Determining which way to go: How G-proteins and membrane microdomains direct cell polarization and the chemotropic response

The ability to sense and respond correctly to extracellular cues is paramount for cells such as those in the immune system and nervous system that navigate through these environments using ligand gradients as their guide. Understanding how these subtle spatial directions are transduced through the membrane, traverse signaling networks, and modulate the cell's internal compass are paramount to discovering how invading pathogens are tracked, axons find their targets, and differentiating cells find their niche.;I set out to test the ability of the yeast mating system to accurately sense and project in microfluidically generated spatial gradients of mating pheromone. I found that yeast cells exhibit good projection accuracy, and have excellent sensitivity to shallow gradients. Their ability to robustly sense gradients over a 1000-fold range of pheromone concentrations, places the mating pathway's dynamic range on par with other G-protein coupled systems. Mutations that disrupt heterotrimeric G-protein signaling caused shifts in the system's dynamic range. Mathematical modeling revealed insights into mechanisms to amplify external spatial signals and produce highly polarized proteins, and how pathway sensitivity can disrupt accurate polarization.;Interestingly, yeast cells adaptively responded to microfluidically generated gradients in which the direction was switched 180°, increasing robustness and accuracy by bending their projection or forming a second projection. This behavior is dependent on the levels of G-protein (heterotrimeric or Cdc42) activity and mediated by spatial competition for Cdc24 and polarisome activation. Similar mechanisms may also underlie other chemotropic responses, such as neutrophil tracking, where signaling environments change as the cell progresses through its environment.;These findings show how G-protein activity affects directional sensing and polarization through signaling and spatial protein localization. But what is the role of the cell membrane itself and does membrane heterogeneity (lipid rafts) contribute to polarization and directional sensing? Lipid raft markers were found to polarize in response to pheromone resulting in a "lipid macrodomain" where mating proteins localize. Disruption of ergosterol results in modest defects in polarization and the inability to directionally sense. Mating proteins exhibit two lateral membrane diffusion rates that do not differ between the front and back of the polarized cell.