(
2011-08)
Shimabukuro, Katsuya; Noda, Naoki; Stewart, Murray; Roberts, Thomas M.
Crawling movement in eukaryotic cells requires coordination of leading edge protrusion
with cell body retraction [1-3]. 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 [6-9]. In Ascaris sperm, protrusion
and retraction are powered by the major sperm protein (MSP) motility system instead of
the conventional actin apparatus [10-11]. 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. Thus, this work provides direct evidence for motorindependent
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.
