The Calmodulin Regulator Protein,PEP-19, Sensitizes ATP-induced Ca2+ Release

PEP-19 is a small, intrinsically disordered protein that binds to the C-domain of calmodulin (CaM) via an IQ motif and tunes its Ca²⁺ binding properties via an acidic sequence. We show here that the acidic sequence of PEP-19 has intrinsic Ca²⁺ binding activity, which may modulate Ca²⁺ binding to CaM by stabilizing an initial Ca²⁺-CaM complex or by electrostatically steering Ca²⁺ to and from CaM. Because PEP-19 is expressed in cells that exhibit highly active Ca²⁺ dynamics, we tested the hypothesis that it influences ligand-dependent Ca²⁺ release. We show that PEP-19 increases the sensitivity of HeLa cells to ATP-induced Ca²⁺ release to greatly increase the percentage of cells responding to sub-saturating doses of ATP and increases the frequency of Ca²⁺ oscillations. Mutations in the acidic sequence of PEP-19 that inhibit or prevent it from modulating Ca²⁺ binding to CaM greatly inhibit its effect on ATP-induced Ca²⁺ release. Thus, this cellular effect of PEP-19 does not depend simply on binding to CaM via the IQ motif but requires its acidic metal binding domain. Tuning the activities of Ca²⁺ mobilization pathways places PEP-19 at the top of CaM signaling cascades, with great potential to exert broad effects on downstream CaM targets, thus expanding the biological significance of this small regulator of CaM signaling.