Tyrosine kinase activity of the ret proto-oncogene products in vitro

We investigated tyrosine kinase activity of the ret proto-oncogene products (proto-Ret proteins), using a cell lysate of NB-39-nu neuroblastoma cells. The 150 kDa and 170 kDa proto-Ret proteins immunoprecipitated with antibodies against their carboxy-terminal 20 amino acids were shown to be phosphorylated predominantly on tyrosine residues in immunocomplex kinase assay. The level of tyrosine phosphorylation of the 150 kDa proto-Ret protein was approximately 10-fold higher than that of the 170 kDa proto-Ret protein, although both proteins were expressed at similar levels in neuroblastoma cells. This result was confirmed by using a lysate of SK-N-MC human primitive neuroectodermal tumor cells transfected with the ret proto-oncogene. The kinase activity of proto-Ret proteins was significantly inhibited by antibodies against their kinase domain, indicating that these antibodies recognize crucial epitopes for the enzymatic activity. On the other hand, the proto-Ret proteins were not phosphorylated in vivo in NB-39-nu cells and SK-N-MC transfectants.