The
Eph receptor tyrosine kinases and their
ephrin ligands regulate many physiological and
pathological processes. EphA4 plays important roles in nervous system development and adult homeostasis, while aberrant EphA4 signaling has been implicated in neurodegeneration. EphA4 may also affect
cancer malignancy, but the regulation and effects of EphA4 signaling in
cancer are poorly understood. A correlation between decreased patient survival and high EphA4
mRNA expression in
melanoma tumors that also highly express ephrinA
ligands suggests that enhanced EphA4 signaling may contribute to
melanoma progression. A search for EphA4 gain-of-function mutations in
melanoma uncovered a mutation of the highly conserved
leucine 920 in the EphA4 sterile alpha motif (SAM) domain. We found that mutation of L920 to
phenylalanine (L920F) potentiates EphA4 autophosphorylation and signaling, making it the first documented EphA4
cancer mutation that increases
kinase activity. Quantitative Föster resonance energy transfer and fluorescence intensity fluctuation (FIF) analyses revealed that the L920F mutation induces a switch in EphA4 oligomer size, from a dimer to a trimer. We propose this switch in oligomer size as a novel mechanism underlying EphA4-linked
tumorigenesis. Molecular dynamics simulations suggest that the L920F mutation alters EphA4 SAM domain conformation, leading to the formation of EphA4 trimers that assemble through two aberrant SAM domain interfaces. Accordingly, EphA4 wild-type and the L920F mutant are affected differently by the SAM domain and are differentially regulated by
ephrin ligand stimulation. The increased EphA4 activation induced by the L920F mutation, through the novel mechanism we uncovered, supports a functional role for EphA4 in promoting pathogenesis.