Spn4A construct [58] into an adenoviral shuttle vector to generate Spn4A.RRKR(r) ( Figure 1A ). Spn4A is a unique ''retained'' (r) serpin that presents, at its C-terminus, an HDEL sequence ( Figure 1A) , a functional variant of the C-terminal KDEL retention signal that directs secretory protein retention in the ER [59] . Because SKI-1/S1P cleavage of SREBP substrates occurs in the Golgi apparatus [35] , we next needed to generate a ''secreted'' (s) variant of Spn4A, Spn4A.RRKR(s) ( Figure 1A ). We hypothesized that only an Spn4A (s) variant, which traffics through the late secretory pathway prior to secretion in the extracellular space, would encounter active SKI-1/S1P molecules in the early Golgi compartment. This was accomplished by inserting a stop codon before the C-terminal ER-retention signal, HDEL. Next, we employed site-directed mutagenesis to optimize the interactions between the RCL of Spn4A and the substrate binding sites of SKI-1/S1P. Residues at positions P2 and P1 of the Spn4A RCL ''bait'' region Arg P4 -Arg P3 -Lys P2 -Arg P1Q were substituted to generate Arg P4 -Arg P3 -Leu P2 -Leu P1Q , thereby mimicking the Lassa virus glycoprotein precursor GP-C cleavage site [50] ( Figure 1A and 1B: Spn4A.RRLL(r) and Spn4A.RRLL(s), respectively).