Folding of malate dehydrogenase inside the GroEL-GroES cavity

The chaperonin GroEL binds nonnative substrate protein in the hydrophobic c entral cavity of an open ring. ATP and GroES binding to the same ring conve rts this cavity into an encapsulated, hydrophilic chamber that mediates pro ductive folding. A 'rack' mechanism of initial protein unfolding proposes t hat, upon GroES and ATP binding, the polypeptide is stretched between the b inding sites on the twisting apical domains of GroEL before complete releas e into the chamber. Here, the structure of malate dehydrogenase (MDH) subun it during folding is monitored by deuterium exchange, peptic fragment produ ction and mass spectrometry. When bound to GroEL, MDH exhibits a core of pa rtially protected secondary structure that is only modestly deprotected upo n ATP and GroES binding. Moreover, deprotection is broadly distributed thro ughout MDH, suggesting that it results from breaking hydrogen bonds between MDH and the cavity wall or global destabilization, as opposed to forced me chanical unfolding.