The propeptide of the transforming growth factor-beta superfamily member, macrophage inhibitory cytokine-1 (MIC-1), is a multifunctional domain that can facilitate protein folding and secretion

Macrophage inhibitory cytokine-1 (MIC-1) is a divergent member of the trans forming growth factor-beta (TGF-beta) superfamily, While it is synthesized in a pre-pro form, it is unique among superfamily members because it does n ot require its propeptide for correct folding or secretion of the mature pe ptide. To investigate! factors that enable these propeptide independent eve nts to occur, we constructed MIC-1/TGF-beta1 chimeras, both with and withou t a propeptide, All chimeras without a propeptide secreted less efficiently compared with the corresponding constructs with propeptide. Folding and se cretion were most affected after replacement of the predicted major alpha - helix in the mature protein, residues 56-68, Exchanging the human propeptid e in this chimera with either the murine MIC-1 or TGF-beta1 propeptide resu lted in secretion of the unprocessed, monomeric chimera, suggesting a speci fic interaction between the human MIC-1 propeptide and Mature peptide. Prop eptide deletion mutants enabled identification of a region between residues 56 and 78, which is important for the interaction between the propeptide a nd the mature peptide. Cotransfection experiments demonstrated that the pro peptide must be in cia with the mature peptide for this phenomenon to occur . These results suggest a model for TGF-beta superfamily protein folding.