Figure 7. Schematic of mechanical actions underpinning cell turning.

A mechanical model of cell turning in keratocytes. (left panel) The act of turning creates the “centrifugal” accumulation of myosin bound to the actin filaments to the outer side of the cell. (center panel) Myosin accumulation on the outer side of a turning cell, increases local myosin contractility and the centripetal flow of actin slipping over the substrate on the outer side of the cell driving turning. Typical actin meshwork flow from the substrate’s frame of reference is shown. Positive connectors indicate positive feedback between asymmetric actin flow and myosin accumulation.

(right panel) Increased myosin contractility and actin flow on the outer side of the turning cell breaks adhesions on the outer edge of the cell (SLIP), weak traction forces (colored vectors) and further promoting the centripetal sliding of the outer actin network over the substrate and consequently turning. Conversely, STICK conditions on the inner side of a turning cell, create strong adhesion and large traction forces. Pinching traction forces perpendicular to the direction of movement are asymmetric. Traction forces at the leading edge are propulsive, and at the rear – resistive. Thus, the elevated contractility and flow on the outer edge inhibit the formation of strong adhesion on the outer edge, which would otherwise inhibit turning (Negative connectors indicate double negative feedback between asymmetric actin flow and adhesion strength). The traction forces and asymmetric adhesions are explained further in Box 1.