Self-Assembly & Spatial Structure in Actin Networks

Actin is an abundant cellular protein that self-assembles into filaments with lengths on the order of several microns. Because actin filaments are linear chain polymers, their mechanics are well described by polymer physics.

The assembly process is well-characterized, but coupling of energy dissipative reactions to polymerization complicates the picture. Actin filaments in solution form complex networks in which interactions between polymer chains become important. Interesting concentration-dependent phenomena occur in these systems, which are currently being investigated via a simple experimental in vitro model. The concentration regime in which a partial phase transition to a nematic liquid crystalline state occurs is focused upon. As predicted by Onsager, isotropic and anisotropic domains coexist in this region. Larger scale spatial structure, which is not predicted by theory, has also been observed. This structure is suggestive of pattern formation.

The dissipative biochemical reactions responsible for the phenomenon know as treadmilling guarantee that the actin networks under consideration are not in thermodynamic equilibrium. Preliminary results suggest that the presence of molecules involved in energy dissipative processes is correlated with the observed large scale structural variation. Therefore, investigation of the role of nonequilibrium processes in the creation of large scale ordering in this in vitro system are being pursued via advanced microscopy techniques. Using such methods, including polarization, confocal and multi-photon fluorescence, the structural variation can be quantitatively characterized. In this way it can be determined if the system can be truly classified as pattern forming.