Lassa virions are pleomorphic lipid-enveloped particles that contain two single-stranded RNA segments, designated L (large) and S (small), encoding four viral proteins in a unique ambisense coding strategy. The L segment encodes the viral RNA-dependent RNA polymerase (L) and the small zinc finger matrix protein (Z) [18] ; the S segment encodes the virus nucleoprotein (NP) and the virus surface glycoprotein precursor (preGP-C) [19] . preGP-C is cleaved co-translationally into a stable signal peptide and GP-C [20] . Post-translational maturation cleavage of GP-C by the proprotein convertase site 1 protease (S1P, [21] ), also known as subtilisin kexin isozyme-1 (SKI-1, [22] ), leads then to the generation of the distal receptor-binding subunit GP-1 and the transmembrane-spanning fusion competent subunit GP-2 [23] . Together with the signal peptide these subunits form the tripartite glycoprotein spike complex on the viral surface [24, 25] .