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 divalent cations (Ca²⁺, Mg²⁺, Mn²⁺, and Zn²⁺) significantly increase hydrophobic exposure on GroEL, whereas monovalent cations (K⁺ and Na⁺) have little effect. Zn²⁺ always induced the largest amount of hydrophobic exposure on GroEL. By using a new method based on interactions of GroEL with octyl-Sepharose, it was demonstrated that Zn²⁺ binding strengthens GroEL hydrophobic binding interactions and increases the efficiency of substrate release upon the addition of MgATP and GroES. The binding of 4,4'-bis(1- anilino-8-naphthalenesulfonic acid) to GroEL in the presence of Zn²⁺ has a Kd ≃ 1 μM, which is similar to that observed previously for the GroEL 4,4'- bis(1-anilino-8-naphthalenesulfonic acid) complex. Urea denaturation, sedimentation velocity ultracentrifugation, and electron microscopy revealed that the quaternary structure of GroEL in the presence of Zn²⁺ had a stability and morphology equivalent to unliganded GroEL. In contrast, circular dichroism suggested some loss in both α-helical and β-sheet secondary structure in the presence of Zn²⁺. These data suggest that divalent cations can modulate the amount of hydrophobic surface presented by GroEL. Furthermore, the influence of Zn²⁺ on GroEL hydrophobic surface exposure as well as substrate binding and release appears to be distinct from the stabilizing effects of Mg²⁺ on GroEL quaternary structure.