  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.
