Ubiquitination of K-Ras Enhances Activation and Facilitates Binding to Select Downstream Effectors

Cancers in which K-Ras activation drives tumor growth could be targeted by treatments blocking K-Ras ubiquitination. Ubiquitination for Activity Mutational activation of the guanosine triphosphatase (GTPase) Ras occurs frequently in various cancers. Although the three mammalian isoforms—N-Ras, H-Ras, and K-Ras—couple to the same set of downstream signaling pathways, mutations in each isoform are associated with different types of cancers. In contrast to previous work showing that ubiquitination of N-Ras and H-Ras restricts their activity, Sasaki et al. found that monoubiquitination of Lys147 in K-Ras increased its activity and its ability to bind to downstream effector proteins. Cells expressing the oncogenic G12V mutant of Ras form tumors when injected into mice; however, cells expressing G12V Ras with an additional mutation abolishing ubiquitination of Lys147 formed smaller tumors in mice. Thus, cancers in which K-Ras activation drives tumor growth and survival could be targeted by treatments blocking K-Ras ubiquitination. The accompanying Perspective by Pfleger provides context on the distinct effects of ubiquitination on the abundance, activity, and access to effectors of different Ras isoforms. The guanosine triphosphate (GTP)–loaded form of the guanosine triphosphatase (GTPase) Ras initiates multiple signaling pathways by binding to various effectors, such as the kinase Raf and phosphatidylinositol 3-kinase (PI3K). Ras activity is increased by guanine nucleotide exchange factors that stimulate guanosine diphosphate release and GTP loading and is inhibited by GTPase-activating proteins that stimulate GTP hydrolysis. KRAS is the most frequently mutated RAS gene in cancer. Here, we report that monoubiquitination of lysine-147 in the guanine nucleotide–binding motif of wild-type K-Ras could lead to enhanced GTP loading. Furthermore, ubiquitination increased the binding of the oncogenic Gly12Val mutant of K-Ras to the downstream effectors PI3K and Raf. Thus, monoubiquitination could enhance GTP loading on K-Ras and increase its affinity for specific downstream effectors, providing a previously unidentified mechanism for Ras activation.

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