The C-terminal Kinase Domain-Binding and Suppression Motif Prevents Constitutive Activation of FGFR2

Genetic alterations in receptor tyrosine kinase genes can generate potent oncogenic drivers. Truncation of the FGFR2 gene by its last exon 18 (E18) is caused by structural alterations, such as focal amplifications and gene fusions/rearrangements, as well as by mutations. All the E18-truncating FGFR2 variants (FGFR2^ΔE18) act as strong driver alterations in cancer, and they commonly encode a receptor lacking the carboxy (C) terminal tail. In this study, we analyzed a compendium of Fgfr2-E18 variants to uncover the mechanism by which loss of the C-tail renders FGFR2 oncogenic. Although permutation of previously annotated C-terminal FGFR motifs did not recapitulate the tumorigenicity of FGFR2^ΔE18, the functional annotation efforts led to the discovery of a C-terminal phenylalanine–serine motif that mediates binding of the C-tail to the kinase domain and thereby suppresses FGFR2 kinase activity. The permutation of this kinase domain–binding and suppression motif in conjunction with other FGFR2-regulatory C-terminal sites fully phenocopied the oncogenic competence of FGFR2^ΔE18. Together, these findings delineate how the C-terminal tail prevents FGFR2 from aberrant oncogenic activation.