A quantitative analysis of contractility in active cytoskeletal protein networks

¹ Niels Bohr Institute/Harvard University
² Harvard University
³ Brandais University/Rowland Insititute
⁴ Harvard Medical School

^* To whom correspondence should be addressed. E-mail: weitz{at}seas.harvard.edu.

Submitted on July 24, 2007
Revised on August 19, 2007
Accepted on 6 December 2007

	Abstract

Cells actively produce contractile forces for a variety of processes including cytokinesis and motility. Contractility is known to rely on myosin II motors which convert chemical energy from ATP hydrolysis into forces on actin filaments. However the basic physical principles of cell contractility remain poorly understood. We reconstitute contractility in a simplified model system of purified F-actin, muscle myosin II motors, and -actinin cross-linkers. We show that contractility occurs above a threshold motor concentration and within a window of cross-linker concentrations. We also quantify the pore size of the bundled networks and find contractility to occur at a critical distance between the bundles. We propose a simple mechanism of contraction based on myosin filaments pulling neighboring bundles together into an aggregated structure. Observations of this reconstituted system in both bulk and low-dimensional geometries show that the contracting gels pull on and deform their surface with a contractile force of about 1 µN, or about 100 pN per F-actin bundle. Cytoplasmic extracts contracting in identical environments show the same type of behavior and dependence on myosin as the reconstituted system. Our results suggest that cellular contractility can be sensitively regulated by tuning the (local) activity of molecular motors and the cross-linker density and binding affinity.

Key Words: Actin, Active cytoskeletal networks, Contractility, Myosin II, Xenopus, alpha-actinin