Bio-filament polymerization driven phenomena : experiments and simulations

Abstract

Actin, as one of the most abundant proteins in eukaryotic cells, has been known to play essential roles in many important biological processes. For example, the movement of living cells is driven by the polymerizing actin network formed at their leading edges. Significant progresses have been made over the past decade in identifying key proteins involved in actin-driven motility allowing researchers to examine such phenomenon with a reconstituted system. Nevertheless, fundamental questions like whether self-spinning/rotating will take place in actin-propelled cargos, what kind of trajectories will the cargos follow and whether the movement will be influenced by the cargo size still remain unclear.

In this study, we systematically examined the actin-driven movements of different sized microspheres, coated with polymerization activator N-WASP, in both 2D and 3D environments. Small fluorescent markers were attached to the beads enabling us to precisely measure their spinning/rotation when moving forward. Interestingly, it was found that the curvature probability distribution for trajectories of beads moving within a confined plane is Gaussian-like while that for 3D trajectories exhibits a peak at a non-zero curvature value. Furthermore, the torsion distribution for paths traced out by beads moving in 3D is symmetric with respect to zero torsion, that is, it shows no bias towards right-handed or left-handed motion. Surprisingly, no apparent self-spinning of actin-propelled beads was observed, in direct contrast to the moving Listeria which has been known to rotate along its long axis. We also demonstrated that cargo size has a profound effect on all aspects of actin-propelled motility, with larger beads leading to slower movements coupled with straighter traveling paths. Interestingly, lager cargos will also inhibit symmetry breaking of the actin cloud and hence suppress the initiation of movement powered by actin polymerization. In addition to greatly enhance our basic understanding on actin-based motility, results here are also expected to provide critical guidance in the development of future theories.

Besides cargo movement, the unsettling question of how plasma membrane of cells deforms in response to propulsive forces generated by polymerization was also considered in this thesis. Specifically, choosing closed mitosis as a representative example, we showed that the dramatic morphology change of the nucleus of Schizosaccharomyces pombe is due to the poleward force generated by growing microtubules that is transmitted to the nuclear membrane through its physical contact with the separating chromatids. Depending on the size of load-transmitting regions near the two spindle poles, the nucleus can undergo symmetric or asymmetric division where the occurrence of shapes like spherical cylinder, dumbbell, pear and tether have all been predicted. On the other hand, improper separation of chromosomes in genetically deficient cells can lead to membrane tethering, in excellent agreement with our experimental observations.