Association between the melanoma-inducing receptor tyrosine kinase Xmrk and src family tyrosine kinases in Xiphophorus.

Melanoma formation in the fish Xiphophorus is an in vivo model for the function of receptor tyrosine kinases (RTKs) in tumor development. The overexpression and high activity of the RTK Xmrk (Xiphophorus melanoma receptor kinase) is responsible for the formation of hereditary malignant melanoma in this fish, but the mechanism by which Xmrk signals cell proliferation has not been elucidated. Remarkably, in earlier experiments an elevated level of a pp60c-src related kinase activity was found in the melanomas. In order to evaluate the significance of src family SH2 domain interactions in the intracellular signalling of Xmrk, we determined its relative binding affinity to the ubiquitous general RTK substrate, PLC gamma, and to the Xiphophorus cytoplasmic kinases Xsrc, Xfyn and Xyes. Recombinant Xmrk purified from baculovirus infected Sf9 cells bound with high affinity to the SH2 domains of PLC gamma and Xfyn in vitro. The affinity of Xmrk to Xsrc and Xyes SH2 domains was 5- to 10-fold lower. Coprecipitation experiments revealed that the Xmrk/Xfyn interaction occurred also in melanoma cells. Moreover, stimulation of the Xmrk kinase activity was paralleled by an increase in Xfyn activity. These results suggest that in malignant melanoma of Xiphophorus the highly activated Xmrk may enhance the activity of Xfyn through direct interaction and that both kinases are linked in a signal transduction pathway.