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.