Figure 4.

Tyrosine phosphorylation of IP₃R1 increases the receptor's IP₃ sensitivity. (A) IP₃ sensitivity of the channel in the planar lipid bilayer. Channel activity of Fyn-phosphorylated IP₃R1 (WT-IP₃R1/Fyn Y528F) and phosphorylated IP₃R1-Y353F (IP₃R1-Y353F/Fyn Y528F) was measured at 173 nM of free cytosolic Ca²⁺ in the presence of 2 μM ruthenium red, 1 mM Na-ATP, and various IP₃ concentrations (10 nM to 10 μM). Open probability values at each IP₃ concentration were calculated as a mean from several independent experiments (n = 4 for WT-IP₃R1/Fyn Y528F; n = 7 for IP₃R1-Y353F/Fyn Y528F; *, P < 0.05 WT vs. Y353F by t test) and fitted by the equation described by Tang et al. (2003). (B) Comparison of open probability for Fyn-phosphorylated WT-IP₃R1, Fyn-phosphorylated IP₃R1-Y353F, and WT-IP₃R1 coexpressed with kinase-dead Fyn (unphosphorylated WT-IP₃R1). At 173 nM Ca²⁺ and 2 μM IP₃, Fyn-phosphorylated WT-IP₃R1 exhibits an approximately threefold increase in channel open probability as compared with both IP₃R1-Y353F and unphosphorylated WT-IP₃R1 (*, P < 0.05 phosphorylated WT vs. Y353F and phosphorylated vs. unphosphorylated WT by t test). Error bars represent the SEM.