Role of major sperm protein (MSP) in the protrusion and retraction of Ascaris sperm.

Nematode sperm offer a unique perspective for investigating amoeboid cell motility. These cells display the hallmark features of amoeboid movement but power their locomotion with a cytoskeleton composed of major sperm protein (MSP) filaments in place of the familiar actin cytoskeleton found in other crawling cells. Thus, properties of sperm can be compared to those of actin-rich cells to identify the shared features that are essential to motility. Sperm are simple cells in which cytoskeletal dynamics are tightly coupled to protrusion of the leading edge and retraction of the cell body. These features have facilitated reconstitution of both protrusion and retraction in cell-free extracts and enabled identification of accessory components in the motility apparatus as well as elucidation of the mechanical basis of movement. Six MSP accessory proteins have been isolated including four components of the sperm cytoskeleton and two enzymes that play key roles in regulating cytoskeletal dynamics and locomotion. Analysis of this versatile in vitro motility system has identified motor-independent mechanisms for protrusion and retraction that are based on changes in filament-packing density. These changes result in expansion and contraction of the MSP-filament network that generate the forces for movement. We discuss how the mechanisms of motility that operate in nematode sperm may contribute generally to the movement of crawling cells.