MITOGENIC SIGNALING BY RET PTC2 REQUIRES ASSOCIATION WITH ENIGMA VIA A LIM DOMAIN/

The ret/ptc2 papillary thyroid cancer oncogene, an oncogenic form of t he c-Ret receptor tyrosine kinase, is the product of a somatic crossov er event fusing the dimerization domain of the type I alpha regulatory subunit of cyclic AMP-dependent protein kinase (RI) with the tyrosine kinase domain of c-Ret. Mitogenic activity of Ret/ptc2 required dimer ization via the N terminus of RI and a tyrosine residue located C-term inal to the kinase core of Ret, Tyr-586 (Durick, K., Yao, V. J., Borre llo, M. G., Bongarzone, I., Pierotti, M. A. and Taylor, S. S. (1995) J . Biol. Chem. 270, 24642-24645). Using the yeast two-hybrid system, Re t/ptc2 binding proteins were identified, and the sites of interaction with Ret/ptc2 were mapped. The SH2 domains of phospholipase C gamma an d Grb10 were both identified, and binding depended on phosphorylation of Tyr-539 and Tyr-429, respectively. These interactions, however, wer e not required for mitogenic signaling. The second of the three LIM do mains in Enigma (Wu, R. Y., and Gill, G. N. (1994) J. Biol. Chem. 269, 25085-25090) was also identified as a Ret/ptc2 binding domain. Enigma , a 455-residue protein, was discovered based on its interaction with the insulin receptor through the C-terminal LIM domain. Although the a ssociation with Enigma required Tyr-586 of Ret/ptc2, the interaction w as phosphorylation-independent. In contrast to the SH2 interactions, d isruption of the interaction with Enigma abolished Ret/ptc2 mitogenic signaling, suggesting that LIM domain recognition of an unphosphorylat ed tyrosine-based motif is required for Ret signal transduction.