Multiple Roles of the Signal Peptide for Arenavirus Glycoprotein Maturation

Lymphocytic choriomeningitis virus (LCMV) is a member of the Arenaviridae family of enveloped, ambisense RNA viruses that persist within distinct rodent hosts. These viruses are highly adapted with their rodent reservoirs, making arenaviruses useful tools to understand mechanisms for virus-host interactions and eukaryotic cell biology. Our goal is to define the non-canonical functions of the LCMV glycoprotein signal peptide in the context of glycoprotein maturation. The viral glycoprotein is expressed as a polyprotein precursor that is processed into three non-covalently interacting subunits where the signal peptide is required for proper downstream cleavage of precursor GPC. The three mature glycoprotein subunits form a complex that coats infectious virions and work in concert to mediate receptor recognition, pH-dependent fusion and viral entry into host cells. For proper glycoprotein expression, cellular signal peptidase first cleaves the LCMV signal peptide from the remaining glycoprotein precursor, GPC. Within the Golgi stacks, cellular SKI-1/S1P protease cleaves the precursor GPC into GP1 and GP2 subunits. Our data indicate a role for the signal peptide in mediating the downstream cleavage events. Signal peptide expression in trans rescues cleavage when precursor GPC is directed to the secretory pathway via a foreign signal peptide. This in trans rescue of GPC cleavage occurs with wild type signal peptide whereas epitope-tagged wild type signal peptide does not rescue GPC cleavage. These results suggest a key function for the C-terminal region of the signal peptide. Additionally, the LCMV signal peptide contains a highly conserved variant (FLxL) of the YxxL motif used by cellular proteins for endosomal or intracellular trafficking. The introduction of double point mutations within this FLxL motif  (ALxA, AAxL, YLxA) failed to cleave GPC into mature GP1 and GP2. Variants FAxA and YAxL did yield mature subunits, though at reduced levels. These results indicate a highly conserved and intricate mechanism where multiple viral protein subunits, and possibly cellular factors, coordinate the maturation and cell biology of arenavirus glycoprotein synthesis. A better understanding of arenavirus biology may improve our understanding of virus-host interactions. This project is supported by NIH grant AI-065359 and NSF grant DGE-0638751.