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Protein transport in plants
CHLOROPLAST PROTEIN TARGETING Welcome to my lab Im looking for highly motivated
students who want to become independent researchers. I offer you an
interesting research project that will lead you to a PhD in Plant Molecular
Biology. You will have possibilities to visit other labs and international
research conferences/meetings. I will introduce you to the scientific
field of chloroplast protein targeting (both practical and theoretical)
at regular weekly meetings and journal clubs. Please dont hesitate
to contact me henrik.aronsson@botany.gu.se
for more information about preliminary data, work plan/methodology,
equipments and collaborators.
Research
aim The
main aim is to uncover the function of translocon components in the
chloroplast import machinery and for this purpose wild type and mutant
Arabidopsis plants will be used. There are already knockout (KO)
mutants available for several of the components of the translocation
machinery and the mechanisms of targeting will be studied both in
vitro and in vivo. The major components we presently are
focusing on are called Toc12, Toc34, Toc64, Tic32, Tic55 and Tic62.
Importance Since protein targeting happens
in all organisms, this is a fundamental process that is important to
understand. In plants, chloroplast protein targeting is especially important
since most proteins in leaves are found in the chloroplasts, and most
chloroplast proteins are encoded in the nucleus and translated on cytosolic
ribosomes [1]. Since chloroplasts are the site for the photosynthesis
process, that provides the earth with organic carbon and oxygen needed
for the survival of the human population, protein targeting to them
is important on a global scale. The chloroplasts contain proteins necessary
to perform photosynthesis, biosynthesis of amino acids, lipids, pigments
and also work as storage places for starch and proteins. To modify these
important processes in an economical important crop, we have to know
more about the targeting process. The use of different knockout genes
of the translocation machinery provides us with a unique chance to study
the role of the chloroplast protein targeting. The project will help
us to understand the role of docking preproteins, the redox sensitivity
of protein targeting, the lipids role for protein association as well
as the targeting process in general.
Current state-of-the-art
in the research field
Arabidopsis recently emerged as an excellent
model system for studying chloroplast protein import, and the necessary
techniques are now well established [2-4]. Most chloroplast proteins
are encoded in the nucleus and imported post-translationally into chloroplasts.
A protein complex in the chloroplast envelope, called the translocon,
mediates import. Components of the translocon are called Toc or Tic,
for translocon at the outer or inner envelope membrane of chloroplasts
[5-6]. A lot of Toc and Tic components have been identified and characterised,
but many of their functions remain to be determined, e.g. Tic55 and
Toc64. Proteins imported by the translocon have a cleavable, amino terminal
region in their primary structure, the transit peptide. There is a guidance
complex, comprising Hsp70 and 14-3-3 proteins, which interacts with
the transit peptide and escorts proteins to the chloroplast where the
main receptors are Toc34 and Toc159 [5-6]. The transit peptide is removed
upon transfer across the envelope channel, consisting of Toc75 and recently
suggested Tic110 [5-6]. Toc64 contains three tetratricopeptide repeat
motifs (TPRs) that are exposed at the chloroplast cytosol interface.
The TPR motif of Toc64 suggests that Toc64 are acting on a protein-protein
interaction base with the guidance complex, docking the guidance complex
to the envelope [7]. However, recently one of the three Toc64 homologues
in Arabidopsis where suggested to be localized at the mitochondrial
surface which might then be part of the dual targeting mechanism [8].
The
Tic translocon, Tic55, includes a Rieske-type iron-sulfur cluster and
a mononuclear iron-binding site and is thought to be involved in redox
regulation of import [9]. Redox regulated import has been shown to occur
in maize [10]. Recently Tic62 and Tic32 was identified and also suggested
to be involved in redox sensing [11-12]. Thus, Tic32, Tic55 and Tic62
might be fine-tuning the import capacity based on the metabolic status
of the chloroplast [5-6].
Some techniques/methods that
will be used in the project
References 1. Abdallah F, Salamini F, Leister
D (2000) Trends Plant Sci. 5: 141-2 2. Jarvis P, Chen L-J, Li H-m, Peto C, Fankhauser C, Chory J (1998) Science
282: 100-3 3. Bauer J, Chen KH, Hiltbunner A, Wehrli E, Eugster M,
Schnell D, Kessler F (2000) Nature 403: 203-7 4. Aronsson H, Jarvis P (2002) FEBS Lett. 529: 215-20 5. Schleiff E, Soll J (2004) Nature Reviews Mol. Cell Biol. 5: 198-208 6.
Jarvis P, Robinson C (2004) Curr Biol. 14: 1064-77 7. Sohrt K, Soll J (2000) J Cell Biol. 148:
1213-21 8. Chew
O, Lister R, Qbadou S, Heazlewood JL, Soll J, Schleiff E, Millar AH,
Whelan J (2004) FEBS Lett. 557: 109-114 9. Caliebe A, Grimm R, Kaiser G, Lübeck J, Soll J, Heins
L (1997) EMBO J. 16: 7342-7350 10. Hirohashi T, Hase T, Nakai M (2001) Plant Phys. 125:
2154-63 11. Küchler M, Decker S, Hörmann F, Soll J, Heins L (2002)
EMBO J. 21: 6136-6145 12. Hormann F, Kuchler
M, Sveshnikov D, Oppermann U, Li Y, Soll J (2004).J Biol Chem. 279: 34756-62 Contact information: Henrik Aronsson,
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