Revisiting RAS family GTPase signaling: effector selectivity and oncogenic bypass

Distinct effector-binding preferences among RAS family GTPases challenge the longstanding view that canonical RAS proteins uniformly bind and activate RAF, PI3Kα, RalGDS, and other downstream effectors. Quantitative binding data, supported by structural insights into effector recognition, instead reveal a division of labor: the canonical RAS subfamily (KRAS, HRAS, NRAS) binds RAF kinases with high affinity, the RRAS subfamily (RRAS2 and MRAS) preferentially engages PI3Kα, and the RAP subfamily (RAP1A and RAP1B) shows the strongest binding to RalGDS. These intrinsic preferences, encoded in the switch regions and further shaped by isoform and effector expression, as well as subcellular localization, establish a hierarchy in which canonical RAS, RRAS2/MRAS, and RAP1A/B primarily activate RAF, PI3Kα, and RalGDS, respectively, in normal cells. Oncogenic mutations at codons G12, G13, or Q61 disrupt this hierarchy by driving sustained accumulation of GTP-bound canonical RAS, enabling engagement of lower-affinity effectors such as PI3Kα and RalGDS. In addition, certain mutations, including KRAS-G12D and -G12V, modestly enhance PI3Kα binding, representing a neomorphic expansion of effector engagement. Together, these effects bypass intrinsic effector selectivity, allowing canonical RAS to co-opt effectors normally associated with other RAS subfamilies and broaden downstream signaling. This framework explains how inherent effector preferences govern normal signaling and how oncogenic mutations override these constraints to expand effector engagement in RAS-driven cancers.