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Curvature and torsion in growing actin networks

Joshua W Shaevitz1,3 and Daniel A Fletcher2

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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 s. 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 three-dimensional (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, possibly associated with filament debranching, rather than a fixed torque.


PACS

87.17.Jj Cell locomotion, chemotaxis

87.16.Ka Filaments, microtubules, their networks, and supramolecular assemblies

82.35.Pq Biopolymers, biopolymerization

87.14.E- Proteins

87.15.H- Dynamics of biomolecules

Subjects

Soft matter, liquids and polymers

Biological physics

Chemical physics and physical chemistry

Dates

Issue 2 (June 2008)

Received 5 September 2007, accepted for publication 19 May 2008

Published 16 June 2008

 
Image from Curvature and torsion in growing actin networks


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