We investigated the dissociation of tetradecameric GroEL by high hydrostatic pressure in the range of 1-2.5 kbar. Kinetics of the dissociation of GroEL in the absence and presence of Mg2+ and/or KC1 were monitored using light scattering. All of the kinetics were biphasic in nature. At any given pressure, only monomers and 14mers were produced, and below 2.5 kbar, the 14mers only partially dissociated to monomers, which did not significantly reassemble on depressurization. Under identical reaction conditions, the observed dissociation rates decreased by only 2-fold when the concentration of GroEL was increased by 20-fold. At 2.5 kbar the observed rates decreased exponentially with the increase in [KC1] and reached a minimum at ∼75 mM. Similarly, the rates decreased with the increase in [Mg2+] and reached a minimum at ∼3 mM Mg2+. In the presence of saturating amounts of Mg2+ (10 mM) and KC1 (100 mM), the rates were much faster than with 10 mM Mg2+ alone. The results could be rationalized in terms of the presence of GroEL heterogeneity, which could not be assessed easily by common techniques such as sedimentation velocity, HPLC, gel electrophoresis, and dissociation by chaotropes. This heterogeneity is evidence of subpopulations of GroEL that dissociate at different pressures. At low pressures, the oligomer without added Mg2+ only partially dissociates to monomers, leading to an apparent plateau in the kinetics, whereas in the presence of Mg2+ the species are converted to a tighter Mg2+-bound species, leading to a much slower dissociation process. The presence of KC1 in the sample also leads to similar heterogeneity.
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