Receptor tyrosine kinases (RTKs) are a class of cell-surface receptors that activate Ras and Raf-MEK-ERK signaling pathways to mediate many important cell-cell communication processes. In the absence of signal, RTKs exist predominantly as individual monomers, and their intracellular kinase domains are in an inactive conformation. Upon binding signal, RTKs dimerize, leading to the activation of their kinase domains and triggering downstream events. Protein structure models suggest that the activated dimers are asymmetric, in which one monomer acts as an "activator," and the other as a substrate "receiver." Using C. elegans, we have discovered a system in which simple, but powerful genetic tools can be used to probe this mechanism of kinase activation in RTKs. Genetic analysis of EGL-15, the C. elegans fibroblast growth factor receptor (FGFR), has provided many insights into the molecular mechanisms of RTK signaling. In the hypodermis, EGL-15 dysregulation causes defects that include the hypermorphic "clear" (Clr) phenotype and the hypomorphic "suppressor of clear" (Soc) phenotype. We have used
egl-15 heterozygous genotypes which express two different forms of EGL-15 on the surface of their cells to understand how the structures of EGL-15 heterodimers affect kinase activation.
egl-15(
n1457) is a nonsense mutation that truncates the EGL-15 carboxy-terminal domain (CTD). Based on the
egl-15(
n1457 deltaCTD) homozygote phenotype, this truncated protein forms EGL-15 homodimers with compromised activity, but that are sufficiently active to be at the Clr/Soc phenotypic threshold. Interestingly, we discovered that heterozygotes between
egl-15(
n1457 deltaCTD) and several
egl-15 substitution mutations have less activity than
egl-15(
n1457 deltaCTD)/nDf19, a deletion that eliminates
egl-15. These results suggest that certain EGL-15 mutant monomers can inhibit the activity of EGL-15(deltaCTD) monomers when the two dimerize. We have utilized a quantitative fluorescent biosensor of ERK activation, ERK-nKTR, to visualize the impact of
egl-15 dysregulation on signal transduction. We find that ERK is strongly activated in the hypodermis of genotypes that display the hypermorphic Clr phenotype. By combining this quantitative assay with our analysis of mutant alleles that inhibit
egl-15(
n1457 deltaCTD), we hope to gain insight into the molecular mechanism of kinase activation.