Binding and Functional Folding (BFF): A Physiological Framework for Studying Biomolecular Interactions and Allostery

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Signaling

Background:

• EF-hand calcium-binding proteins (CBPs), including S100 proteins (S100s) and calmodulin (CaM), are crucial signaling molecules.
• These proteins undergo conformational changes upon binding intracellular calcium (Ca2+), modulating their interactions with target proteins and initiating biological responses.
• The Ca2+ binding affinity of CaM and most S100s is typically weak in the absence of a target, but increases significantly upon effector protein binding through heterotropic allostery.

Purpose of the Study:

• To review and synthesize the mechanisms of allostery in EF-hand CBPs.
• To introduce a new physiological framework, "binding and functional folding (BFF)", for understanding CBP-target interactions and calcium signaling.
• To explore the evolutionary basis of allosteric regulation and CBP-target specificity.

Main Methods:

• Literature review and synthesis of existing research on EF-hand CBPs, allostery, and calcium signaling.
• Development of the "binding and functional folding (BFF)" framework to explain molecular mechanisms.
• Analysis of evolutionary aspects of CBP-target interactions.

Main Results:

• Allostery is physiologically essential for maintaining calcium homeostasis and regulating signaling within a narrow dynamic range.
• The BFF framework provides a molecular-level understanding of folding, binding, and functional events in CBP-target complexes.
• Two types of BFF mechanisms (concerted and stepwise) are proposed for protein interactions.
• Evolutionary analysis highlights how amino acid residues contribute to both allosteric Ca2+ affinity tightening and specific/promiscuous CBP-target complex formation.

Conclusions:

• The BFF framework offers a unified view of allosteric regulation in calcium-binding proteins.
• Understanding these mechanisms is critical for comprehending cellular calcium dynamics and signaling pathways.
• The interplay between allosteric Ca2+ binding and target interaction is finely tuned by evolution for proper cellular function.