Review
Function of a chloroplast SRP in thylakoid protein export
L.A. Eichacker
aY
*, R. Henry
b
a
Botanisches Institut der Universita
«
tMu
«
nchen, Menzinger Strasse 67, 80638 Munich, Germany
b
Biological Sciences Department, University of Arkansas, B2 Ferritor Building, Fayetteville, AR 72701, USA
Received 13 August 2001; accepted 13 August 2001
Abstract
Protein export systems derived from prokaryotes are used to transport proteins into or across the endoplasmic reticulum,
the mitochondrial inner membrane, and the chloroplast thylakoid membrane. Signal recognition particle (SRP) and its
receptor are essential components used exclusively for cotranslational export of endomembrane and secretory proteins to the
endoplasmic reticulum in eukaryotes and export of polytopic membrane proteins to the cytoplasmic membrane in
prokaryotes. An organellar SRP in chloroplasts (cpSRP) participates in cotranslational targeting of chloroplast synthesized
integral thylakoid proteins. Remarkably, cpSRP is also used to posttranslationally localize a subset of nuclear encoded
thylakoid proteins. Recent work has begun to reveal the basis for cpSRP's unique ability to function in co- and
posttranslational protein localization, yet much is left to question. This review will attempt to highlight these advances and
will also focus on the role of other soluble and membrane components that are part of this novel organellar SRP targeting
pathway. ß 2001 Elsevier Science B.V. All rights reserved.
Keywords: Signal recognition particle; Signal recognition particle ; Oxa1; Alb3; Protein transport ; FtsY; Thylakoid ; Chloroplast
1. Introduction
The thylakoid membrane of chloroplasts is the
marker compartment of photosynthetic eukarya. At
least four supramolecular protein complexes, each
containing 14^26 protein subunits, are assembled in
the thylakoid where they function as a quantum-,
electron- and proton-transfer machine essential for
sustaining life on earth. Biogenesis of the photosyn-
thetic complexes in the thylakoid requires this mem-
brane to be one of the major protein export sites of
the photosynthetic cell. Proteins from two genetic
origins, the nucleus and the chloroplast, are exported
from the stroma into or across the thylakoid. The
stroma, much like the bacterial cytosol or mitochon-
drial matrix, is the site of transcription and trans-
lation of chloroplast encoded proteins. In contrast,
nuclear encoded thylakoid proteins are expressed in
the cytoplasm, and imported into the organelle be-
fore entering thylakoid export pathways that origi-
nate in the stroma. Based on the evolutionary origin
of organelles, it is not surprising that protein export
from the stroma to the thylakoid resembles export to
the endoplasmic reticulum, to the mitochondrial in-
ner membrane, and to the bacterial cytoplasmic
membrane [1].
In general, proteins gain access to export pathways
through the use of a signal sequence at the N-termi-
nal end of the unfolded or partly folded polypeptide
0167-4889 / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.
PII: S0167-4889(01)00151-3
* Corresponding author. Fax: +49-89-17861-185.
E-mail addresses: eichacker@botanik.biologie.uni-
muenchen.de (L.A. Eichacker), rahenry@comp.uark.edu
(R. Henry).
BBAMCR 14804 11-12-01
Cyaan Magenta Geel Zwart
Biochimica et Biophysica Acta 1541 (2001) 120^134
www.bba-direct.com