Zonulin is a physiologic epithelial and endothelial permeability modulator. Zonulin increases antigen trafficking from the gut lumen into the bloodstream and in between body compartments, a mechanism linked to many chronic inflammatory diseases. Upon its initial discovery, it was noted that zonulin was not a single protein, but rather a family of structurally and functionally related proteins referred to as the zonulin family proteins (ZFPs). ZFPs are members of the mannose associated serine proteases (MASP) family and are the result of high mutation rates leading to many zonulin polymorphisms. Pre-haptoglobin 2, the precursor of haptoglobin 2, was identified as the first eukaryotic member of the ZFPs, and properdin, a key positive regulator of the alternative pathway, as a second member. In this study, we report two additional proteins that are likely ZFPs. Human coagulation factor X (FX) and CD5 antigen-like (CD5L). Both FX and CD5L recombinant proteins were detected by anti-zonulin antibody in Western immunoblot analysis, and both proteins decreased epithelial barrier competency of Caco-2聽cell monolayers as established by the Trans Epithelial Electrical Resistance (TEER) assay. These results indicate that FX and CD5L have structural and functional similarities with previously identified ZFPs and, therefore, can be considered new members of this family of proteins.
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