Differences in the Regulation of K-Ras and H-Ras Isoforms by Monoubiquitination

Ras GTPases are signaling switches that control critical cellular processes including gene expression, differentiation, and apoptosis. The major Ras isoforms (K, H, and N) contain a conserved core GTPase domain, but have distinct biological functions. Among the three Ras isoforms there are clear differences in post-translational regulation, which contribute to differences in localization and signaling output. Modification by ubiquitination was recently reported to activate Ras signaling in cells, but the mechanisms of activation are not well understood. Here, we show that H-Ras is activated by monoubiquitination and that ubiquitination at Lys-117 accelerates intrinsic nucleotide exchange, thereby promoting GTP loading. This mechanism of Ras activation is distinct from K-Ras monoubiquitination at Lys-147, which leads to impaired regulator-mediated GTP hydrolysis. These findings reveal that different Ras isoforms are monoubiquitinated at distinct sites, with distinct mechanisms of action, but with a common ability to chronically activate the protein in the absence of a receptor signal or oncogenic mutation. Background: Ras proteins are critical regulators of cellular growth and are differentially modified by ubiquitination. Results: Chemical ubiquitination and immunoprecipitation assays demonstrate that monoubiquitination causes sustained H-Ras activation in the absence of oncogenic mutations. Conclusion: The mechanism by which H-Ras is activated by monoubiquitination is both isoform-specific and site-specific. Significance: Monoubiquitination adds a new level of regulation and complexity to isoform-specific Ras signaling.