Dual targeting to mitochondria and chloroplasts
*, Ian Small
Department of Molecular Biology and Genetics, 321 Biotechnology Building, Cornell University, Ithaca, NY 14853-2703, USA
de Recherches en Ge
tale (INRA-URGV), 2, rue Gaston Cre
mieux, CP 5708, 91057 Evry cedex, France
Received 13 August 2001; accepted 13 August 2001
Plant cells contain two organelles originally derived from endosymbiotic bacteria: mitochondria and plastids. Their
endosymbiotic origin explains why these organelles contain their own DNA, nonetheless only a few dozens of genes are
actually encoded by these genomes. Many of the other genes originally present have been transferred to the nuclear genome
of the host, the product of their expression being targeted back to the corresponding organelle. Although targeting of
proteins to mitochondria and chloroplasts is generally highly specific, an increasing number of examples have been
discovered where the same protein is imported into both organelles. The object of this review is to compare and discuss these
examples in order to try and identify common features of dual-targeted proteins. The study helps throw some light on the
factors determining organelle targeting specificity, and suggests that dual-targeted proteins may well be far more common
than once thought. ß 2001 Elsevier Science B.V. All rights reserved.
Keywords: Mitochondria; Chloroplast ; Protein import; Dual targeting
Plant cells contain two organelles originally de-
rived from endosymbiotic bacteria: mitochondria
and plastids. Amongst many other functions, these
two organelles are best known for their roles in en-
ergy metabolism, notably respiration and photosyn-
thesis. The closest bacterial organisms to the endo-
symbiotic ancestors of these organelles have nearly a
thousand genes (Rickettsia ) or several thousands
(cyanobacteria ). Since the endosymbiosis, many
of the genes of the endosymbiotic bacteria have
been lost, leaving the organelle genomes with less
than a hundred protein-coding genes each [3,4].
The vast majority of mitochondrial and plastid pro-
teins are encoded in the nucleus, synthesized by cy-
tosolic ribosomes and subsequently imported into the
organelles via active protein transport systems. The
total number of proteins present in mitochondria and
chloroplasts is thought to be about 2000^3000 for
each of them .
Mitochondria originated much earlier than plas-
tids and thus the ¢rst plastids arose in cells that al-
ready contained an e¤cient system for targeting cy-
tosolically synthesized proteins to mitochondria. One
might have expected evolution to have seized this
opportunity to reuse the same machinery for target-
ing proteins to plastids, but in fact this seems not to
be the case; the two protein import systems have
clearly been derived independently and do not share
homology. In this situation, it is thus easy to under-
stand why protein targeting is usually highly speci¢c.
0167-4889 / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.
* Corresponding author.
E-mail address: email@example.com (N. Peeters).
BBAMCR 14799 11-12-01
Biochimica et Biophysica Acta 1541 (2001) 54^63