Uni-directional growth of F-actin

It was shown that the growth of F-actin occurs uni-directionally in vitro; that is, actin monomers (G. ADP-actin) polymerize onto only one of the two ends of a preexisting seed in the absence of ATP. Fragments of the F-actin-heavy meromyosin complex prepared by sonication were used as seeds. By studying a large number of electron micrographs, we concluded that growth occurs on the tail end of the arrowhead structure of the seed. This result suggests that the thin filament grows in vivo from the center of the sarcomere towards the Z-line. It was further shown that the fragments of F-actin combine with both ends of the seed with nearly equal probability. These results are well interpreted according to the theory deveoloped by Oosawa(1,2).