Reconstitution of Amoeboid Motility In Vitro Identifies a Motor-Independent Mechanism for Cell Body Retraction

SummaryCrawling movement in eukaryotic cells requires coordination of leading-edge protrusion with cell body retraction [1, 2 and 3]. Protrusion is driven by actin polymerization along the leading edge [4]. The mechanism of retraction is less clear; myosin contractility may be involved in some cells [5] but is not essential in others [6, 7, 8 and 9]. In Ascaris sperm, protrusion and retraction are powered by the major sperm protein (MSP) motility system instead of the conventional actin apparatus [ 10 and 11]. These cells lack motor proteins [12] and so are well suited to explore motor-independent mechanisms of retraction. We reconstituted protrusion and retraction simultaneously in MSP filament meshworks, called fibers, that assemble behind plasma membrane-derived vesicles. Retraction is triggered by depolymerization of complete filaments in the rear of the fiber [13]. The surviving filaments reorganize to maintain their packing density. By packing fewer filaments into a smaller volume, the depolymerizing network shrinks and thereby generates sufficient force to move an attached load. Our work provides direct evidence for motor-independent retraction in the reconstituted MSP motility system of nematode sperm. This mechanism could also apply to actin-based cells and may explain reports of cells that crawl even when their myosin activity is compromised.

► MSP comet tail fibers reconstitute protrusion and retraction simultaneously
► Retraction does not require a mechanochemical motor protein like myosin
► Retracting fibers shrink by packing their surviving filaments into a smaller volume
► Shrinkage of the fiber produces the force to power retraction