The PSI Domain of the MET Oncogene Encodes a Functional Disulfide Isomerase Essential for the Maturation of the Receptor Precursor

Abstract

The tyrosine kinase receptor encoded by the MET oncogene has been extensively studied. Surprisingly, one extracellular domain, PSI, evolutionary conserved between plexins, semaphorins, and integrins, has no established function. The MET PSI sequence contains two CXXC motifs, usually found in protein disulfide isomerases (PDI). Using a scrambled oxidized RNAse enzymatic activity assay in vitro, we show, for the first time, that the MET extracellular domain displays disulfide isomerase activity, abolished by PSI domain antibodies. PSI domain deletion or mutations of CXXC sites to AXXA or SXXS result in a significant impairment of the cleavage of the MET 175 kDa precursor protein, abolishing the maturation of α and β chains, of, respectively, 50 kDa and 145 kDa, disulfide-linked. The uncleaved precursor is stuck in the Golgi apparatus and, interestingly, is constitutively phosphorylated. However, no signal transduction is observed as measured by AKT and MAPK phosphorylation. Consequently, biological responses to the MET ligand—hepatocyte growth factor (HGF)—such as growth and epithelial to mesenchymal transition, are hampered. These data show that the MET PSI domain is functional and is required for the maturation, surface expression, and biological functions of the MET oncogenic protein.

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“…4 C and 4D). Little is known about PSI domains, particularly their direct biochemical function [ 95 ]; however, they are often subunits of larger, functional often extracellular receptor domains [ 93 ] where they have at least a structural role in signal transduction [ 96 ]. We hypothesise that the remaining unannotated protein core represents a larger domain, within which the PSI domain is a subunit.…”

The MET Oncogene: Thirty Years of Insights into Molecular Mechanisms Driving Malignancy

“…The receptor encoded by the MET gene is a dimeric protein proteolytically processed and glycosylated from a precursor of 170kDa ( Figure 1 A): The short α-chain (50kDa) is exposed to the surface of the cell and is covalently bound through disulfide bridges to the long transmembrane β-chain (145kDa) consisting of (i) an extracellular domain forming the functional domain called SEMA (Semaphorin), containing the binding site for the HGF factor ( Figure 1 B), with the α-chain [ 27 , 28 , 29 , 30 ]; (ii) a plexin–semaphorin–integrin (PSI) homology domain endowed with disulfide exchange isomerase activity [ 31 ]; and (iii) four IPT (immunoglobulin-like, plexins, transcription factors) domains, two of which (IPT3 and 4) contain a second high-affinity site for HGF binding [ 32 ].…”

“…exposed to the surface of the cell and is covalently bound through disulfide brid long transmembrane β-chain (145kDa) consisting of (i) an extracellular domai the functional domain called SEMA (Semaphorin), containing the binding site fo factor (Figure 1B), with the α-chain [27][28][29][30]; (ii) a plexin-semaphorin-integrin mology domain endowed with disulfide exchange isomerase activity [31]; and IPT (immunoglobulin-like, plexins, transcription factors) domains, two of wh and 4) contain a second high-affinity site for HGF binding [32]. A short transmembrane segment joins the extracellular to the intracellul that contains the functional domain endowed with tyrosine kinase activity [10].…”

An Observatory for the MET Oncogene: A Guide for Targeted Therapies

“…The extracellular portion of the β chain comprises a semaphorin (SEMA) domain, a Plexin–Semaphorin–Integrin (PSI) homology domain, and four immunoglobulin-like IPT domains [ 29 , 30 ]. SEMA and IPT domains are crucial for ligand binding and receptor dimerization, while the PSI domain is essential for the proper maturation of the receptor through its recently described disulfide isomerase activity [ 31 ]. The intracellular part of MET is composed of a short juxtamembrane domain (JM), followed by the catalytic site and the docking site for signal transducers.…”