Dual function of protein confinement in chaperonin-assisted protein folding

The GroEL/GroES chaperonin system mediates the folding of a range of newly synthesized polypeptides in the bacterial cytosol. Using a rapid biotin-str eptavidin-based inhibition of chaperonin function, we show that the cage fo rmed by GroEL and its cofactor GroES can have a dual role in promoting fold ing. First, enclosure of nonnative protein in the GroEL:GroES complex is es sential for folding to proceed unimpaired by aggregation. Second, folding i nside the cage can be significantly faster than folding in free solution, i ndependently of ATP-driven cycles of GroES binding and release. This sugges ts that confinement of unfolded protein in the narrow hydrophilic space of the chaperonin cage smoothes the energy landscape for the folding of some p roteins, increasing the flux of folding intermediates toward the native sta te.