Curvature and torsion in growing actin networks

TL;DR

This study measures the 3D curvature and torsion of actin-driven bead paths, revealing slow curvature changes and no significant torsion bias, which informs understanding of intracellular pathogen motility.

Contribution

It provides direct 3D measurements of actin-propelled bead trajectories, linking curvature dynamics to motility mechanisms and challenging previous force-based models.

Findings

Bead paths tend to have low curvature at any time.

Path curvature changes slowly over approximately 200 seconds.

No significant torsion bias observed in 3D motion.

Abstract

Intracellular pathogens such as Listeria monocytogenes and Rickettsia rickettsii move within a host cell by polymerizing a comet-tail of actin fibers that ultimately pushes the cell forward. This dense network of cross-linked actin polymers typically exhibits a striking curvature that causes bacteria to move in gently looping paths. Theoretically, tail curvature has been linked to details of motility by considering force and torque balances from a finite number of polymerizing filaments. Here we track beads coated with a prokaryotic activator of actin polymerization in three dimensions to directly quantify the curvature and torsion of bead motility paths. We find that bead paths are more likely to have low rather than high curvature at any given time. Furthermore, path curvature changes very slowly in time, with an autocorrelation decay time of 200 seconds. Paths with a small radius of curvature, therefore, remain so for an extended period resulting in loops when confined to two dimensions. When allowed to explore a 3D space, path loops are less evident. Finally, we quantify the torsion in the bead paths and show that beads do not exhibit a significant left- or right-handed bias to their motion in 3D. These results suggest that paths of actin-propelled objects may be attributed to slow changes in curvature rather than a fixed torque.