DIVALENT-CATIONS CAN INDUCE THE EXPOSURE OF GROEL HYDROPHOBIC SURFACES AND STRENGTHEN GROEL HYDROPHOBIC BINDING INTERACTIONS - NOVEL EFFECTS OF ZN2+ GROEL INTERACTIONS

Fluorescent and non-fluorescent probes have been used to show that div alent cations (Ca2+, Mg2+, Mn2+, and Zn2+) significantly increase hydr ophobic exposure on GroEL, whereas monovalent cations (K+ and Na+) hav e little effect, Zn2+ always induced the largest amount of hydrophobic exposure on GroEL, By using a new method based on interactions of Gro EL with octyl-Sepharose, it was demonstrated that Zn2+ binding strengt hens GroEL hydrophobic binding interactions and increases the efficien cy of substrate release upon the addition of MgATP and GroES. The bind ing of 4,4'-bis(1-anilino-8-naphthalenesulfonic acid) to GroEL in the presence of Zn2+ has a K-d congruent to 1 mu M, which is similar to th at observed previously for the GroEL 4,4'-bis(1-anilino-8-naphthalenes ulfonic acid) complex, Urea denaturation, sedimentation velocity ultra centrifugation, and electron microscopy revealed that the quaternary s tructure of GroEL in the presence of Zn2+ had a stability and morpholo gy equivalent to unliganded GroEL. In contrast, circular dichroism sug gested some loss in both alpha-helical and beta-sheet secondary struct ure in the presence of Zn2+. These data suggest that divalent cations can modulate the amount of hydrophobic surface presented by GroEL, Fur thermore, the influence of Zn2+ on GroEL hydrophobic surface exposure as well as substrate binding and release appears to be distinct from t he stabilizing effects of Mg2+ on GroEL quaternary structure.