The propeptide of macrophage inhibitory cytokine (MIC-1), a TGF-β superfamily member, acts as a quality control determinant for correctly folded MIC-1

Macrophage inhibitory cytokine (MIC-1), a divergent member of the transforming growth facror-beta (TGF-beta) superfamily and activation associated cytokine, is secreted as a 28 kDa dimer, To understand its secretion, we examined its processing in MIC-l-transfected Chinese hamster ovary cells. Mature MIC-1 dimer arises post-endoplasmic reticulum (ER) by proteolytic cleavage of dimeric pro-MIC-l precursor at a furin-like site. Unlike previously characterized TGF-beta superfamily members, MIG-1 dimers are also secreted in constructs lacking the propeptide, A clue to the function of the propeptide came from the observation that a range of proteasome inhibitors, including lactacystin and MG132, cause major increases in levels of undimerized pro-MIC-l precursor. There was no effect of proteasome inhibitors on cells expressing mature MIG-1 without the propeptide, suggesting that the propeptide can signal misfolding of MIG-I, leading to proteasomal degradation, Deletion mutagenesis showed the N-terminal 28 amino acids of the propeptide are necessary for proteasomal degradation. This is the first demonstration, to our knowledge, of a quality control function in a propeptide domain of a secretory protein and represents an additional mechanism to ensure correct folding of proteins leaving the ER.