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

Abstract

Macrophage inhibitory cytokine (MIC‐1), a divergent member of the transforming growth factor‐β (TGF‐β) superfamily and activation associated cytokine, is secreted as a 28 kDa dimer. To understand its secretion, we examined its processing in MIC‐1‐transfected Chinese hamster ovary cells. Mature MIC‐1 dimer arises post‐endoplasmic reticulum (ER) by proteolytic cleavage of dimeric pro‐MIC‐1 precursor at a furin‐like site. Unlike previously characterized TGF‐β superfamily members, MIC‐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‐1 precursor. There was no effect of proteasome inhibitors on cells expressing mature MIC‐1 without the propeptide, suggesting that the propeptide can signal misfolding of MIC‐1, 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.