BIOSYNTHETIC PROCESSING AND QUATERNARY INTERACTIONS OF PROPROTEIN CONVERTASE SPC4 (PACE4)

SPC4 (PACE4), a member of the eukaryotic family of subtilisin-like pro protein convertases, is synthesized as a proenzyme (proSPC4) which und ergoes proteolytic removal of N-terminal propeptide during transit thr ough 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 transfec ted fibroblast cells, the 110-kDa proSPC4 undergoes slow cleavage to g enerate a 103-kDa mature enzyme in the endoplasmic reticulum (ER), Sit e-directed mutagenesis studies demonstrate that the proteolytic activa tion of SPC4 occurs mainly through a unimolecular autocatalytic proces s and propeptide cleavage is a prerequisite for its export from the ER , Sedimentation velocity and chemical cross-linking analysis demonstra te that the precursor protein in the cells exists as both a monomer an d a dimer-sized complex whereas mature SPC4 exists only as a monomer, These results suggest that the cleavage of the ill-terminal propeptide of SPC4 plays a regulatory role in its activation and secretion throu gh the change in its oligomeric state. (C) 1998 Federation of European Biochemical Societies.