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First published online 25 July 2006
doi: 10.1242/jcs.03074

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Complex biosynthesis of the muscle-enriched iron regulator RGMc

Department of Biochemistry and Molecular Biology, Mail code L224, Oregon Health and Science University, 3181 SW Sam Jackson Park Road, Portland, Oregon 97239-3098, USA

^* Author for correspondence (e-mail: rotweinp{at}ohsu.edu)

Accepted 1 June 2006

The recently discovered repulsive guidance molecule c (RGMc or hemojuvelin) gene encodes a putative glycosylphosphatidylinositol (GPI)-anchored protein that is expressed in striated muscle and in liver. Mutations in this gene have been linked to the severe iron storage disease, juvenile hemochromatosis, although the mechanisms of action of RGMc in iron metabolism are unknown. As a first step toward understanding the molecular physiology of this protein, we studied its biosynthesis, processing and maturation. Production of RGMc occurs as an early and sustained event during skeletal muscle differentiation in culture and is secondary to RGMc gene activation. As assessed by pulse-chase studies and cell-surface labeling experiments, two classes of GPI-anchored and glycosylated RGMc molecules are targeted to the membrane and undergo distinct fates. Full-length RGMc is released from the cell surface and accumulates in extracellular fluid, where its half-life exceeds 24 hours. By contrast, the predominant membrane-associated isoform, a disulfide-linked heterodimer composed of N- and C-terminal fragments, is not found in the extracellular fluid, and is short-lived, as it disappears from the cell surface with a half-life of <3 hours after interruption of protein synthesis. A natural disease-associated RGMc mutant, with valine substituted for glycine at residue 320 (313 in mouse RGMc), does not undergo processing to generate the heterodimeric membrane-linked isoform of RGMc, and is found on the cell surface only as larger protein species. Our results define a series of biosynthetic steps leading to the normal production of different RGMc isoforms in cells, and provide a framework for understanding the biochemical basis of defects in the maturation of RGMc in juvenile hemochromatosis.

Key words: RGMc, Hemojuvelin, Muscle, Juvenile hemochromatosis

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