Pro-protein convertases control the maturation and processing of the iron-regulatory protein, RGMc/hemojuvelin

David Kuninger , Robin Kuns-Hashimoto , Mahta Nili and Peter Rotwein

Department of Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, Oregon, 97239-3098, USA

author email corresponding author email

BMC Biochemistry 2008, 9:9doi:10.1186/1471-2091-9-9


Published:	2 April 2008

Abstract

Background

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.

Conclusion

Our results define a role for PCs in the maturation of RGMc that may have implications for the physiological actions of this critical iron-regulatory protein.