Ligand-regulated binding of FAP68 to the hepatocyte growth factor receptor.

We have used the yeast two-hybrid system to identify proteins that interact with the intracellular portion of the hepatocyte growth factor (HGF) recep tor (Met). We isolated a human cDNA encoding a novel protein of 68 kDa, whi ch we termed FAP68. This protein is homologous to a previously described FK 506-binding protein-associated protein, FAP48, which derives from an altern ative spliced form of the same cDNA, lacking an 85-nucleotide exon and lead ing to an early stop codon. Here we show that epithelial cells, in which th e HGF receptor is naturally expressed, contain FAP68 and not FAP48 proteins . FAP68 binding to Met requires the last 30 amino acids of the C-terminal t ail, which are unique to the HGF receptor. Indeed, FAP68 does not interact with related tyrosine kinases of the Met and insulin receptor families. FAP 68 interacts specifically with the inactive form of HGF receptor, such as a kinase-defective receptor or a dephosphorylated wild type receptor. In viv o, endogenous FAP68 can be coimmunoprecipitated with the HGF receptor in th e absence of stimuli and not upon HGF stimulation. Thus, FAP68 represents a novel type of effector that interacts with the inactive HGF receptor and i s released upon receptor phosphorylation. Free FAP68 exerts a specific stim ulatory activity toward the downstream target p70 S6 protein kinase (p70S6K ). Significantly, nonphosphorylated HGF receptor prevents FAP68 from stimul ating p70S6K. These data suggest a role for FAP68 in coupling HGF receptor signaling to the p70S6K pathway.