Actin filaments elongate rapidly. (A) An actin filament is tracked during successive images collected at ∼3-s intervals. It undergoes a breakage event at t = 90 s (arrow) and the newly created filament end (open arrowhead) remains stationary or shrinks slightly during the next 30 s. At t = 116 s, the filament end (open arrowhead) begins to grow and extends out of the field of view at t = 129 s. The average rate of growth for this filament, determined as shown in D, was 0.97 µm/s. A second filament, first appearing at t = 116 s, grows upward and toward the right at a rate of 1.7 µm/s. See Video 4 (available). Bar, 2 µm. (B) Kymograph of the filament highlighted in A. The filament origin or base is at the right and the elongation extends toward the left. (C) Plot of pixel intensity as a function of position along the filament shown in A. The traces show two different time points: gray line, 100 s; black line, 132 s. Filament origin is plotted at the right and growth occurs toward the left. A black arrow marks the location where this filament crosses over another filament (see also Fig. 2 A; 132 s), resulting in a doubling of fluorescence intensity. The solid black line at 2000 represents the upper limit of pixel intensity for an individual actin filament, as determined with the analyses shown in Fig. S3. (D) Actin filament length as a function of time for several representative filaments is plotted and a line of best fit used to estimate rate of elongation. Closed squares, filament marked with open arrowheads in A; open circles, second filament growing upward and toward right in A; closed circles, example filament from a different time-lapse series.