GDF 15 ( growth differentiation factor 15 )

The premature GDF/PDF/MIC-1 protein consists of 308 amino acids that contain a 29 amino acid signal peptide, a 167 amino acid propeptide, and a 112 amino acid mature protein. The mature protein is secreted as a homodimer linked by disulfide bonds and is released from the propeptide following intracellular cleavage at RXXR furine-like cleavage site. The mature peptide of GDF-1/MIC-1 contains two additional cysteine residues in addition to the seven conserved cysteines necessary for the cysteine knot, a structural hallmark of this TGF-β superfamily. The exact function of these two additional cysteine residues is still unknown. The propeptide has a consensus N-linked glycosylation site in it. Unlike all other TGF-β superfamily members, MIC-1 mature peptide can be correctly folded and secreted without a propeptide. The propeptide plays a novel role in proteosomal targeting of the monomeric precursor and ensures that only dimeric precursor exists in the endoplasmic reticulum.