Ca-dependent binding of actin to gelsolin

Ca2+ of 0.3-1.0 muM induces both the exposure of tryptic cleavage sites within the gelsolin molecule inaccessible in the Ca-free conformation, and binding of one actin monomer to the N-terminal half of gelsolin. On the other hand, gelsolin-induced enhancement of pyrene actin fluorescence was observed only above 50 muM Ca2+, and a ternary actin/gelsolin complex preformed in 200 muM Ca2+ was stable only above 30 muM Ca2+. These results provide direct evidence for a Ca-induced transitions from closed to open conformation of the gelsolin molecule in the range of 3 x 10(-7) to 10(-6) M Ca2+. They also suggest that Ca2+ > 10(-5) M is required to stabilize actin-actin contacts in the 2:1 actin/gelsolin complex. (C) 2002 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.