The E. coli chaperonin GroEL and its cofactor GroES promote protein folding by sequestering nonnative polypeptides in a cage-like structure. Here we define the contribution of this system to protein folding across the entire E. coli proteome. Approximately 250 different proteins interact with GroEL, but most of these can utilize either GroEL or the upstream chaperones trigger factor (TF) and DnaK for folding. Obligate GroEL-dependence is limited to only approximately 85 substrates, including 13 essential proteins, and occupying more than 75% of GroEL capacity. These proteins appear to populate kinetically trapped intermediates during folding; they are stabilized by TF/DnaK against aggregation but reach native state only upon transfer to GroEL/GroES. Interestingly, substantially enriched among the GroEL substrates are proteins with (betaalpha)8 TIM-barrel domains. We suggest that the chaperonin system may have facilitated the evolution of this fold into a versatile platform for the implementation of numerous enzymatic functions.