FIGURE 5.

The truncation mutants of Pγ greatly enhanced the affinity of Pγ63–87 but not Pγ71–87. Purified Pαβ (10 pm) was preincubated with the indicated concentration of Pγ63–87 (A) or Pγ71–87 (B) in the presence of Pγ1–60 (♢), Pγ1–70 (▵), or Pγ1–83 (■) or 10 mm Tris (●) for 10 min at room temperature. The PDE activity was measured by the addition of 0.1 mm [³H]cAMP using the radiotracer assay and normalized to the percentage of the activity in the absence of C-terminal peptide (Pγ1–60, no inhibition; Pγ1–70, 10% inhibition; Pγ1–83, 35% inhibition). The data are the means ± S.D. of three experiments. The lines represent the fit to a three-parameter logistic dose-response equation. The following IC₅₀ values were determined for A: 2.1 ± 0.1 μm (no addition), 1.6 ± 0.1 μm (Pγ1–60), 0.018 ± 0.001 μm (Pγ1–70), and 0.045 ± 0.003 μm (Pγ1–83). For B, the IC₅₀ values were: Pγ71–87: 2.6 ± 0.2 μm (no addition), 4.6 ± 0.4 μm (Pγ1–70), and 11.0 ± 0.4 μm (Pγ1–83).