PEP-19 modulates calcium binding to calmodulin by electrostatic steering

PEP-19 is a small protein that increases the rates of CA²⁺ binding to the C-domain of Calmodulin (CaM) by an unknown mechanism. Although an IQ motif promotes binding to CaM, an acidic sequence in PEP-19 is required to modulate CA²⁺ binding and to sensitize HeLa cells to ATP-induced CA²⁺ release. Here, we report the NMR solution structure of a complex between PEP-19 and the C-domain of apo CaM. The acidic sequence of PEP-19 associates between helices E and F of CaM via hydrophobic interactions. This allows the acidic side chains in PEP-19 to extend toward the solvent and form a negatively charged surface that resembles a catcher's mitt near CA²⁺ binding loop III of CaM. The topology and gradients of negative electrostatic surface potential support a mechanism by which PEP-19 increases the rate of CA²⁺ binding to the C-domain of CaM by 'catching' and electrostatically steering CA²⁺ to site III.