Temperature Regulates Stability, Ligand Binding (Mg²⁺ and ATP), and Stoichiometry of GroEL–GroES Complexes

Chaperonins are nanomachines that harness ATP hydrolysis to power and catalyze protein folding, a chemical action that is directly linked to the maintenance of cell function through protein folding/refolding and assembly. GroEL and the GroEL–GroES complex are archetypal examples of such protein folding machines. Here, variable-temperature electrospray ionization (vT-ESI) native mass spectrometry is used to delineate the effects of solution temperature and ATP concentrations on the stabilities of GroEL and GroEL–GroES complexes. The results show clear evidence for destabilization of both GroEL₁₄ and GroES₇ at temperatures of 50 and 45 °C, respectively, substantially below the previously reported melting temperature (Tₘ ∼ 70 °C). This destabilization is accompanied by temperature-dependent reaction products that have previously unreported stoichiometries, viz. GroEL₁₄–GroESy–ATPₙ, where y = 1, 2, 8 and n = 0, 1, 2, 8, that are also dependent on Mg²⁺ and ATP concentrations. Variable-temperature native mass spectrometry reveals new insights about the stability of GroEL in response to temperature effects: (i) temperature-dependent ATP binding to GroEL; (ii) effects of temperature as well as Mg²⁺ and ATP concentrations on the stoichiometry of the GroEL–GroES complex, with Mg²⁺ showing greater effects compared to ATP; and (iii) a change in the temperature-dependent stoichiometries of the GroEL–GroES complex (GroEL₁₄–GroES₇ vs GroEL₁₄–GroES₈) between 24 and 40 °C. The similarities between results obtained by using native MS and cryo-EM [Clare et al. An expanded protein folding cage in the GroEL–gp31 complex. J. Mol. Biol. 2006, 358, 905–911; Ranson et al. Allosteric signaling of ATP hydrolysis in GroEL–GroES complexes.Nat. Struct. Mol. Biol. 2006, 13, 147–152] underscore the utility of native MS for investigations of molecular machines as well as identification of key intermediates involved in the chaperonin-assisted protein folding cycle.