THE PLANT CELL, Vol 1, Issue 8 783-791, Copyright © 1989 by American Society of Plant Biologists
A Yeast Mitochondrial Presequence Functions as a Signal for Targeting to Plant Mitochondria in Vivo
U. K. Schmitz and D. M. Lonsdale
Molecular Genetics Department, Institute of Plant Science Research, Cambridge Laboratory, Maris Lane, Trumpington, Cambridge CB2 2LQ, United Kingdom
To date, the presequence of the mitochondrial [beta]-subunit of ATPase from
tobacco is the only signal sequence that has been shown to target a foreign
protein into plant mitochondria in vivo. Here we report that the
presequence of a yeast mitochondrial protein directs bacterial
[beta]-glucuronidase (GUS) specifically into the mitochondrial compartment
of transgenic tobacco plants. Fusions between the presequence of the
mitochondrial tryptophanyl-tRNA-synthetase gene from yeast and the GUS gene
have been introduced into tobacco plants and yeast cells. In both systems,
proteins containing the complete yeast mitochondrial presequence are
efficiently imported in the mitochondria. Measurements of GUS activity in
different subcellular fractions indicate that there is no substantial
misrouting of the chimeric proteins in plant cells. In vitro synthesized
GUS fusion proteins have a higher molecular weight than those found inside
yeast and tobacco mitochondria, suggesting a processing of the precursors
during import. Interestingly, fusion proteins translocated across the
mitochondrial membranes of tobacco have the same size as those that are
imported into yeast mitochondria. We conclude that the processing enzyme in
plant mitochondria may recognize a proximate or even the same cleavage site
within the mitochondrial tryptophanyl-tRNA-synthetase presequence as the
matrix protease from yeast.
