The Propeptide of the Transforming Growth Factor-β Superfamily Member, Macrophage Inhibitory Cytokine-1 (MIC-1), Is a Multifunctional Domain That Can Facilitate Protein Folding and Secretion

• Published in: The Journal of biological chemistry 2001-05, Vol.276 (20), p.16911
• Main Authors: W. Douglas Fairlie, Hong-Ping Zhang, Wan M. Wu, Susan L. Pankhurst, Asne R. Bauskin, Patricia K. Russell, Peter K. Brown, Samuel N. Breit
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M010000200

Macrophage inhibitory cytokine-1 (MIC-1) is a divergent member of the transforming growth factor-β (TGF-β) superfamily. While it is synthesized in a pre-pro form, it is unique among superfamily members because it does not require its propeptide for correct folding or secretion of the mature peptide. To investigate factors that enable these propeptide independent events to occur, we constructed MIC-1/TGF-β1 chimeras, both with and without a propeptide. All chimeras without a propeptide secreted less efficiently compared with the corresponding constructs with propeptide. Folding and secretion were most affected after replacement of the predicted major α-helix in the mature protein, residues 56–68. Exchanging the human propeptide in this chimera with either the murine MIC-1 or TGF-β1 propeptide resulted in secretion of the unprocessed, monomeric chimera, suggesting a specific interaction between the human MIC-1 propeptide and mature peptide. Propeptide deletion mutants enabled identification of a region between residues 56 and 78, which is important for the interaction between the propeptide and the mature peptide. Cotransfection experiments demonstrated that the propeptide must be in cis with the mature peptide for this phenomenon to occur. These results suggest a model for TGF-β superfamily protein folding.