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

Abstract

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.