Electrostatic Forces Mediate Fast Association of Calmodulin and the Intrinsically Disordered Regulatory Domain of Calcineurin

TL;DR

This study reveals that electrostatic interactions primarily drive the rapid association between calmodulin and the intrinsically disordered regulatory domain of calcineurin, elucidating a key mechanism in signal transduction involving IDPs.

Contribution

The paper demonstrates how electrostatic forces govern the kinetics of IDP-mediated protein interactions, combining experimental and computational methods to quantify these effects.

Findings

Association rates depend strongly on ionic strength.

Electrostatic interactions are the main determinant of binding kinetics.

The recognition domain's electrostatic properties control signal transduction speed.

Abstract

Intrinsically disordered proteins (IDPs) and proteins with intrinsically disordered regions (IDRs) govern a daunting number of physiological processes. For such proteins, molecular mechanisms governing their interactions with proteins involved in signal transduction pathways remain unclear. Using the folded, calcium-loaded calmodulin (CaM) interaction with the calcineurin regulatory IDP as a prototype for IDP-mediated signal transduction events, we uncover the interplay of IDP structure and electrostatic interactions in determining the kinetics of protein-protein association. Using an array of biophysical approaches including stopped-flow and computational simulation, we quantify the relative contributions of electrostatic interactions and conformational ensemble characteristics in determining association kinetics of calmodulin (CaM) and the calcineurin regulatory domain (CaN RD). Our chief findings are that CaM/CN RD association rates are strongly dependent on ionic strength and that observed rates are largely determined by the electrostatically-driven interaction strength between CaM and the narrow CaN RD calmodulin recognition domain. These studies highlight a molecular mechanism of controlling signal transduction kinetics that may be utilized in wide-ranging signaling cascades that involve IDPs.