Inter-ring communication allows the GroEL chaperonin complex to distinguish between different substrates

The productive folding of substrate proteins by the GroEL complex of Escherichia coli requires the activity of both the chaperonin rings. These heptameric rings were shown to regulate the chaperonins' affinity for substrates and co-chaperonin via inter-ring communications; however, the molecular details of the interactions are not well understood. We have investigated the effect of substrate binding on inter-ring communications of the chaperonin complex, both the double-ring GroEL as well as the single-ring SR1 chaperonin in complex with four different substrates by using mass spectrometry. This approach shows that whereas SR1 is unable to distinguish between Rubisco, gp23, gp5, and MDH, GroEL shows clear differences upon binding these substrates. The most distinctive binding behavior is observed for Rubisco, which only occupies one GroEL ring. Both bacteriophage capsid proteins (gp23 and gp5) as well as MDH are able to bind to the two GroEL rings simultaneously. Our data suggest that inter-ring communication allows the chaperonin complex to differentiate between substrates. Using collision induced dissociation in the gas phase, differences between the chaperonin(substrate) complexes are observed only when both rings are present. The data indicate that the size of the substrate is an important factor that determines the degree of stabilization of the chaperonin complex.