MEN 2B (
multiple endocrine neoplasia type 2B) is an autosomal dominant
cancer syndrome caused by an oncogenic form of the
receptor tyrosine kinase REarranged during transfection (RET). The
MEN 2B syndrome is associated with an abnormal autophosphorylation of the mutated receptor even without
ligand-stimulation. Here, we characterize the activation of a RET(
MEN 2B) variant carrying the point mutation Met918Thr, and show that the 150 kDa precursor of RET(
MEN 2B) becomes phosphorylated already during synthesis in the endoplasmic reticulum (ER). At least three different
tyrosine residues (Tyr905, Tyr1062, Tyr1096) of the RET(
MEN 2B) precursor are phosphorylated before the oncogenic receptor reaches the cell surface. We also demonstrate that the precursor of RET(
MEN 2B) interacts with both
growth factor receptor-bound
protein and Src homology 2 domain-containing already in the ER, and that this interaction is dependent on the
kinase activity of RET. With the aid of two RET mutants (RET(
MEN 2B/S32L) and RET(
MEN 2B/F393L)), which accumulate in the ER, we show that the oncogenic precursor of the receptor has the capacity to activate AKT,
extracellular signal-regulated kinase and
signal transducer and activator of transcription 3 from the ER. Taken together, our data demonstrate that the oncogenic precursor of RET(
MEN 2B) is phosphorylated, interacts with adapter
proteins and induces downstream signalling from the ER.