Functional characterization of ProSAAS: similarities and differences with 7B2

Prohormone convertases (PC) 1 and 2, enzymes found primarily in neuroendocrine tissues, are thought to mediate the proteolytic cleavage of many peptide precursors. To date, endogenous binding proteins for both PC2 (7B2) and PC1 (proSAAS) have been identified. Although 7B2 represents a potent inhibitor of PC2, the most important function of 7B2 as regards this enzyme appears to be the absolute requirement of PC2 for 7B2 in the generation of active enzyme, recently corroborated through production of a null animal that lacks PC2 activity. The purpose of the present study was to determine whether proSAAS exerts effects on PC1 other than inhibition, and to establish functional similarities and differences between 7B2 and proSAAS. We first asked whether the N-terminal domain of proSAAS (proSAAS-(1-180)) could stabilize PC1 activity, similar to the effect of the N-terminal domain of 7B2 on PC2. Recombinant His-tagged proSAAS-(1-180) had no effect on PC1 activity in vitro and was unable to protect PC1 from thermal denaturation. Transient cotransfection of proSAAS-(1-225) cDNA with PC1 cDNA into HEK 293 cells reduced the amount of PC1 activity detected in the medium. Surprisingly, cotransfection of proSAAS-(1-180) cDNA, encoding a protein that lacks the inhibitory C-terminal domain peptide, also reduced the activity of PC1 detected in the medium, but the mass of PC1 secreted into the medium was increased, suggesting a proSAAS-mediated inactivation reaction. Similar results were observed in CHO/PC1 cells stably transfected with pro-SAAS-(1-180). Stable transfection of SAAS cDNAs into AtT-20 cells was used to examine the role of proSAAS in a neuroendocrine setting. Unlike 7B2, proSAAS-(1-225) was able to slow convertase-mediated processing of proopiomelanocortin and proenkephalin; however, similarly to 7B2, proSAAS expression did not result in any accumulated differences in the content of cellular processed peptide. In summary, although both proSAAS and 7B2 potently inhibit PC enzymes via a C-terminal peptide, their intracellular interactions with PCs appear to differ significantly, with each binding protein exhibiting unique properties.