In Vivo Observation of Polypeptide Flux through the Bacterial Chaperonin System

The quantitative contribution of chaperonin GroEL to protein folding in E. coli was analyzed. A diverse set of newly synthesized polypeptides, predominantly between 10-55 kDa, interacts with GroEL, accounting for 10%-15% of all cytoplasmic protein under normal growth conditions, and for 30% or more upon exposure to heat stress. Most proteins leave GroEL rapidly within 10-30 s. We distinguish three classes of substrate proteins: (I) proteins with a chaperonin-independent folding pathway; (II) proteins, more than 50% of total, with an intermediate chaperonin dependence for which normally only a small fraction transits GroEL; and (III) a set of highly chaperonin-dependent proteins, many of which dissociate slowly from GroEL and probably require sequestration of aggregation-sensitive intermediates within the GroEL cavity for successful folding.