ROLE OF SALT BRIDGES IN GROEL ALLOSTERY

Abstract

Chaperonin GroEL facilitates protein folding with two stacked back-to-back, identical rings and the "lid", co-chaperonin GroES. The mis-folded/unfolded substrate protein (SP) adjusts the chaperonin cycling from an asymmetric to a symmetric cycle by catalyzing the release of ADP from the <italic> trans </italic> ring of GroEL, thus promoting the <bold> R </bold> to <bold> T </bold> allosteric transition. ATP binding to the SP bound ring promotes the association of a second GroES and subsequently a GroEL-GroES ₂ "football" complex is formed as the folding functional form. However, ADP does release spontaneously, albeit at very slow rate, in the absence of SPs.

The intrinsic mechanism by which GroEL relaxes to the lower potential energy <bold> T </bold> state remains poorly understood. A network of salt bridges forms and breaks during the allosteric transitions of GroEL. Residue D83 in the equatorial domain forms an intra-subunit salt bridge with K327 in the apical domain, and R197 in the apical domain forms an inter-subunit salt bridge with E386 in the intermediate domain. These two salt bridges stabilize the <bold> T </bold> state and break during the <bold> T </bold> to <bold> R </bold> state transition. Removal of these salt bridges by mutation destabilizes the <bold> T </bold> state and favors the <bold> R </bold> state of GroEL. These mutations do not alter the intrinsic ATPase activity of GroEL. However, the affinity for nucleotides becomes enhanced and ADP release is hindered such that SP cannot displace the equilibrium to the <bold> T </bold> state, as normally it does in the wild type. The exchange of ADP to ATP and association of a second GroES is compromised with the following GroEL-GroES ₂ "football" formation is hindered. These mutations do not completely eliminate the <bold> T </bold> state, in the absence of nucleotide, as shown biochemically and by crystal structures. The biased allosteric equilibrium hampers the formation of folding active "football" complex as the mutant GroEL's incompetency to revisit <bold> T </bold> state in the presence of nucleotide, but not due to the elimination of its <bold> T </bold> state. This study revealed the critical role of salt bridges in regulating the allosteric transitions of GroEL and conjugated formation of the "football" complex.