Mammalian mitochondria have the innate ability to import tRNAs by a mechanism distinct from protein import.
Rubio MA, Rinehart JJ, Krett B, Duvezin-Caubet S, Reichert AS, Söll D, Alfonzo JD.
Department of Microbiology, Ohio State RNA Group, Ohio State University, Columbus, OH 43210-1292, USA.
Abstract
Mitochondrial
genomes generally encode a minimal set of tRNAs necessary for protein
synthesis. However, a number of eukaryotes import tRNAs from the
cytoplasm into their mitochondria. For instance, Saccharomyces
cerevisiae imports cytoplasmic tRNA(Gln) into the mitochondrion without
any added protein factors. Here, we examine the existence of a similar
active tRNA import system in mammalian mitochondria. We have used
subcellular RNA fractions from rat liver and human cells to perform
RT-PCR with oligonucleotide primers specific for nucleus-encoded
tRNA(CUG)(Gln) and tRNA(UUG)(Gln) species, and we show that these tRNAs
are present in rat and human mitochondria in vivo. Import of in vitro
transcribed tRNAs, but not of heterologous RNAs, into isolated
mitochondria also demonstrates that this process is tRNA-specific and
does not require the addition of cytosolic factors. Although this in
vitro system requires ATP, it is resistant to inhibitors of the
mitochondrial electrochemical gradient, a key component of protein
import. tRNA(Gln) import into mammalian mitochondria proceeds by a
mechanism distinct from protein import. We also show that both tRNA(Gln)
species and a bacterial pre-tRNA(Asp) can be imported in vitro into
mitochondria isolated from myoclonic epilepsy with ragged-red fiber
cells if provided with sufficient ATP (2 mM). This work suggests that
tRNA import is more widespread than previously thought and may be a
universal trait of mitochondria. Mutations in mitochondrial tRNA genes
have been associated with human disease; the tRNA import system
described here could possibly be exploited for the manipulation of
defective mitochondria.
