Molecular chaperones involved in chloroplast protein import
, Mitsuru Akita
, Kenneth Keegstra
Department of Energy Plant Research Laboratory, Michigan State University, East Lansing, MI 48824-1312, USA
Genetics Program, Michigan State University, East Lansing, MI 48824, USA
Department of Plant Biology, Michigan State University, East Lansing, MI 48824, USA
Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA
Plant Biophysics/Biochemistry Research Laboratory, College of Agriculture, Ehime University, 3-5-7 Tarumi,
Matsuyama, Ehime, 790-8566, Japan
Received 25 July 2001; accepted 1 August 2001
Transport of cytoplasmically synthesized precursor proteins into chloroplasts, like the protein transport systems of
mitochondria and the endoplasmic reticulum, appears to require the action of molecular chaperones. These molecules are
likely to be the sites of the ATP hydrolysis required for precursor proteins to bind to and be translocated across the two
membranes of the chloroplast envelope. Over the past decade, several different chaperones have been identified, based mainly
on their association with precursor proteins and/or components of the chloroplast import complex, as putative factors
mediating chloroplast protein import. These factors include cytoplasmic, chloroplast envelope-associated and stromal
members of the Hsp70 family of chaperones, as well as stromal Hsp100 and Hsp60 chaperones and a cytoplasmic 14-3-3
protein. While many of the findings regarding the action of chaperones during chloroplast protein import parallel those seen
for mitochondrial and endoplasmic reticulum protein transport, the chloroplast import system also has unique aspects,
including its hypothesized use of an Hsp100 chaperone to drive translocation into the organelle interior. Many questions
concerning the specific functions of chaperones during protein import into chloroplasts still remain that future studies, both
biochemical and genetic, will need to address. ß 2001 Elsevier Science B.V. All rights reserved.
Keywords: Chloroplast ; Protein targeting ; Precursor protein; Molecular chaperone; Heat shock protein
Plastids import the vast majority of their resident
proteins post-translationally from the cytoplasm [1^
5]. Most of the knowledge concerning the transport
of proteins into plastids has been obtained through
experiments with isolated pea chloroplasts, although
it is assumed that all types of plastids utilize the same
general import apparatus. The import process re-
quires a variety of membrane-bound and soluble fac-
tors. Membrane proteins of the chloroplast envelope
that mediate import are discussed in an accompany-
ing review by Jarvis and Soll. This review will focus
on a major class of soluble factors important in chlo-
roplast protein import, the molecular chaperones.
Chloroplast protein import can be divided into
two stages, based on their di¡ering nucleotide tri-
phosphate requirements. The `binding' or `docking'
stage of import involves the hydrolysis of low levels
0167-4889 / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.
* Corresponding author, at address a. Fax: +1-517-353-9168.
E-mail address: firstname.lastname@example.org (K. Keegstra).
These authors contributed equally to this work.
BBAMCR 14801 11-12-01
Biochimica et Biophysica Acta 1541 (2001) 102^113