Fig. 1. Effect of pyrene modification of actin on the binding of caldesmon to actin and actin-tropomyosin.

Varied concentrations of ¹⁴C-labeled caldesmon were incubated with 10 μm actin (○) or pyrene-labeled actin (•) at 25 °C in solutions containing 42 mm NaCl, 9.6 mm imidazole-HCl, pH 7.0, 4.6 mm MgCl₂,0.25 mm EGTA, and 1 mm DTT. A, absence of tropomyosin: apparent association constants are 3.6 × 10⁶ and 1.3 × 10⁶ M⁻¹ for normal and pyrenyl actin, respectively. Inset: SDS gels of co-sedimentation of unlabeled caldesmon with normal (n) and pyrenyl (p) actin. B, presence of 2.5 μm smooth muscle tropomyosin. Apparent association constants are 7.5 × 10⁶ and 2.0 × 10⁶ M⁻¹ for normal and pyrenyl actin.