Fig. 1.

Inhibition of InsP₃R1 by CaBP1. (A) CaBP1 inhibits InsP₃-evoked Ca²⁺ release. Permeabilized DT40–IP₃R1 cells in CLM with a [Ca²⁺]_c of 1.2 μM were incubated with CaBP1 (10 min) before adding InsP₃. Results (means ± SEM; n = 3, with duplicate determinations in each) show the concentration-dependent release of Ca²⁺ by InsP₃. (B) Effects of CaBP1 and CaBP1₁₃₄ (50 μM) on InsP₃-evoked Ca²⁺ release at the indicated [Ca²⁺]_c show that CaBP1 is not the only Ca²⁺ sensor. For each [Ca²⁺]_c, results (percentage of control, means ± SEM; n = 3–4, with duplicate determinations in each) show the Ca²⁺ release evoked by the concentration of InsP₃ that evoked half-maximal Ca²⁺ release (EC₅₀) under control conditions. *P < 0.05 relative to control. (C) Typical patch-clamp recordings from single InsP₃R1 in medium with [Ca²⁺]_c of 1.5 μM stimulated with InsP₃ (10 μM) alone or with CaBP1 (10 μM). Bars show the closed state. The holding potential was +40 mV. (D) Summary data (mean ± SEM; n given in Fig. S1F) show NP_o, mean channel open (τ_o) and closed (τ_c) times and unitary conductance (γ_Cs).