Thiol isomerase ERp57 targets and modulates the lectin pathway of complement activation

J Biol Chem. 2019 Mar 29;294(13):4878-4888. doi: 10.1074/jbc.RA118.006792. Epub 2019 Jan 22.


ER protein 57 (ERp57), a thiol isomerase secreted from vascular cells, is essential for complete thrombus formation in vivo, but other extracellular ERp57 functions remain unexplored. Here, we employed a kinetic substrate-trapping approach to identify extracellular protein substrates of ERp57 in platelet-rich plasma. MS-based identification with immunochemical confirmation combined with gene ontology enrichment analysis revealed that ERp57 targets, among other substrates, components of the lectin pathway of complement activation: mannose-binding lectin, ficolin-2, ficolin-3, collectin-10, collectin-11, mannose-binding lectin-associated serine protease-1, and mannose-binding lectin-associated serine protease-2. Ficolin-3, the most abundant lectin pathway initiator in humans, circulates as disulfide-linked multimers of a monomer. ERp57 attenuated ficolin-3 ligand recognition and complement activation by cleaving intermolecular disulfide bonds in large ficolin-3 multimers, thereby reducing multimer size and ligand-binding affinity. We used MS to identify the disulfide-bonding pattern in ficolin-3 multimers and the disulfide bonds targeted by ERp57 and found that Cys6 and Cys23 in the N-terminal region of ficolin-3 form the intermolecular disulfide bonds in ficolin-3 multimers that are reduced by ERp57. Our results not only demonstrate that ERp57 can negatively regulate complement activation, but also identify a control mechanism for lectin pathway initiation in the vasculature. We conclude that extensive multimerization in large ficolin-3 multimers leads to a high affinity for ligands and strong complement-activating potential and that ERp57 suppresses complement activation by cleaving disulfide bonds in ficolin-3 and reducing its multimer size.

Keywords: ER protein 57 (ERp57); complement system; disulfide; ficolin; innate immunity; lectin pathway; mass spectrometry (MS); redox regulation; thiol isomerase.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Complement Pathway, Mannose-Binding Lectin*
  • Glycoproteins / genetics
  • Glycoproteins / metabolism*
  • Humans
  • Lectins / genetics
  • Lectins / metabolism*
  • Protein Disulfide-Isomerases / genetics
  • Protein Disulfide-Isomerases / metabolism*
  • Protein Multimerization*
  • Proteolysis*


  • FCN3 protein, human
  • Glycoproteins
  • Lectins
  • Protein Disulfide-Isomerases
  • PDIA3 protein, human