The stable signal peptide (SSP) of the lymphocytic choriomeningitis virus surface glycoprotein precursor has several unique characteristics. The SSP is unusually long, at 58 amino acids, and contains two hydrophobic domains, and its sequence is highly conserved among both Old and New World arenaviruses. To better understand the functions of the SSP, a panel of point and deletion mutants was created by in vitro mutagenesis to target the highly conserved elements within the SSP. We were also able to confirm critical residues required for separate SSP functions by trans-complementation. Using these approaches, it was possible to resolve functional domains of the SSP. In characterizing our SSP mutants, we discovered that the SSP is involved in several distinct functions within the viral life cycle, beyond translocation of the viral surface glycoprotein precursor into the endoplasmic reticulum lumen. The SSP is required for efficient glycoprotein expression, posttranslational maturation cleavage of GP1 and GP2 by SKI-1/S1P protease, glycoprotein transport to the cell surface plasma membrane, formation of infectious virus particles, and acid pH-dependent glycoproteinmediated cell fusion.The Arenaviridae comprise a group of enveloped RNA viruses that include several causative agents of hemorrhagic fevers in the New World and Africa. Among these are Lassa fever virus (LASV) and Junín virus (JUNV). The prototypic arenavirus, lymphocytic choriomeningitis virus (LCMV), has a bisegmented, single-stranded, negative-sense RNA genome. Each of the two segments uses an ambisense coding strategy to direct the synthesis of two polypeptides. The large (L) segment (7.2 kb) encodes a small RING finger protein, Z, and the RNA-dependent RNA polymerase, the L protein, while the small (S) segment (3.4 kb) encodes the nucleoprotein, NP, and the glycoprotein (GP) precursor pGPC (5). The 498-aminoacid pGPC of LCMV consists of three domains (Fig. 1A) that are produced as independent polypeptides by posttranslational processing: residues 1 to 58 comprise the stable signal peptide (SSP), which is cotranslationally cleaved by signal peptidase and followed by the GP precursor GPC (residues 59 to 498), which is further processed into GP1 (residues 59 to 265) and GP2 (residues 266 to 498) (5, 7, 26) by the cellular protease, SKI-1/S1P (3). GP1 is heavily glycosylated, contains the receptor binding site and antibody neutralization sites, and is noncovalently associated with GP2. GP2 contains a transmembrane region and anchors the GP complex in the lipid bilayer of the cell membrane and virus envelope (6). LCMV cell entry involves a fusion event that requires exposure to acidic pH (9) to trigger GP1 dissociation from GP2 and to induce irreversible conformational changes in GP2 that mediate membrane fusion (10).Signal peptides (SPs) mediate protein translocation into the lumen of the endoplasmic reticulum (ER), promoting proper folding and posttranslational modifications of these polypeptides. Following translocation, an SP is typically cleaved and r...

The pseudo-fourfold homotetrameric synapse formed by Cre protein and target DNA restricts site-specific recombination to sequences containing dyad-symmetric Cre-binding repeats. Mixtures of engineered altered-specificity Cre monomers can form heterotetramers that recombine nonidentical asymmetric sequences, allowing greater flexibility for target site selection in the genome of interest. However, the variety of tetramers allowed by random subunit association increases the chances of unintended reactivity at nontarget sites. This problem can be circumvented by specifying a unique spatial arrangement of heterotetramer subunits. By reconfiguring intersubunit protein-protein contacts, we directed the assembly of two different Cre monomers, each having a distinct DNA sequence specificity, in an alternating (ABAB) configuration. This designed heterotetramer preferentially recombined a particular pair of asymmetric Lox sites over other pairs, whereas a mixture of freely associating subunits showed little bias. Alone, the engineered monomers had reduced reactivity towards both dyad-symmetric and asymmetric sites. Specificity arose because the organization of Cre-binding repeats of the preferred substrate matched the programmed arrangement of the subunits in the heterotetrameric synapse. When this "spatial matching" principle is applied, Cre-mediated recombination can be directed to asymmetric DNA sequences with greater fidelity.