Fig. 4.

Channel-deficient type 1 IP₃Rs rescue Ca²⁺ entry in DT40 IP₃R^–/– B cells, whereas IP₃-binding-deficient type 1 IP₃Rs do not. Free Ca²⁺ measurements were made in DT40 IP₃R^–/– knockout cells transfected with either YFP plus WT IP₃R (WT; A), YFP plus ΔC-terminal IP₃R mutant (ΔC; B), YFP plus Δ90–110 IP₃R mutant (Δ90–110; C), YFP plus D2550A IP₃R mutant (D2550A; D), YFP plus R265Q IP₃R mutant (R265Q; E), or YFP alone (F). Ca²⁺ pools were released in cells by 3 μg/ml anti-IgM (arrow) in nominally Ca²⁺-free medium followed by replacement with anti-IgM and normal 1 mM Ca²⁺ medium (bar). DT40 IP₃R^–/– knockout cells, transfected as above, were prepared for immunocytochemistry and visualized by confocal microscopy, and they appear directly below their respective Ca²⁺ trace. (A–F) The Left images corresponds to YFP expression, and the Right images corresponds to IP₃R staining. All traces are averages of ≈15 cells because the mutants Δ90–110 or R265Q IP₃Rs do not respond and ΔC or D2550A only displays the NRE response.