Abstract
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.
Highlights
PEP-19 is a small protein that increases the rates of Ca2 þ binding to the C-domain of calmodulin (CaM) by an unknown mechanism
We focused our structural analysis on interactions between PEP-19 and apo CaM since increasing the rate-limiting association of Ca2 þ with the C-domain of CaM has the potential to increase its rate of association with other Ca2 þ -dependent target proteins
We used the isolated C-domain of CaM (C-CaM) since we showed previously that PEP-19 binds preferentially to the C-domain[6,18], and that PEP-19 has the same effect on the Ca2 þ binding properties of both intact CaM and C-CaM19
Summary
PEP-19 is a small protein that increases the rates of Ca2 þ binding to the C-domain of calmodulin (CaM) by an unknown mechanism. An IQ motif promotes binding to CaM, an acidic sequence in PEP-19 is required to modulate Ca2 þ binding and to sensitize HeLa cells to ATP-induced Ca2 þ release. Proteins that could potentially modulate or tune the Ca2 þ binding properties of CaM would enhance its ability to sense diverse Ca2 þ signals that vary greatly in amplitude and frequency. Our search for such regulators of CaM signalling led us to a small, intrinsically disordered protein called PEP-19. PEP-19 sensitizes HeLa cells to ATP-dependent Ca2 þ release[17]
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