Leader (L) of Theiler's murine encephalomyelitis virus (TMEV) is required for virus growth in a murine macrophage-like cell line

Theiler's murine encephalomyelitis virus is divided into two subgroups on the basis of their different biological activities. GDVII subgroup strains cause acute and fatal encephalomyelitis in mice, while TO or DA subgroup strains cause non-fatal polioencephalomyelitis in weanling mice followed by virus persistence and demyelination in the spinal cords. Nonstructural leader (L) protein is encoded at the most N-terminus of the polyprotein. The L coding region of TO or DA subgroup strains has another out-of-frame open reading frame, which produces another nonstructural protein, L*. L* protein is reported to be essential for virus growth in macrophage cells. In the present report, we studied the role of L protein in virus growth in macrophage-like cell line, J774-1, by using a series of deletion mutant viruses. In J774-1 cells (the absence of L* protein), the mutant virus [deleting the entire L coding region (ΔL), N-terminal zinc-finger domain (ΔZ), acidic domain (ΔA), or C-terminal serine/threonine (S/T)-rich domain (ΔS/T)] did not grow. The mutant virus disrupting zinc-finger motif (Lcys) did not grow, either. However, in L*-expressing J774-1 cells (the presence of L* protein), Lcys, ΔZ and ΔS/T had a rescue of the growth activity, while ΔL or ΔA had no rescue. The data suggest that L protein is required for virus growth in J774-1 cells and also suggest that the site responsible for virus growth in those cells, is the acidic domain of L protein.