The N-Glycans Modulate the Binding of ADAMTS13 with von Willebrand Factor.

ADAMTS13, a circulating multidomain protease consisting a metalloprotease (MP), a disintegrin, a thrombospondin type 1 repeat [TSR], a cysteine rich region, a spacer, 7 additional TSR and 2 CUB domains, is critical for maintaining the homeostasis of von Willebrand factor (VWF). Severe deficiency of the enzyme can lead to VWF-platelet thrombosis in the microcirculation characteristic of thrombotic thrombocytopenic purpura (TTP). Sequence analysis predicts that ADAMTS13 contains 2 and 6 likely N-glycosylation sites respectively at or near the MP and spacer domains, and 2 possible sites in the CUB domains. To investigate the N-glycosylation status and the role of N-glycans in the function of ADAMTS13, we digested plasma derived ADAMTS13 and recombinant ADAMTS13 or its truncated variants with peptide-N-glycosidase F (PNGase F) or endoglycosidase H. SDS PAGE analysis showed that digestion with PNGase F, but not with endo H, decreased the molecular weights of plasma derived ADAMTS13 as well as recombinant ADAMTS13 proteins, including AD7 (residues 1-1427, full-length), AD5 (residues 1-745, MP-TSR#1), AD2 (residues 1-448, MP-TSR#1), AD8 (residues 556-748, spacer-TSR#2) and AD13 (residues 1016-1427, TSR#8-CUB#2), indicating that the MP, spacer and CUB domains were all N-glycosylated as predicted, and contained complex type carbohydrate chains. Enzyme linked immunoassay showed that removal of the N-glycans did not affect the binding of proteases to immobilized VWF multimers or VWF73 peptide. Removal of the N-glycans modestly affected the protease-TTP IgG binding, increasing the Kd by 190% for AD7 and by 50% for AD5. Expression experiments in Lec1 mutant Chinese hamster ovary (CHO) cells with defective GlcNAc-TI activity due to mutations in the Mgat1 gene showed that the resulting high-mannose type AD7 and AD5 variant proteases were secreted normally, showed no difference in VWF cleaving and TTP IgG binding activities, but exhibited increased binding to VWF multimers or VWF73 peptide. In summary, ADAMTS13 are N-glycosylated with complex type oligosaccharides at the MP, spacer and CUB domains. Intracellular conversion from high mannose to complex type glycans modulates the binding of the protease to VWF, suggesting that this process may help prevent depletion of the protease under conditions of high VWF levels in the circulation.