The C-terminal region of proSAAS is a potent inhibitor of prohormone convertase 1

ProSAAS is a recently discovered 26-kDa neuroendocrine protein that was pre viously found to inhibit prohormone convertase (PC) 1 and not PC2, In the p resent study, the specificity of proSAAS toward other members of the prohor mone convertase family was determined. Two mu M proSAAS selectively inhibit s PC1 but not furin, PACE4, PC5A, or PC7. The PC1 inhibitory region of proS AAS was mapped to an 8-12-residue region near the C terminus that includes a critical Lys-Arg sequence. Synthetic peptides corresponding to this regio n are competitive inhibitors of PC1 with apparent K-i values of 14-40 nM. T he inhibition becomes more effective with incubation time, indicating that the inhibitor is slow binding. A fusion protein containing the inhibitory r egion of proSAAS linked to the C terminus of glutathione S-transferase bind s the 71-kDa form but not the 85-kDa form of PC1. This binding, which occur s at pH 5.5 and not at pH 7.4, is stable to incubation at room temperature for 1 h in the presence or absence of 0.5% Triton X-100 and/or 0.5 M NaCl. The removal of Ca2+ with chelating agents partially releases the bound PC1. High concentrations of the inhibitory peptide quantitatively release the b ound PC1, Taken together, these data support the proposal that proSAAS func tions as an endogenous inhibitor of PC1.