Ligand-Linked Disorder-to-Order Transitions in Allosteric Communication
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Ligand-linked disorder-to-order transitions are integral to the function of numerous cellular proteins. Furthermore, these transitions can contribute to allosteric regulation, a principal mechanism controlling protein function. Despite their ubiquity, the relationship between the sequence and function of these regions and their mechanisms of achieving allosteric regulation are not well understood. The Escherichia coli biotin repressor, BirA, is a bifunctional protein that provides a model system to investigate these questions. Binding of the corepressor, biotinyl-5'-AMP, is coupled to a disorder-to-order transition resulting in a complex network of hydrophobic residues packing over the adenylate moiety. Additionally, this binding event is coupled to BirA dimerization, enhancing the self-association free energy by -4.0 kcal/mol. In this work, the sequence-function relationship of the disorder-to-order transition was investigated using several combinations of alanine substitutions in the hydrophobic network. Equilibrium binding and kinetic measurements show that the full functional response in the disorder-to-order transition is achieved through the appropriate packing of hydrophobic residues in the hydrophobic network. In addition to the disorder-to-order transition on the ligand binding surface, the dimerization interface contains several regions that, while disordered in the unliganded monomer, are folded in the liganded dimer. Through structural and thermodynamic analysis of the G142A variant, long-distance reciprocal communication between disorder-to-order transitions on the ligand binding and dimerization surfaces is identified as central to allostery. Together, these results demonstrate the functional versatility of disorder-to-order transitions and how the sequences of these regions dictate protein function and allosteric regulation.