Cell surface detachment of pregnancy-associated plasma protein-A requires the formation of intermolecular proteinase-inhibitor disulfide bonds and glycosaminoglycan covalently bound to the inhibitor

The metzincin metalloproteinase pregnancy-associated plasma protein-A (PAPP-A, pappalysin-1) promotes cell growth by proteolytic cleavage of insulin-like growth factor-binding proteins 4 and 5, causing the release of bound insulin-like growth factors. PAPP-A binds an unknown cell-surface heparan sulfate proteoglycan, suggesting that it controls insulin-like growth factor signaling spatially. In human pregnancy, the majority of PAPP-A circulates as a disulfide-bonded complex with its inhibitor, the proform of eosinophil major basic protein (proMBP). Interestingly, Ser-62 of proMBP is substituted with a glycosaminoglycan (GAG) chain, possibly a heparan sulfate type, and the PAPP-A ·proMBP complex is unable to bind to the cell surface. We show here that proMBP detaches surface-bound PAPP-A in a process that depends on the proMBP GAG and also on the formation of intermolecular disulfide bonds between PAPP-A and proMBP. Unlike what was expected, we demonstrate that the GAG of proMBP is not required for PAPP-A ·proMBP complex formation and that proMBP residues His-137, Ser-178, Arg-179, and Asn-181 are important for the recognition of PAPP-A. Using a mouse model, we find that the half-life of circulating PAPP-A and proMBP in complex is severalfold higher than both of the uncomplexed proteins, further suggesting that the PAPP-A·proMBP complex is formed at the cell surface in vivo rather than in the circulation. Further supporting this, we show that formation of the PAPP-A·proMBP complex at the cell surface proceeds rapidly compared with the slow rate of complex formation in solution. Because both PAPP-A and proMBP are expressed ubiquitously, this model may be applicable to many tissues in which insulin-like growth factor bioavailability is locally regulated.