between mo nomers within the GroEL(7) heptameric layers. The effect of divalent c ations on the structure of the chaperonin molecule is quantitatively a nd qualitatively distinct from that of magnesium ions on the chaperoni n ATPase activity.