1.G.8 The Arenavirus Fusion Protein (AV-FP) Family
Arenaviruses are important agents of zoonotic disease worldwide. The virions expose a tripartite envelope glycoprotein complex at their surface, formed by the glycoprotein subunits GP1, GP2 and the stable signal peptide. This complex is responsible for binding to target cells and for the subsequent fusion of viral and host-cell membranes for entry. During this process, the acidic environment of the endosome triggers a fusogenic conformational change in the transmembrane GP2 subunit of the complex. Igonet et. al (2011) reported the crystal structure of the recombinant GP2 ectodomain of the lymphocytic choriomeningitis virus, the arenavirus type species, at 1.8-Å resolution. The structure shows the characteristic trimeric coiled coil present in class I viral fusion proteins, with a central stutter that allows a close structural alignment with most of the available structures of class I and III viral fusion proteins. The structure further shows a number of intrachain salt bridges stabilizing the postfusion hairpin conformation, one of which involves an aspartic acid that appears released from a critical interaction with the stable signal peptide upon low pH activation.
The mature arenavirus envelope glycoprotein complex (GPC) is a tripartite complex comprising a stable signal peptide (SSP) in addition to the receptor-binding (G1) and transmembrane fusion (G2) subunits. SSP is a key element in GPC-mediated membrane fusion, and GPC sensitivity to acidic pH is modulated in part through the lysine residue at position 33 in the ectodomain loop of SSP (York and Nunberg, 2009). A glutamine substitution at this position stabilizes the native GPC complex and thereby prevents the induction of pH-dependent membrane fusion. The pH-dependent interaction is vulnerable to small compounds that stabilize the native complex and prevent the activation of membrane fusion.