Mechanism of transfer of NO from extracellular S-nitrosothiols into the cytosol by cell-surface protein disulfide isomerase

The ADAM disintegrin metalloproteinases (where ADAM is ‘a disintegrin and metalloproteinase’) are a family of transmembrane cell-surface proteins with essential roles in adhesion and proteolytic processing in all animals. The archetypal family member is ADAM17 {also known as TACE [TNFα (tumour necrosis factor α)-converting enzyme]}, which is involved in processing pro-TNFα and in the activation of ligands for the EGFR [EGF (epidermal growth factor) receptor], as well as cleavage of diverse cell-surface receptors and adhesion molecules. ADAM-mediated shedding is itself influenced via cell signalling pathways. In this issue of the Biochemical Journal, Willems et al. make the observation that phorbol ester activates shedding by ADAM17 by affecting the activity of PDI (protein disulfide isomerase). They propose that PDI maintains ADAM17 in an inactive ‘closed’ state and PMA stimulation generates ROS (reactive oxygen species) and thus an altered redox environment, which in turn inactivates PDI and allows ADAM17 to adopt an ‘open’ active conformation. This activation is accompanied by changes in disulfide bonds in the ADAM17 ectodomain. This is a novel and exciting finding that could help to unlock the actions of ADAM sheddases, as well as a host of other mechanisms that rely upon rapid alterations in protein conformation on the cell surface.

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Thrombin-induced shedding of tumour endothelial marker 5 and exposure of its RGD motif are regulated by cell-surface protein disulfide-isomerase

Biochemical Journal ◽

10.1042/bj20111682 ◽

2012 ◽

Vol 441 (3) ◽

pp. 937-944 ◽

Cited By ~ 14

Author(s):

Mario Vallon ◽

Philipp Aubele ◽

Klaus-Peter Janssen ◽

Markus Essler

Keyword(s):

Cell Surface ◽

Disulfide Bonds ◽

Protein Disulfide Isomerase ◽

Surface Protein ◽

Surface Binding ◽

Disulfide Isomerase ◽

Rgd Motif ◽

Thrombin Cleavage ◽

Protein Disulfide ◽

Endothelial Marker

TEM5 (tumour endothelial marker 5; also known as GPR124) is an adhesion G-protein-coupled receptor containing a cryptic RGD motif in its extracellular domain. TEM5 is expressed in endothelial cells and pericytes during angiogenesis. In the present paper, we report that thrombin mediates shedding of an N-terminal TEM5 fragment of 60 kDa (termed N60) containing the RGD motif in an open conformation. Thrombin directly cleaved rsTEM5 (recombinant soluble TEM5) 5 and 34 residues downstream of the RGD motif, resulting in formation of N60 and its C-terminal counterpart (termed C50). Interestingly, N60 derived from thrombin cleavage of rsTEM5 was covalently linked to C50 by disulfide bonds, whereas N60 shed from thrombin-treated cells was not associated with its membrane-bound C-terminal counterpart. Inhibition of the reducing function of cell-surface PDI (protein disulfide-isomerase) abrogated thrombin-induced N60 shedding. Conversely, addition of reduced PDI enhanced N60 shedding. Furthermore, thrombin cleavage of rsTEM5 was increased by reduced PDI and resulted in dissociation of the N60–C50 heterodimer. We conclude that PDI regulates thrombin-induced shedding of N60 and exposure of the TEM5 RGD motif by catalysing the reduction of crucial disulfide bonds of TEM5 on the cell surface. Binding of N60 to RGD-dependent integrins may modulate cellular functions such as adhesion and migration during angiogenesis.

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