Mutagenic analysis of the multifunctional stable signal peptide of the arenaviruses

Lymphocytic choriomeningitis virus (LCMV) is the prototype arenavirus and a human pathogen found worldwide. The viral surface glycoprotein is expressed as a polypeptide precursor (pGPC) and is cleaved into three separate proteins: stable signal peptide (SSP), GP1 and GP2. These three subunits form a GP complex that localizes to the cell surface where viral budding takes place. While the traditional role of a signal peptide (SP) is to mediate translation initiation and translocation into the lumen of the endoplasmic reticulum, SSP of LCMV is 58 amino acids in length and its sequence is highly conserved among all arenaviruses, suggesting that it performs multiple functions during the virus lifecycle. In order to better understand the function(s) of this unique viral protein a panel of SSP point and deletion mutants was designed and tested in a variety of assays. Our findings indicate that not only is SSP critical for pGPC expression, but it also plays a vital role in GPC processing, cell surface localization, infectious particle formation, and GP-mediated cell fusion. It is also of interest to note that each of these identified SSP functions is distinct from the others and can be inhibited selectively. Finally, we were able to map specific functional domains within the SSP using in vitro mutagenesis and trans-complementation.