Biosynthetic processing and quaternary interactions of proprotein convertase SPC4 (PACE4)

Abstract

SPC4 (PACE4), a member of the eukaryotic family of subtilisin‐like proprotein convertases, is synthesized as a proenzyme (proSPC4) which undergoes proteolytic removal of N‐terminal propeptide during transit through the secretory pathway. As this propeptide processing seems to be a key event in the functional expression of SPC4, we have investigated its mechanism and the intracellular site where it occurs. In transfected fibroblast cells, the 110‐kDa proSPC4 undergoes slow cleavage to generate a 103‐kDa mature enzyme in the endoplasmic reticulum (ER). Site‐directed mutagenesis studies demonstrate that the proteolytic activation of SPC4 occurs mainly through a unimolecular autocatalytic process and propeptide cleavage is a prerequisite for its export from the ER. Sedimentation velocity and chemical cross‐linking analysis demonstrate that the precursor protein in the cells exists as both a monomer and a dimer‐sized complex whereas mature SPC4 exists only as a monomer. These results suggest that the cleavage of the N‐terminal propeptide of SPC4 plays a regulatory role in its activation and secretion through the change in its oligomeric state.