Plant Molecular Biology 39: 641–645, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
News in chloroplast protein import
Alexander Caliebe and Jürgen Soill
Botanisches Institut, Christian-Albrechts-Universität Kiel, Am Botanischen Garten 1–9, 24118 Kiel, Germany
author for correspondence)
Received 27 October 1998; accepted 2 November 1998
During the co-evolution of ancestor plastids and their
hosts, the vast majority of genes encoding plastidic
proteins were transferred to the genome of the host.
Therefore, systems evolved to mediate the import of
nuclear-encoded proteins across two envelope mem-
branes into the plastids. This transport system had to
be able to distinguish between proteins destined for
chloroplasts and other organelles. The challenge was
mainly met by the interaction of two protein translo-
cation systems located in the outer (Toc complex)
and inner (Tic complex) envelope membranes, respec-
tively. Additional proteins in the cytosol, the envelope
intermembrane space and the stroma also support the
translocation process [4, 5, 13].
Surprisingly, the Toc and Tic proteins share no
structural homology with those of other eukaryotic im-
port/export systems, for example those of mitochon-
dria and peroxisomes, whereas intraplastidic transport
processes into and across the thylakoids by the so-
called SRP, Sec or pH-dependent pathway, which
are beyond the scope of this review, are of prokaryotic
origin and resemble export mechanisms in bacteria
[13, and references therein]. Like other import/export
systems, import into chloroplasts requires different
chaperones such as the HSP70, HSP100 and the
GroEL/GroES homologue proteins.
Most of the nuclear-encoded chloroplast proteins
are synthesized in the cytosol as precursor proteins
with a cleavable transit sequence at the N-terminus.
The transit sequence is both necessary and sufﬁcient
for the translocation of the precursor protein into the
chloroplast. In general, the transit sequence has an
overall positive charge (see below) due to the lack
of acidic amino acids, while the N-terminus is un-
charged and does not contain glycine or proline [4, 5].
Furthermore, the transit peptide contains a consensus
motif for a cytosolic protein kinase, which phosphory-
lates serine or threonine residues. This protein kinase
speciﬁcally phosphorylates chloroplastidic precursor
proteins, but not mitochondrial and/or peroxisomal
precursor proteins .
The phosphorylated precursor may bind to the Toc
complex, but is arrested in this stage. Before translo-
cation can resume, it has to be dephosphorylated by
a not yet identiﬁed phosphatase, which is probably in
close vicinity to the Toc complex. This cycle of phos-
phorylation/dephosphorylation might be a regulatory
step in import.
The precursor protein ﬁrst contacts the surface
of the chloroplast reversibly, interacting with galac-
tolipids speciﬁc for the outer envelope membrane. The
transit sequence-lipid interaction results in structural
changes of the transit sequence, which might be rele-
vant for speciﬁc precursor receptor recognition and its
further interaction with the Toc complex . Such a
mechanism would add to the correct targeting process
of precursor proteins to chloroplasts. The subsequent
translocation step requires ATP, either at the outer
envelope membrane or in the intermembrane space,
and results in a precursor that is irreversibly inserted
into the translocation channel of the Toc complex. At
this stage, translocation intermediates of the precur-
sor protein cofractionate with both outer and inner
envelope membranes of chloroplasts. ATP is most
likely necessary for the action of HSP70 homologues
which co-operate in this step (see below). Additional
GTP-dependent reactions could occur during the early
stages of translocation. Both Toc34 and Toc86 have
GTP-binding domains [7, 9] and Toc34 clearly shows
GTPase activity in vitro (see below).
Complete translocation of the precursor protein
demands the joint interaction of the Toc and Tic
complex and requires stromal ATP. During or shortly