Structural insights into the complex of oncogenic KRas4B^G12V and Rgl2, a RalA/B activator

About a quarter of total human cancers carry mutations in Ras isoforms. Accumulating evidence suggests that small GTPases, RalA, and RalB, and their activators, Ral guanine nucleotide exchange factors (RalGEFs), play an essential role in oncogenic Rasinduced signalling. We studied the interaction between human KRas4B and the Ras association (RA) domain of Rgl2 (Rgl2(RA)), one of the RA-containing RalGEFs. We show that the G12V oncogenic KRas4B mutation changes the interaction kinetics with Rgl2(RA). The crystal structure of the KRas4B(G12V): Rgl2(RA) complex shows a 2:2 heterotetramer where the switch I and switch II regions of each KRasG12V interact with both Rgl2(RA) molecules. This structural arrangement is highly similar to the HRas(E31K):RALGDS(RA) crystal structure and is distinct from the well-characterised Ras: Raf complex. Interestingly, the G12V mutation was found at the dimer interface of KRas4B(G12V) with its partner. Our study reveals a potentially distinct mode of Ras:effector complex formation by RalGEFs and offers a possible mechanistic explanation for how the oncogenic KRas4B(G12V) hyperactivates the RalA/B pathway.