The Balance Model explains Ras-Isoform Specificity by Sequence Variations on Helix alpha-4 and the Hypervariable Region

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 GTPases are known, which are divided into four major subfamilies. Each of the subfamilies contains between 22 to 63 structurally related, but functionally distinct isoforms. For almost two decades, the lipid modified C-terminal HyperVariable Region (HVR) of small GTPases was recognized as the primary structural determinant for isoform specificity. However, a mechanistic explanation as to how the HVR realizes this was missing.

Using a combination of computational biology, molecular cell biology and quantitative fluorescence imaging techniques, we recently provided new structural insight on how Ras operates in the context of the membrane. We provided evidence 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 HVR. We propose the ‘balance model’, where different combinations of the HVR and helix alpha-4 tune the membrane orientation equilibrium of small GTPases to direct isoform specific functions.