Co-operative binding of Ca2+ ions to the regulatory binding sites of gelsolin.
The rate of association of actin with gelsolin was measured at various Ca2+ and ATP concentrations. The fraction of Ca2+-activated gelsolin was determined by quantitative evaluation of the association rates thereby assuming that Ca2+-binding gelsolin associates with actin and Ca2+-free gelsolin does not. A plot of the fraction of Ca2+-activated gelsolin vs. the free Ca2+ concentration revealed a sigmoidal shape suggesting that co-operative binding of Ca2+ ions is required for activation of gelsolin. A good fit of the experimental data by calculated binding curves was obtained if two Ca2+ ions were assumed to bind to actin in a highly co-operative manner. ATP decreased the rate of association of gelsolin with actin and bound to gelsolin at a low affinity (Kd = 32 microm for Ca2+-free and Kd = 400 microm for Ca2+-activated gelsolin). In contrast, a 1 : 1 gelsolin-actin complex was found to be activated for association with actin by a single Ca2+ ion in a non-co-operative manner.
Gremm
D
D
Institute of Physiological Chemistry, Ruhr-University, Buchum, Germany.
Wegner
A
A
D000199:Actins / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D018260:Gelsolin / Q000378:metabolism*; D007700:Kinetics; D018482:Muscle, Skeletal / Q000378:metabolism; D011485:Protein Binding; D011817:Rabbits
D000199:Actins; D018260:Gelsolin; D000255:Adenosine Triphosphate; D002118:Calcium