title: Allosteric activation of the Par-6 PDZ via a partial unfolding transition.
abstract: Proteins exist in a delicate balance between the native and unfolded states, where thermodynamic stability may be sacrificed to attain the flexibility required for efficient catalysis, binding, or allosteric control. Partition-defective 6 (Par-6) regulates the Par polarity complex by transmitting a GTPase signal through the Cdc42/Rac interaction binding PSD-95/Dlg/ZO-1 (CRIB-PDZ) module that alters PDZ ligand binding. Allosteric activation of the PDZ is achieved by local rearrangement of the L164 and K165 side chains to stabilize the interdomain CRIB:PDZ interface and reposition a conserved element of the ligand binding pocket. However, microsecond to millisecond dynamics measurements revealed that L164/K165 exchange requires a larger rearrangement than expected. The margin of thermodynamic stability for the PDZ domain is modest (∼3 kcal/mol) and further reduced by transient interactions with the disordered CRIB domain. Measurements of local structural stability revealed that tertiary contacts within the PDZ are disrupted by a partial unfolding transition that enables interconversion of the L/K switch. The unexpected participation of partial PDZ unfolding in the allosteric mechanism of Par-6 suggests that native-state unfolding may be essential for the function of other marginally stable proteins.
authors.affiliation: Department of Biochemistry, Medical College of Wisconsin, Milwaukee, Wisconsin 53226, United States.
authors.forename: Dustin S
authors.initials: DS
authors.lastname: Whitney
authors.forename: Francis C
authors.initials: FC
authors.lastname: Peterson
authors.forename: Evgenii L
authors.initials: EL
authors.lastname: Kovrigin
authors.forename: Brian F
authors.initials: BF
authors.lastname: Volkman
mesh_terms: D000494:Allosteric Regulation; D029721:Drosophila Proteins / Q000737:chemistry*; D054731:PDZ Domains; D011493:Protein Kinase C / Q000737:chemistry*; D058767:Protein Unfolding; D013816:Thermodynamics
chemical_list: D029721:Drosophila Proteins; C532584:Par-6 protein, Drosophila; D011493:Protein Kinase C