Cells contain at least three different types of contractile actin-based stress fibers. Transverse arcs do not associate with focal adhesions on either end, whereas ventral stress fibers contact focal adhesions on both ends. Dorsal stress fibers have one end tied to a focal adhesion on the ventral side of the cell and frequently attach to a transverse arc with the other end.
Using live cell microscopy and a variety of fluorescently tagged proteins, Hotulainen and Lappalainen found that dorsal stress fibers and transverse arcs underwent continuous de novo formation and disassembly. The ventral fibers, in contrast, were formed by fusion of dorsal and transverse fibers.
Dorsal fiber formation initiated at focal adhesions and was dependent on the mDia1 formin, which has been implicated previously in assembly of unbranched actin filaments. Transverse arcs arose from the fusion of short actin bundles, which themselves formed in the leading edge of the lamellipodium and then drifted back into the center of the cell. Depletion of mDia1 formin had no impact on transverse arc formation, but disruption of Arp2/3 activity prohibited assembly of the short actin bundles in the leading edge and their aggregation into transverse arcs.
Additionally, transverse arcs fell apart rapidly in response to myosin II inactivation. The dorsal stress fibers also eventually broke down in the myosin-depleted cells, but appeared to rely less on the protein for their structural integrity.
The RhoA GTPase signaling pathway is known to be involved in stress fiber assembly, activating formins and inhibiting the activity of actin depolymerization agents. The question now, say the researchers, is how RhoA and other signaling pathways control two distinct stress fiber assembly processes.