Pro-protein convertases control the maturation and processing of the iron-regulatory protein, RGMc/hemojuvelin
Repulsive guidance molecule c (RGMc or hemojuvelin), a glycosylphosphatidylinositol-linked glycoprotein expressed in liver and striated muscle, plays a central role in systemic iron balance. Inactivating mutations in the RGMc gene cause juvenile hemochromatosis (JH), a rapidly progressing iron storage disorder with severe systemic manifestations.
RGMc undergoes complex biosynthetic steps leading to membrane-bound and soluble forms of the protein, including both 50 and 40 kDa single-chain species.
Results: We now show that pro-protein convertases (PC) are responsible for conversion of 50 kDa RGMc to a 40 kDa protein with a truncated COOH-terminus.
Unlike related molecules RGMa and RGMb, RGMc encodes a conserved PC recognition and cleavage site, and JH-associated RGMc frame-shift mutants undergo COOH-terminal cleavage only if this site is present. A cell-impermeable peptide PC inhibitor blocks the appearance of 40 kDa RGMc in extra-cellular fluid, as does an engineered mutation in the conserved PC recognition sequence, while the PC furin cleaves 50 kDa RGMc in vitro into a 40 kDa molecule with an intact NH2-terminus.
Iron loading reduces release of RGMc from the cell membrane, and diminishes accumulation of the 40 kDa species in cell culture medium.
Conclusions: Our results support a key role for PCs in maturation of RGMc that has implications for the physiological actions of this critical iron-regulatory protein.
Author: David Kuninger, Robin Kuns-Hashimoto, Mahta Nili and Peter Rotwein
Credits/Source: BMC Biochemistry 2008, 9:9
Published on: 2008-04-02
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