The versatile mutational "repertoire" of Escherichia coli GroEL, a multidomain chaperonin nanomachine.

The bacterial chaperonins are highly sophisticated molecular nanomachines, controlled by the hydrolysis of ATP to dynamically trap and remove from the environment unstable protein molecules that are susceptible to denaturation and aggregation. Chaperonins also act to assist in the refolding of these unstable proteins, providing a means by which these proteins may return in active form to the complex environment of the cell. The Escherichia coli GroE chaperonin system is one of the largest protein supramolecular complexes known, whose quaternary structure is required for segregating aggregation-prone proteins. Over the course of more than two decades of research on GroE, it has become accepted that GroE, more specifically the GroEL subunit, is a "high-tolerance" molecular system, capable of accommodating numerous mutations, while retaining its molecular integrity. In some cases, a given site of mutation was revealed to be absolutely required for GroEL function, providing hints regarding the network of signals and triggers that propel this unique system. In other instances, however, a mutation has produced a more delicate response, altering only part of, or in some cases, only a single facet of, the molecular mechanism, and these mutants have often provided invaluable hints on the extent of the complexity underlying chaperonin-assisted protein folding. In this review, we highlight some examples of the latter type of GroEL mutants which compose the unique "mutational repertoire" of GroEL and touch upon the important clues that each mutant provided to the overall effort to elucidate the details of GroE action.