Protein folding in living cells is a fascinating process. After the ribosome-mediated translation of the linear genetic information into polypeptide chains, proteins undergo multiple steps to achieve their functional three-dimensional conformation. During the phase of maturation unfolded proteins and folding intermediates are particularly volatile for misfolding and for the formation of unwanted interactions in the crowded environment of the cell. Failures in protein folding may result in the loss of essential functions and the formation of toxic protein aggregates.
Throughout their lifespan, proteins are guided by a number of factors that facilitate their maturation and that help proteins to maintain their functionality even under stress and denaturing conditions. Many of these factors belong to the family of molecular chaperones. Cells harbor structurally and functionally distinct classes of chaperones that vary in size and complexity. The diversity ranges from chaperones that only bind to misfolded polypeptides to prevent their aggregation to those that recognize specific substrate proteins and facilitate their folding to the native state. Further, chaperones are involved in the translocation of partially unfolded proteins across membranes, complex assembly and disassembly, and many other regulatory processes within the cell.
Why study protein folding in chloroplasts?
Chloroplasts are important organelles of algae and higher plants harboring essential biochemical pathways that provide the base for virtually all life on our planet.
Plastids evolved from ancient photosynthetic prokaryotes and are thought to have entered the eukaryotic lineage through endosymbiotic events. During evolution few genes encoding for about 100 different proteins remained in the plastid genome and are expressed by the chloroplast transcription and translation machinery. The majority of the 3000 chloroplast proteins are encoded in the nucleus, synthesized as precursors in the cytoplasm and translocated post-translationally into the chloroplast. Besides the fascinating composition of the chloroplasts for basic research, chloroplasts further bear the potential for biotechnological applications to use processes of energy conversion into biochemical modules for the production of bio-derived resources.
Previous work from others and myself indicate that chloroplast chaperones have an equally important function for protein homeostasis as in other cellular compartments. However, many aspects of the chaperone mediated folding and quality control network in the chloroplast are not clear. Thus, understanding the molecular chaperone-network contributing to de novo protein folding and quality control is key for a better understanding of chloroplast biosynthesis and maintenance.
Chlamydomonas reinhardtii as model organism
Chlamydomonas reinhardtii is a single-celled green algae with a size of about 10 µm, belonging to the phylum of Chlorophyta. The cells are characterized by a single, large and cup-shaped chloroplast, a large pyronoid, an eyespot and two flagella.
Complementary to higher plants, Chlamydomonas serves for many years as model system to study basic processes of photosynthetic active eukaryotes. Many components of the photosynthetic pathways are closely homolog to higher plants. Importantly, Chlamydomonas is not restricted to photosynthesis as sole carbon source as it is capable to heterotrophically metabolite acetate.
All three genomes (nuclear, mitochondrial, plastidic) have been fully sequenced and Chlamydomonas is the only organism where protocols exist to transform all three genomes. Despite considerable morphological differences of this green algae to higher plants, the organism is very well suited for the investigation of principle biochemical and cell biological processes.