It has been widely believed that an asymmetric GroEL-GroES complex (termed the bullet-shaped complex) is formed solely throughout the chaperonin reaction cycle, whereas we have recently revealed that a symmetric GroEL-(GroES) 2 complex (the football-shaped complex) can form in the presence of denatured proteins. However, the dynamics of the GroEL-GroES interaction, including the football-shaped complex, is unclear. We investigated the decay process of the football-shaped complex at a single-molecule level. Because submicromolar concentrations of fluorescent GroES are required in solution to form saturated amounts of the football-shaped complex, single-molecule fluorescence imaging was carried out using zero-mode waveguides. The single-molecule study revealed two insights into the GroEL-GroES reaction. First, the first GroES to interact with GroEL does not always dissociate from the football-shaped complex prior to the dissociation of a second GroES. Second, there are two cycles, the "football cycle " and the "bullet cycle," in the chaperonin reaction, and the lifetimes of the footballshaped and the bullet-shaped complexes were determined to be 3-5 s and about 6 s, respectively. These findings shed new light on the molecular mechanism of protein folding mediated by the GroEL-GroES chaperonin system.The chaperonin is an essential molecular chaperone that assists protein folding in the cell (1, 2). The Escherichia coli chaperonin GroEL is composed of 14 identical 57-kDa subunits arranged in two heptameric rings stacked back-to-back with each containing a cavity. The cofactor, GroES, consists of a dome-shaped heptameric ring of identical 10-kDa subunits.The widely accepted model of protein folding mediated by the GroEL-GroES chaperonin system is as follows. (i) The denatured protein is injected into one of GroEL rings (cis-ring) upon the ATP-dependent formation of the GroEL-GroES complex.(ii) ATP hydrolysis in the cis-ring results in the formation of the GroEL-GroES complex with bound ADP. (iii) Subsequent ATP binding to the opposite ring (trans-ring) induces the release of GroES, ADP, and the encapsulated protein from the cis-ring.(iv) The trans-ring is reoriented to a new cis-ring, thereby allowing the next ATPase cycle (1-5). In other words, an asymmetric GroEL-GroES complex (termed the bullet-shaped complex) is formed throughout the cycle, whereas a symmetric GroEL-(GroES) 2 complex (termed the football-shaped complex) does not exist.The accepted model has been challenged by the findings that indicate the existence of the football-shaped complex (6 -14). In particular, we have recently revealed that the bullet-and the football-shaped complexes coexist during the reaction cycle (14). We have also found that the formation of the footballshaped complex is regulated by the ATP/ADP ratio (14) and the amount of denatured proteins in solution (13). However, the dynamics of the GroEL-GroES interaction via the footballshaped complex remains elusive. Here, we have developed a single-molecule assay using zero-mode waveguid...