The ATPase activity of GroEL is supported at high temperatures by divalent cations that stabilize its structure

Previously, we reported that the ATPase activity of GroEL that requires potassium and magnesium was highly temperature dependent in the 25-60 degrees C range. Here, we report that the monovalent cations, rubidium and ammonium were able to fully substitute for potassium; while the divalent cations manganese, cobalt, and nickel supported the ATPase activity of GroEL albeit to a lesser degree than magnesium. ATPase activities with manganese, cobalt, and nickel were 64%, 41%, and 29%, respectively, of the maximum activity (100%) when utilizing magnesium. Interestingly, the ability of all the cations to support the GroEL ATPase activity was somewhat consistent over the entire 25-60 degrees C range. Maximum ATPase activities were observed at 49 degrees C. Here, the influence of these cations on the thermal denaturation of GroEL was also monitored using bisANS binding as an indication of the exposure of hydrophobic surfaces during thermal denaturation of GroEL. Maximum exposure of hydrophobic surfaces on GroEL alone or in the presence of each of the monovalent cations was determined to occur at 65 degrees C. However, the maximum exposure of hydrophobic surfaces on GroEL in the presence of magnesium, manganese, cobalt or nickel was found to occur at 71 degrees C indicating that GroEL is significantly stabilized against thermal denaturation by these divalent cations.