Development of protein-based inhibitors of the proprotein of convertase SKI-1/S1P: processing of SREBP-2, ATF6, and a viral glycoprotein.

Processing of membrane-bound transcription factors such as sterol regulatory element-binding proteins (SREBPs) and the ER-stress response factor ATF6, and glycoproteins of some hemorrhagic fever viruses are initiated by the proprotein convertase SKI-1/S1P. So far, no cellular protein-based inhibitor of the hydrophobic-amino acid specific SKI-1 is known. The prosegment of the basic-amino acid specific convertases (e.g. furin and PC5) or alpha(1)-PDX, a variant of alpha(1)-antitrypsin (alpha(1)-AT) exhibiting an RIPR(358) sequence at the reactive site loop, were shown to potently inhibit these secretory proteinases. Accordingly, we tested the SKI-1-inhibitory potential of various point mutants of either the 198 amino acid preprosegment of SKI-1-(1-198) or alpha(1)-AT. Transient transfections data showed that, out of numerous mutants studied, the R134E prosegment mutant or the alpha(1)-AT reactive site loop variants RRVL(358), RRYL(358) and RRIL(358) are the best specific cellular inhibitors of SKI-1. The observed inhibition of the processing of endogenous SREBP-2, exogenous ATF6 and a PDGF-A (RRLL(86)) variant were >55% and reach approximately 80% in stable transfectants. We also show that SKI-1 forms SDS-stable complexes with these alpha(1)-AT variants, but not with wild-type alpha(1)-AT or alpha(1)-PDX. Finally, these inhibitors were also shown to affect the processing and stability of the Crimean-Congo hemorrhagic fever virus glycoprotein.