The gelation of actin by actin-binding protein.

The rigidity and sedimentation rate of purified rabbit skeletal muscle F-actin was measured in the presence of three actin-gelling proteins: rabbit macrophage ac- tin-binding protein, chicken gizzard filamin, and rabbit skeletal muscle myosin. The rigidity at the yield pres- sure of 1 mg/ml of F-actin in 0.1 M KCI, pH 7.5, increased by 27 x lo-’ dynes/cm’ for each microgram/ml of actin-binding protein added, when the actin-binding protein concentration exceeded 3.2 X lo+’ M. The rigidity of F- actin under identical conditions increased by only 0.96 x lo-’ dynes/cm’/pg/ml of added filamin or myosin, and the minimum gelling concentrations of filamin and myosin were 1.2 X 10m7 and 2.3 x 10e7 M, respectively. The sedimentation rate of F-actin, 2 mg/ml, in 0.1 M KCI, pH 7.5, increased by 120% of control for each microgram/ml of added actin-binding protein and by 6% for each microgram/ml of