The Most Oncogenic Ras Isoform

The small GTPase Ras is a bimolecular switch protein involved in a multitude of cellular signaling pathways, including those that control cell proliferation, differentiation, and apoptosis.

Guanine nucleotide exchange factors (GEFs) promote the exchange of GDP to GTP in the active site to yield signaling active Ras(GTP), while GTPase activating proteins (GAPs) accelerate the rate of hydrolysis of GTP to GDP on Ras to return the protein to its signaling inactive conformation.

The three isoforms of Ras, H-Ras, K-Ras, and N-Ras, share 100% sequence identity in their effector lobes, which contain the active site and switch regions responsible for interactions with effector and regulatory proteins.

The C-terminus of Ras is posttranslationally modified for insertion into the plasma membrane, where Ras interacts with a variety of effector proteins, such as Raf, PI3K, and RalGEF.

Importantly, residue differences in the allosteric lobe, which is 90% similar among the isoforms, could contribute to isoform-specific Ras-membrane interactions, though this is an understudied area of Ras biology.

Additionally, recent reports of dimerization and nanocluster formation at the membrane have emerged as important signaling units of Ras in the cell.

The work presented herein provides unique insights into Ras dimerization, with a particular focus on understanding dimerization at a molecular level, and its signaling implications.

Further, despite the fact that H-Ras has been used as a model for the other isoforms, we continue to show that K-Ras and N-Ras are structurally, biochemically, and functionally distinct.

The first crystal structure of wild-type K-Ras bound to a GTP analogue is presented, along with complementary NMR data to assess this isoforms' unique balance of conformational states in solution.

Additionally, a novel co-crystal structure of WT K-Ras and oncogenic K-RasG12D bound to a small protein inhibitor molecule provides unprecedented insight into how to target one of the most prevalent oncogenes in human cancers.