Structural insights into the covalent regulation of PAPP-A activity by proMBP and STC2

Abstract

Originally discovered in the circulation of pregnant women as a protein secreted by placental trophoblasts, the metalloprotease pregnancy-associated plasma protein A (PAPP-A) is also widely expressed by many other tissues. It cleaves insulin-like growth factor-binding proteins (IGFBPs) to increase the bioavailability of IGFs and plays essential roles in multiple growth-promoting processes. While the vast majority of the circulatory PAPP-A in pregnancy is proteolytically inactive due to covalent inhibition by proform of eosinophil major basic protein (proMBP), the activity of PAPP-A can also be covalently inhibited by another less characterized modulator, stanniocalcin-2 (STC2). However, the structural basis of PAPP-A proteolysis and the mechanistic differences between these two modulators are poorly understood. Here we present two cryo-EM structures of endogenous purified PAPP-A in complex with either proMBP or STC2. Both modulators form 2:2 heterotetramer with PAPP-A and establish extensive interactions with multiple domains of PAPP-A that are distal to the catalytic cleft. This exosite-binding property results in a steric hindrance to prevent the binding and cleavage of IGFBPs, while the IGFBP linker region-derived peptides harboring the cleavage sites are no longer sensitive to the modulator treatment. Functional investigation into proMBP-mediated PAPP-A regulation in selective intrauterine growth restriction (sIUGR) pregnancy elucidates that PAPP-A and proMBP collaboratively regulate extravillous trophoblast invasion and the consequent fetal growth. Collectively, our work reveals a novel covalent exosite-competitive inhibition mechanism of PAPP-A and its regulatory effect on placental function.

Cryo-EM structure of human PAPP-A2 and mechanism of substrate recognition

“…We also observed calcium bound to two sites (Supplementary Fig. 2g ), analogous to calcium binding sites found in PAPP-A 18 , 19 .…”

“…Sequestered IGFs are liberated by cleavage of IGFBP proteins by PAPP-A2 and PAPP-A. While recent publications have described the structure of apo PAPP-A, PAPP-A bound to the anchor peptide region of IGFBP5, and PAPP-A bound to STC2 and proMBP proteins 17 – 19 , much less is known about PAPP-A2. Unraveling the non-redundant role of PAPP-A2 is important to understanding its modulation for IGF signaling.…”

“…Also, PAPP-A binds to cell surface glycosaminoglycans via its CCP3 and CCP4 domains, but PAPP-A2 does not 15 . PAPP-A was first experimentally shown to form a trans -homodimer 16 and was later visualized as a dimer by cryo-EM reconstruction in the apo form and when bound to IGFBP5 17 , and when bound to inhibitory proteins STC2 and proMBP 18 , 19 . Unlike PAPP-A, PAPP-A2 was originally shown to not form a covalent dimer by non-reducing polyacrylamide gel electrophoresis (PAGE) 6 , yet the majority of PAPP-A2 was shown to run at a similar migration as dimeric PAPP-A on native PAGE 16 .…”

Spatial proteomics and transcriptomics of placenta accreta spectrum

“…PRG2 overexpression has been previously described in PAS (11, 24). PAPPA is a metalloproteinase which acts as a growth promoter in many tissues through release of IGF (25). In normal placentation, EVTs fuse into placental bed giant cells at the maternal interface, where they upregulate the PGR2–PAPPA complex (21), suggesting this interaction between PGR2 and PAPPA/IGF may play a role in excess trophoblast invasion in PAS.…”

Unveiling the Role of Pregnancy-Associated Plasma Protein A (PAPP-A) in Pregnancy-Associated Breast Cancer: A Comprehensive Review

“…As a glycoprotein synthesized in the placenta, its concentration in maternal circulation escalates progressively throughout gestation [ 19 ]. PAPP-A assumes a crucial function in fetal development by catalyzing the cleavage of IGFBP-4 in the presence of IGF, thereby regulating fetal growth [ 20 ]. Furthermore, PAPP-A's significance transcends its role in pregnancy.…”