Interaction of Calponin with Phospholipids

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The interaction between chicken gizzard calponin and phospholipids was examined by sedimentation assay and affinity chromatography. Calponin was sedimented with phosphatidylserine (PS) and phosphatidylinositol (PI) vesicles but not with phosphatidylcholine (PC) vesicles. The apparent K_d values of calponin to PS and PI were calculated to be 1.3×10⁶ and 1.5×10⁶M^-1, respectively. Domain mapping with chymotryptic digestion showed that the phospholipid-binding site resided within the N-terminal 22-kDa fragment, in which the bindings of actin, calmodulin, S100, and tropomyosin also occur. The amount of calponin bound to PS and PI vesicles decreased with increasing ionic strength or Ca²⁺ concentrations. The presence of MgCl₂ was needed for the calponin-PS vesicle interaction. Calponin-binding proteins including actin, calmodulin, and S100 inhibited calponin binding to the phospholipid vesicles in a concentration-dependent manner, while tropomyosin had little effect on the binding. The inhibitory effects of calmodulin and S100 were found only in the presence of CaCl₂. Neither caldesmon nor SM22 affected the binding.