We study the rheological properties of reconstituted actin networks by tracking the thermal motion of embedded micron-sized probe beads with four types of surface coatings. For the most slippery beads, thermal motion causes those smaller than the network mesh size to percolate through the network or hop from one confinement "cage" to another. Consequently, the smaller beads sense a weaker network. This trend is reversed for three other types of beads, which detect an apparently stiffer network due to the physisorption of nearby filaments to the bead surface. We also confirm the existence of a depletion layer around non- or weakly-sticky probe surfaces by confocal imaging. Analysis of these effects is necessary in order to accurately define the actin network rheology both in vitro and in vivo. We investigate microrheological properties of F-actin across the isotropic-nematic phase transition region by both video particle tracking and laser deflection particle tracking. As the nematic order parameter increases with actin concentration, the storage modulus in the perpendicular direction grows faster and larger than that in the perpendicular direction. Furthermore, we find that the viscoelasticity of F-actin network varies with the magnesium concentration more sensitively in the nematic phase than in the isotropic phase. In all, particle tracking microrheology reveals rich rheological features of F-actin affected by the isotropic-nematic phase transition and by tuning weak electrostatic interactions among the protein filaments. To address the network dynamics, we observe an abnormal slowdown of the longitudinal diffusion of F-actin across the isotropic-nematic phase transition region. We also find that the F-actin diffusion across the transition region markedly differs from the diffusion of microtubule and fd virus in F-actin solutions. Additionally, the viscous drag probed by F-actin is found to increase sharply with magnesium concentration in the nematic but not in the isotropic state. Based on these results, we propose that the abnormal slowdown is caused by the counterion induced transient association between parallel actin filaments in the nematic phase.
He, Jun,
"Microrheology and Dynamics of F-actin Networks"
(2009).
Physics Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://doi.org/10.7301/Z01J9812
