Protein transport in Plant Cells

Project Report from the year 2014 in the subject Biology - Cytology, grade: 2.0, LMU Munich, language: English, abstract: The aim of this study is to check for an import of the radioactive marked protein monodehydroascorbate reductase, translated in vitro from an Arabidopsis thaliana gene, into chloroplasts and mitochondria isolated from pea. MDHAR is an oxidoreductase, which participates in different metabolic pathways, especially in the ascorbate and alderate metabolism. The protein consists of 441 amino acids, which correlates to a molecular weight of 48.4 kDa. The biological role of MDHAR is related to salt stress, water deprivation and symbiosis with fungi.In plant cells, there are two organelles which were once freely living prokaryotes. Due to two independent endosymbiotic events, they became semi-autonomous cell organelles. Mitochondria are thought to be derived from -proteobacteria and chloroplasts evolved from cyanobacteria. Indeed those organelles have kept their prokaryotic biochemistry but plastidal and mitochondrial genomes have been reduced to plasmid size. This is an effect of intracellular gene transfer in which genes from the organelle are transferred to the nucleus. The driving force of this transfer is not proofed yet but there are several theories. Mutational pressure on the organelle genome, such as DNA damage and lack of recombination, might lead to an accumulation of deleterious mutations and therefore to a relocation of essential genes to the nucleus. Once transferred to the nucleus, the gene must be expressed and translocated back to the respective organelle. Therefore, the protein has to be translocated over several membranes and through different suborganellar compartments. Generally, mitochondria contain an inner and an outer membrane, the intermembrane space and the matrix. Chloroplasts are more complex and contain outer and inner envelopes, the inter membrane space, the stroma, the thylakoid membranes and the thylakoid lumen. Complex import mechanisms are known for both organelles. The chloroplast import machinery has an independent origin similar but different to the mitochondrial import machinery, but in general chloroplasts and mitochondria share a common principle of protein translocation over their membranes, ToC/TiC and ToM/TiM complexes. The ToC/TiC complexes are the translocators over the outer and inner envelope of the chloroplast and the ToM/TiM complexes translocate proteins over the outer and inner membrane of mitochondria.