LOCALIZATION OF THE CALCIUM-SENSITIVE ACTIN MONOMER BINDING-SITE IN GELSOLIN TO SEGMENT-4 AND IDENTIFICATION OF CALCIUM-BINDING SITES

Gelsolin is composed of six repeating segments of sequence (G1-6) and contains three distinct actin binding sites, two that bind to G-actin and one that binds to filaments. The calcium-dependent actin monomer b inding site present in the carboxyl-terminal half of the protein (G4-6 ) plays a critical role both in the cooperative binding of actin by ge lsolin and in its nucleating activity. Here we have localized this act in binding site to segment 4 (G4) by expressing the segments G4, G4-5, G5, and G5-6 in Escherichia coli and analyzing their actin binding pr operties. In addition we have measured their calcium binding. G4-5 and G5-6 each bind a single calcium ion, but there is no binding by G4 or G5. The affinity of binding by G5-6 is 10 times higher than that of G 4-5, and calcium binding by G4-6 shows two sites of different affinity . Thus each actin binding site of gelsolin is restricted to a single s egment (G1, G2, and G4), but the nonbinding segments G5 and G6 play an important role in the calcium regulation of actin binding and other a ctivities of gelsolin.