Enzyme coated beads can grow multiple “comet tails”
FEM simulations showing formation of multiple “comet tails”
Actin Based Motility
Animal cells are rich in a protein called actin that performs many key functions from providing mechanical structure (as a key component cytoskeleton), to generating forces needed for cell movement. Pathogenic bacteria, such as listeria, rickettsia and salmonella move by co-opting this actin machinery. An enzyme coating causes a host cell's actin to preferentially polymerize at the surface of the bacteria. The gel which forms exerts force both on the bacteria and the surrounding medium. This propels the bacterium, leaving behind a "comet tail" of gelled actin. Understanding these motility mechanisms may give us new treatments for infection, and also help elucidate other, more ubiquitous aspects of actin function and motility.
The biophysical processes involved are often studied through experiments with polystyrene beads coated with the enzymes present on motile bacteria. Even in a mixture of pure proteins resembling the cytoplasm, these beads will spontaneously break symmetry and exhibit bacteria-like motility. The main part of my thesis addressed how this symmetry breaking occurs. Working from data on beads that formed multiple tails, I modeled the process as a surface instability driven by the competition between elastic energy and chemical energy at the surface of the expanding actin gel. My results on tail number selection agree with data showing that this process is controlled by bead size, enzyme coating density and the concentration of the actin related protein gelsolin, which enhances actin gel degradation. In order to explain the formation of regular, symmetric helical comet tails on beads with two tails, I did computer simulations based on slender body theory. I showed that under certain conditions, hydrodynamic forces alone can induce a helical shape in a growing rigid object (Balter, Tang Phys. Rev. E 71, 051912 (2005)).