Uni-directional growth of F-actin.

In 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 pre-existing 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 arrow-head 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 developed by Oosawa (1,2).