A role for actin arc contraction in cell flattening. A diagram depicting actin arc dynamics in a crawling cell. Actin filaments in the lamellipodium are created by polymerization at the leading edge of the cell (bottom of the image). A subset of these actin filaments are condensed into the actin filament bundles that make up a new actin arc (“actin arc formation”). Myosin II loads on to the newly forming actin arc, giving the actin arc the ability to contract. Dual green dots along the actin arc denote the two motor domains of a myosin II filament as revealed by SIM imaging. An actin arc contracts through myosin II activity and shortens along its length (“actin arc contraction”), thus leading to a decrease in the circumference of the actin arc as it moves away from the edge. As its shrinks, the actin arc pulls the non-adhesion-attached ends of DSFs closer together (“dorsal stress fibers brought together”). The force the actin arc exerts on DSFs is balanced by the attachment of the DSFs to the substrate. Thus, as the actin arc pulls the DSFs together, the dorsal contractile system moves closer to the ventral side of the cell, creating a flat lamella (“flattened lamella”). The broken line depicts the shape of the cell without actin arcs or DSFs (“no dorsal contractile network”).