Symmetry breaking in navigating cells: The role of heterotrimeric and monomeric G-proteins in Dictyostelium chemotaxis

Chemotaxis is the property of cells to migrate towards gradients of chemicals. It is a highly dynamic process that involves directional sensing, cellular polarity and cell motility. Chemotaxis is important for all organisms for many processes including, immune reponse, embryo organization and it’s also rsponsible for development of many diseases including asthma, cancer, and cardiovascular diseases. It is mediated by a complex network of interconnecting signaling pathways, amplification loops, and feedback control mechanisms. The aim of this thesis was to identify and characterize essential components of the chemotactic signaling pathway. We proposed a basal signal module for Dictyostelium chemotaxis, which serves as model organism for chemotaxis studies. The basal signaling module consists of G-proteins and cytoskeletal machinery which is sufficient for bringing a chemotactic response. Next we found that activation of monomeric G-protein Ras has three phases; each dependent upon activation of different G-protein subunits, specific RasGEFs, RasGAPs, cGMP formation, and cytoskeleton proteins. Our novel mass-pull down proteomic strategy led us to discover several additional components of this basal signaling module. Ric8 was characterized as a non-receptor GEF for Galpha proteins, thus playing an important role in non-canonical G-protein cycle. Likewise, several other proteins that can serve as important link in the signaling pathways were identified, and they still need to be characterized. Therefore this thesis not only provides new insights in the molecular mechanisms underlying regulation of G-protein signaling and chemotaxis, but also establishes the foundation for new exciting research