The RET proto-oncogene encodes two isoforms of a receptor tyrosine kinase which 
plays a role in neural crest and kidney development. Ret ligands have been 
recently identified as the neuron survival factor GDNF (Glial-Derived 
Neurotrophic Factor) and Neurturin. Somatic rearrangements of RET, designated 
RET/PTCs, have been frequently detected in papillary thyroid carcinomas. In 
addition, distinct germ-line mutations of RET gene have been associated with the 
inherited cancer syndromes MEN (Multiple Endocrine Neoplasia) 2A, 2B and FMTC 
(Familial Medullar Thyroid Carcinomas) as well as with the congenital megacolon 
or Hirschsprung's disease, thus enlightening a significant role of this receptor 
gene in diverse human pathologic conditions. In this study, by performing 
classical inhibition experiments using synthetic phosphopeptides and by 
site-directed mutagenesis of the putative docking site, we have determined that 
for Grb2 the latter is provided by the tyrosine 620 of Ret/ptc2 long isoform 
(corresponding to Tyr 1096 on proto-Ret). However, in intact cells, the 
interaction of Grb2 with the two short and long Ret isoforms expressed 
separately is of similar strength, thus suggesting that Ret short isoform 
interaction with Grb2 could be mediated not only by Shc but also by a molecule 
that binds preferentially to this isoform. This possibility is supported by the 
evidence that the mutant Ret/ptc2Y620F long isoform displays a weak 
coimmunoprecipitation with Grb2 and that this mutant, lacking the docking site 
for Grb2 but owing all the others phosphotyrosines, surprisingly displays a 
reduced transforming activity compared to that of the two WTs oncogenes. We thus 
conclude that in intact cells both Ret isoforms bind to Grb2, although with 
different modalities. In addition, the present results are in agreement with the 
possibility that different signal transduction pathways are associated with the 
two isoforms of Ret.