Thermal activation of F-actin and reconstituted thin filaments at pCa 9. (A) Effects of IR laser irradiation on the motility of F-actin (top) or reconstituted thin filaments (bottom). Merged fluorescence images before (left), during (middle), and after (right) heating are shown. Heat source is indicated as a small white circle (denoted as “Heat source”) in the middle panels marked in pink. Images show 2 s before heating, at the onset of heating, and 2 s after heating, respectively, in the left, middle, and right panels (as indicated on the bottom in green). Images in red represent trajectories of the movement of filaments during 2 s (as indicated on bottom). Images in yellow represent immobile filaments. See Videos 1 and 2 for F-actin and reconstituted thin filaments, respectively. Scale bars, 20 μm. (B) Time courses of sliding velocity of F-actin (top) or reconstituted thin filaments (bottom) located at various distances from the heat source (see color map on right) before, during, and after heating. Heating was given for 2 s from 0 to 2 s (marked in pink). Note similar sliding velocities were obtained upon heating for F-actin and reconstituted thin filaments.