A thermodynamic coupling mechanism can explain the GroEL-mediated acceleration of the folding of barstar

Despite extensive structural and kinetic studies, the mechanism by which th e Escherichia coli chaperonin GroEL assists protein folding has remained so mewhat elusive. It appears that GroEL might play an active role in facilita ting folding, in addition to its role in restricting protein aggregation by secluding folding intermediates. We have investigated the kinetic mechanis m of GroEL-mediated refolding of the small protein barstar. GroEL accelerat es the observed fast (millisecond) refolding rate, but it does not affect t he slow refolding kinetics. A thermodynamic coupling mechanism, in which th e concentration of exchange-competent states is increased by the law of mas s action, can explain the enhancement of the fast refolding rates. It is no t necessary to invoke a catalytic role for GroEL, whereby either the intrin sic refolding rate of a productive folding transition or the unfolding rate of a kinetically trapped off-pathway intermediate is increased by the chap eronin. (C) 2000 Academic Press.