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

Area of Science:

• Molecular Biology
• Cell Signaling
• Biochemistry

Background:

• The hepatocyte growth factor (HGF) receptor, p190MET, is a tyrosine kinase crucial for cellular processes.
• An alternative isoform, p140MET, lacks the cytoplasmic kinase domain.
• Both receptor variants arise from a common 170 kDa precursor (Pr170).

Purpose of the Study:

• To elucidate the post-translational processing of the HGF receptor.
• To investigate the generation of p190MET and p140MET isoforms.
• To determine the role of proteolytic cleavage in HGF receptor regulation.

Main Methods:

• Pulse-chase labeling experiments to track protein processing.
• Analysis of protein modifications including glycosylation (high mannose oligosaccharides).
• Inhibition studies using brefeldin A and examination of receptor overexpression and kinase-defective mutants.

Main Results:

• A fraction of Pr170 undergoes initial cleavage in the endoplasmic reticulum to form Pr120.
• Pr120 contains high mannose oligosaccharides and accumulates when ER export is inhibited.
• A second cleavage in the trans-Golgi network converts Pr170 and Pr120 into mature p190MET and p140MET, a process sensitive to brefeldin A.
• Receptor overexpression leads to Pr170 accumulation, increased kinase activation, and Pr120 overproduction.
• Kinase-defective HGF receptors lack the truncated isoform.

Conclusions:

• Proteolytic cleavage of the HGF receptor precursor is a multi-step process occurring in the ER and trans-Golgi network.
• The generation of the truncated p140MET isoform is linked to kinase activity.
• Proteolytic cleavage of the cytoplasmic domain serves as a crucial safety mechanism to prevent ligand-independent HGF receptor kinase activation.