Allosteric coupling via distant disorder-to-order transitions.

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biology

Background:

• Allosteric coupling in proteins is crucial for biological regulation.
• The precise mechanisms by which intrinsic disorder facilitates allostery are not fully understood.
• The Escherichia coli biotin repressor (BirA) exhibits allosteric coupling between effector binding and dimerization.

Purpose of the Study:

• To elucidate the role of intrinsic disorder in mediating allosteric coupling in BirA.
• To investigate the structural and functional consequences of mutations affecting allosteric communication.
• To understand how disorder-to-order transitions contribute to the protein's regulatory function.

Main Methods:

• X-ray crystallography to determine protein structure.
• Site-directed mutagenesis to create protein variants.
• Functional assays to measure allosteric coupling and ligand binding.

Main Results:

• Wild-type BirA exhibits significant allosteric coupling (-4.0 ± 0.3 kcal/mol) driven by loop folding upon effector binding.
• A specific alanine substitution abolished this coupling, resulting in a monomeric, corepressor-bound state.
• Structural analysis revealed that key loops involved in effector binding and dimerization become disordered in the mutant, contrasting with the folded state in the wild-type.

Conclusions:

• Allosteric coupling in BirA is achieved through reciprocal communication of disorder-to-order transitions between distant functional surfaces.
• Intrinsic disorder plays a critical role in transmitting allosteric signals.
• The interplay between ordered and disordered states is essential for the integrated function of BirA.