Multiple equilibria of the Escherichia coli chaperonin GroES revealed by mass spectrometry

Nanospray time-of-flight mass spectrometry has been used to study the assembly of the heptamer of the Escherichia coli cochaperonin protein GroES, a system previously described as a monomer-heptamer equilibrium. In addition to the monomers and heptamers, we have found measurable amounts of dimers and hexamers, the presence of which suggests the following mechanism for heptamer assembly: 2 Monomers <-> Dimer; 3 Dimers <-> Hexamer; Hexamer + Monomer <-> Heptamer. Equilibrium constants for each of these steps, and an overall constant for the Monomer <-> Heptamer equilibrium, have been estimated from the data. These constants imply a standard free-energy change, Delta G(0), of about 9 kcal/mol for each contact surface formed between GroES subunits, except for the addition of the last subunit, where Delta G(0) = 6 kcal/mol. This lower value probably reflects the loss of entropy when the heptamer ring is formed. These experiments illustrate the advantages of electrospray mass spectrometry as a method of measuring all components of a multiple equilibrium system.