National Doctoral Programme in
Informational and Structural Biology

Small GTPases of the Ras superfamily are central to critical cellular functions, such as proliferation, differentiation, migration and trafficking. Therefore, their misregulation is associated with severe diseases. More than 150 Ras-like GTPase are known, which are divided into four major subfamilies. Each of the subfamilies contains between 22 to 63 structurally related, but functionally distinct isoforms. Using a combination of computational biology, molecular cell biology and quantitative fluorescence imaging techniques, we recently provided new fundamental structural insight on how Ras operates in the context of the membrane. We showed that Ras adopts isoform specific orientations on the membrane, which in turn critically regulate Ras activity. These orientations are guided by a new switch III region and are stabilized by the amphipathic helix alpha 4 and the C-terminal HyperVariable Region (HVR). Intriguingly, these structural elements vary from one isoform to another not only for Ras-, but also for Rho- and Rab-proteins. This suggests that different combinations of the HVR and helix alpha 4 tune the membrane orientation of small GTPases to direct isoform specific functions (balance-model). Here we propose to further elucidate molecular mechanistic details of this new orientation-switch in Ras-, Rho- and Rab-proteins. One important aspect will be to identify the link between the orientation-switch and the dynamics of the organisation of Ras into nanoscale assemblies on the membrane (nanocluster). In addition, we want to learn more about the involvement of the orientation-switch in diseases and develop assays to find novel specific drugs against it.

Our results have the potential to substantially transform the current understanding of GTPase functioning and answer the long-standing fundamental biological question of GTPase isoform specificity.