Helical propensity in an intrinsically disordered protein accelerates ligand binding.

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Many intrinsically disordered proteins (IDPs) gain structure upon binding to other molecules.
• The role of transient secondary structure in unbound IDPs during binding is not fully understood.
• The activator for thyroid hormone and retinoid receptors (ACTR) is an IDP that folds upon binding to the CREB binding protein's NCBD.

Purpose of the Study:

• To investigate the functional consequence of transient secondary structure in unbound ACTR on its binding kinetics with NCBD.
• To determine if preformed secondary structure in IDPs influences molecular recognition.

Main Methods:

• Utilized site-directed mutagenesis to create ACTR variants with altered secondary structure content in the unbound state.
• Employed Nuclear Magnetic Resonance (NMR) spectroscopy to assess secondary structure.
• Performed fluorescence-monitored stopped-flow kinetics to measure binding rates.

Main Results:

• Mutants with varying secondary structure in unbound ACTR were successfully generated without affecting intermolecular interactions.
• NMR confirmed differences in secondary structure content, particularly in helix 1 of ACTR.
• Kinetic measurements demonstrated that the degree of secondary structure in unbound ACTR directly impacts binding kinetics with NCBD.

Conclusions:

• Transiently formed secondary structure in intrinsically disordered proteins is not merely a byproduct but actively influences binding.
• Preformed secondary structure in unbound ACTR serves as a crucial determinant for efficient molecular recognition.
• This finding supports the concept of 'recognition by folding' where transient structures guide binding interactions.