Functional Analysis of Rat Acidic Calponin

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Recombinant acidic calponin, a member of the calponin family, interacted with F-actin, but not with microtubules, desmin filaments, tropomyosin, calmodulin, S100 and phosphatidylserine (PS) vesicles with significant affinity. The bindings of acidic calponin to F-actin occurred in a concentration-dependent manner and were saturated at a molar ratio of about 1 acidic calponin to 1—2 actin molecules. The apparent Kd value of acidic calponin to F-actin was calculated to be 1.6×10⁵ M⁻¹. Chemical cross-linking experiments indicated that a 1 : 1 molar covalent complex of acidic calponin and actin monomer was produced as in the case of basic calponin–actin binding. No significant morphologic change of F-actin was observed by the addition of acidic calponin. Acidic calponin had little effect on actomyosin Mg²⁺-ATPase activity unlike basic calponin. Basic calponin partially competed with acidic calponin for binding to F-actin. Domain mapping with V8 protease revealed that acidic calponin binding site resided within the C-terminal 16 kDa fragment of actin, where the binding of basic calponin also occurs. However, both calponins showed reversal effects on fluorescence intensity of pyrene-labeled F-actin. Fragments of acidic calponin with 30 and 22 kDa, lacking the C-terminal acidic tail, were bound to F-actin. Interestingly, both the fragments became bound to PS vesicles, but not to other components. Circular dichroism studies showed that limited digestion of acidic calponin resulted in about 30% decrease of α-helix and β contents. The present results suggest that acidic calponin is functionally distinct from basic calponin and expresses a novel characteristic after removal of the acidic tail region.