Folding of maltose binding protein outside of and in GroEL

We used hydrogen exchange–mass spectrometry (HX MS) and fluorescence to compare the folding of maltose binding protein (MBP) in free solution and in the GroEL/ES cavity. Upon refolding, MBP initially collapses into a dynamic molten globule-like ensemble, then forms an obligatory on-pathway native-like folding intermediate (1.2 seconds) that brings together sequentially remote segments and then folds globally after a long delay (30 seconds). A single valine to glycine mutation imposes a definable folding defect, slows early intermediate formation by 20-fold, and therefore subsequent global folding by approximately twofold. Simple encapsulation within GroEL repairs the folding defect and reestablishes fast folding, with or without ATP-driven cycling. Further examination exposes the structural mechanism. The early folding intermediate is stabilized by an organized cluster of 24 hydrophobic side chains. The cluster preexists in the collapsed ensemble before the H-bond formation seen by HX MS. The V9G mutation slows folding by disrupting the preintermediate cluster. GroEL restores wild-type folding rates by restabilizing the preintermediate, perhaps by a nonspecific equilibrium compression effect within its tightly confining central cavity. These results reveal an active GroEL function other than previously proposed mechanisms, suggesting that GroEL possesses different functionalities that are able to relieve different folding problems. The discovery of the preintermediate, its mutational destabilization, and its restoration by GroEL encapsulation was made possible by the measurement of a previously unexpected type of low-level HX protection, apparently not dependent on H-bonding, that may be characteristic of proteins in confined spaces.