Saccharomyces cerevisiae imports the cytosolic pathway for Gln-tRNA synthesis into the mitochondrion.
Rinehart J, Krett B, Rubio MA, Alfonzo JD, Söll D.
Department
of Molecular Biophysics and Biochemistry, and Department of Chemistry,
Yale University, New Haven, Connecticut 06520-8114, USA.
Abstract
Aminoacyl-tRNA
(aa-tRNA) formation, an essential process in protein biosynthesis, is
generally achieved by direct attachment of an amino acid to tRNA by the
aa-tRNA synthetases. An exception is Gln-tRNA synthesis, which in
eukaryotes is catalyzed by glutaminyl-tRNA synthetase (GlnRS), while
most bacteria, archaea, and chloroplasts employ the transamidation
pathway, in which a tRNA-dependent glutamate modification generates
Gln-tRNA. Mitochondrial protein synthesis is carried out normally by
mitochondrial enzymes and organelle-encoded tRNAs that are different
from their cytoplasmic counterparts. Early work suggested that
mitochondria use the transamidation pathway for Gln-tRNA formation. We
found no biochemical support for this in Saccharomyces cerevisiae
mitochondria, but demonstrated the presence of the cytoplasmic GlnRS in
the organelle and its involvement in mitochondrial Gln-tRNA synthesis.
In addition, we showed in vivo localization of cytoplasmic tRNAGln in
mitochondria and demonstrated its role in mitochondrial translation. We
furthermore reconstituted in vitro cytoplasmic tRNAGln import into
mitochondria by a novel mechanism. This tRNA import mechanism expands
our knowledge of RNA trafficking in the eukaryotic cell. These findings
change our view of the evolution of organellar protein synthesis.
