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

Macrophage inhibitory cytokine (MIC-1), a divergent member of the transform ing growth facror-beta (TGF-beta) superfamily and activation associated cyt okine, is secreted as a 28 kDa dimer, To understand its secretion, we exami ned 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 cha racterized 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 expr essing mature MIG-1 without the propeptide, suggesting that the propeptide can signal misfolding of MIG-I, leading to proteasomal degradation, Deletio n mutagenesis showed the N-terminal 28 amino acids of the propeptide are ne cessary for proteasomal degradation. This is the first demonstration, to ou r knowledge, of a quality control function in a propeptide domain of a secr etory protein and represents an additional mechanism to ensure correct fold ing of proteins leaving the ER.