Theories of amoeboid-like cell movement have largely relied on cytoskeleton polymerization to explain membrane protrusion. But in studying filaments of the worm major sperm protein (MSP) using electron tomography, the authors noted that filaments became more loosely packed as they elongated.
“Polymer physicists have known for a long time that shorter rods pack more tightly,” says Roberts. By contrast, longer rods, regardless of composition, enclose more empty space. Measurements from the new images were consistent with this effect, and simulations confirmed that growing rods exhibited the same length-dependent space constraints as static ones. An MSP mutant that polymerized 2.4 times more slowly reduced the protrusion rate by 3.4-fold, indicating that both polymerization and packing-based expansion contribute to protrusion.
“It's plausible that this effect may occur in actin systems as well,” says Roberts, although actin-based protrusion differs in its molecular details, including the use of cross-linking proteins.