The anti-apoptotic protein L* of Theiler's murine encephalomyelitis virus (TMEV) contains a mitochondrial targeting signal

L* protein of TMEV is out-of-frame with the viral polyprotein from an alternative initiation codon AUG, 13 nucleotides downstream from the authentic polyprotein AUG. Anti-apoptotic activity of L* was demonstrated by both ‘loss of function’ and ‘gain of function’ experiments. However, the precise mechanism(s) of anti-apoptotic activity of L* remains to be clarified. In this study, L* was demonstrated to be localized to mitochondria. It was also shown by the GFP fusion protein that N-terminal sequence of L* may contain a mitochondrial targeting signal (MTS). Surprisingly, L*(5–70)–GFP and L*(41–70)–GFP were localized to mitochondria although L*(1–70)–GFP was distributed in the cytosol, suggesting L* has an MTS between amino acid (AA) positions 41 and 70, and that L*(1–4) inhibits its mitochondrial targeting. Furthermore, L*(1–70)–GFP was localized to the mitochondria by co-expression of L*(65–156), indicating that L*(65–156) suppresses the inhibition of mitochondrial targeting by L*(1–4). These results suggest that the intra- or inter-molecular interaction of L* regulates its mitochondrial localization. It is also suggested that L* may inhibit the intrinsic apoptosis through the localization to mitochondria.