In principle, every step in the viral life cycle is a potential target for antiviral inhibitors. While current antiviral strategies in the arenavirus field mainly target virus entry [58, 59, 60, 61] or replication and assembly [62, 63, 64, 65, 66, 67] , inhibition studies of the glycoprotein activating endoprotease and its impact on viral replication are largely unexploited. Due to its central role in the arenavirus life cycle [23, 26, 33, 34] , S1P should be considered as a cellular target for antiviral drug development. In the present study we analyzed the inhibitory effect of S1P-adapted a 1 -antitrypsins on proteolytic processing of LASV GP-C and its consequences for viral replication. To our knowledge, this is the first report that Effect of S1P Inhibition on LASV Replication www.plosntds.org addresses the impact of protein-based S1P inhibition on LASV GP-C cleavage and multicycle replication. Furin-adapted a 1 -ATs have been shown to efficiently inhibit the formation of infectious progeny of other viruses (e.g. HIV, measles virus and human cytomegalovirus) [38, 39, 40, 41, 68, 69] . Using a replication-competent recombinant VSV pseudotyped with the LASV glycoprotein GP [48] , we demonstrate that proteolytic maturation of the precursor GP-C is sensitive to S1Padapted a 1 -ATs. Mutagenesis of the reactive centre loop (RCL) into the S1P recognition motif RRIL resulted in an abrogation of GP-C processing similar to that observed in S1P-deficient SRD-12B cells. The inhibitory activity of the a 1 -AT variant RRIL on LASV GP cleavage described here is in agreement with a previous study showing its inhibitory potential on the processing of the natural S1P substrates SREBP-2 and ATF6 [42] . Also an a 1 -AT variant that contains the LASV GP-C cleavage motif RRLL exhibited a high S1P inhibitory potential and was found to drastically reduce GP processing. Interestingly, this variant exhibited a 100% inhibition activity on maturation cleavage of an artificial pro-PDGF (precursor of platelet-derived growth factor) mutant that is processed by S1P due to introduction of a RRLL cleavage site, but failed to inhibit cleavage of endogenously expressed SREBP-2 [42] . These data indicate that various substrates differ in their sensitivity towards S1P inhibition.