Generation of a truncated hepatocyte growth factor receptor in the endoplasmic reticulum

The hepatocyte growth factor (HGF) receptor (p190(MET)) is a tyrosine kinase composed of two disulfide-linked chains, α of 50 kDa and β of 145 kDa. We have previously described an isoform (p140(MET)) containing a β chain of 85 kDa, lacking the cytoplasmic kinase domain. The two receptor variants originate by post-translational processing of a common single-chain precursor of 170 kDa (Pr170). In the endoplasmic reticulum a fraction of Pr170 is cleaved at the cytosolic side generating an intermediate product of 120 kDa (Pr120). This molecule 1) is already detectable after 15 min of pulse labeling, 2) contains high mannose-branched oligosaccharides, and 3) accumulates upon treatments inhibiting the export from the endoplasmic reticulum. A second cleavage, occurring after 30 min of chase in the trans- Golgi network, converts the single-chain precursors Pr170 and Pr120 into the mature heterodimers p190(MET) and p140(MET). This process is inhibited by brefeldin A treatment. Conditions leading to Pr170 accumulation in the endoplasmic reticulum, such as receptor overexpression, induce kinase activation and overproduction of Pr120. Conversely, cells expressing a kinase-defective HGF receptor lack the truncated isoform. The proteolytic cleavage of the cytoplasmic domain may thus represent a safety mechanism aimed at preventing ligand-independent intracellular activation of the HGF receptor kinase.