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Molecular chaperons in algal chloroplasts

Chaperons ensure that cellular proteins fold correctly. But it is not their only function. Michael Schroda from the Department of Plant Biochemistry at the University of Freiburg is examining the role of HSP70B, a chaperon in the chloroplasts of Chlamydomonas reinhardtii algae, which is also important for protecting the algae against high light intensity.

Chamydomonas reinhardtii under the light microscope (Photo: Michael Schroda)
Chaperons are found in all groups of organisms, from bacteria to plants and animals. They were discovered as a result of an important function that they have: when proteins are damaged due to high heat (heat shock), the chaperons return the proteins to the correct form. But chaperons are capable of a lot more. “The heat-shock response is most likely a special function of HSP70 chaperons,” said Michael Schroda, junior professor in the Department of Plant Biochemistry at the Institute of Biology II at the University of Freiburg. They guide proteins through the membranes of organelles or control the assembly of protein complexes. In the chloroplasts of unicellular Chlamydomonas reinhardtii algae, HSP70 proteins also protect the algae against light, as Schroda’s group of researchers has found out.
Back in 1989, Schroda’s supervisor, Professor Christoph Beck from Freiburg, discovered that the production of the chloroplast HSP70B chaperon (HSP stands for heat-shock protein) in Chlamydomonas is induced by light. Schroda started his current work on the biological function of light induction back in 1999 during his doctorate and has in the meantime found out that this has something to do with the protection of the photo system II protein complex.

The photo system II, which is located at the innermost of the three chloroplast membranes, is the core of photosynthesis and hence fundamental for the production of energy in plants. It contains, amongst other things, chlorophyll molecules. The absorption of light leads to a reaction during which light energy is transmitted to cellular energy carriers by way of electrons. But the photo system II is very sensitive and can even be damaged at a low light intensity of a normal summer day. Schroda and his team of researchers found out that the chaperon HSP70B protects the algae against this inhibition and also have an idea as to how this protection works.

A complex protein network

Electron microscope image of the VIPP1 rods (Photo: Jochen Golecki and Michael Schroda)
“To understand the complex function of chaperons, it is necessary to find out with which other proteins they interact,” said Schroda. HSP70 chaperons are only able to affect the structure of proteins, but their reaction partners, which are known as co-chaperons, determine which specific cellular targets they modify by mediating their binding to these targets. The protein CDJ2 is such a co-chaperon in the Chlamydomonas chloroplasts. Schroda and his colleagues discovered that a complex of CDJ2 and HSP70B accelerates the formation and degradation of the ring and rod structures of the VIPP1 protein. Such rod structures are normally found inside the chloroplasts. Nothing is so far known about their function.
“We believe that HSP70B protects the photo system II from light damage by interacting with the VIPP1 proteins,” said Schroda. An observation by his work group supports this hypothesis: genetically modified Chlamydomonas cells, which only produce small quantities of VIPP1, are very light sensitive and fade even at the lowest light intensity. Schroda plans to investigate how the VIPP1 rods and the chaperon HSP70B protect the photo system II.

A groundbreaking method

Schroda’s research has already been fruitful: he has developed a method that enables the expression of foreign genes in Chlamydomonas algae. This was previously impossible because the inserted genes could not be accessed by the transcription enzymes due to the dense packaging of DNA. Pure chance led Schroda to solve the problem; he attached the promoter of the HSP70A gene in front of the inserted gene. This promoter normally controls the expression of the HSP70A gene.

Schroda assumes that the success of his method is due to the fact that chaperons play an important role in correct protein folding, which requires their genes to be accessible at any time. That is why they interact with factors that keep their environment accessible. Schroda and his colleagues are currently investigating the molecular mechanism behind this.

Almost all scientists working with the unicellular Chlamydomonas algae use Schroda’s method. His research on the function of HSP70 chaperons in chloroplasts might also be advantageous for biotechnology, for example in what is known as molecular farming. In future, it might be possible to produce industrially important proteins in plant chloroplasts. Supported by chaperons for correct folding, the plants will then be grown in fields in large quantities and at low cost.

mn – 02.04.2008
© BIOPRO Baden-Württemberg GmbH
Further information:
PD Dr. Michael Schroda
University of Freiburg
Institute of Biology II
Department of Plant Biochemistry
Schänzlestraße 1
79104 Freiburg
Tel.: ++49 (0)761 203 2708
Fax: ++49 (0)761 203 2601
E-mail: michael.schroda@biologie.uni-freiburg.de
Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/molecular-chaperons-in-algal-chloroplasts