Rolle und Funktion von Peroxisomen in der Cryptomonade Guillardia theta

Peroxisomes are involved in several important metabolic processes in eukaryotic cells, such as detoxification of reactive oxygen species (ROS) or degradation of fatty acids by β-oxidation, but also in the glyoxylate cycle and photorespiration. The identification of peroxisome-specific peroxins in the cryptomonad Guillardia theta and other chromalveolates by Mix et al. (2018) confirmed the presence of peroxisomes in protists with the so-called complex plastids. In contrast to yeast, plant, and mammalian cells, little is known about the peroxisomal participation in metabolic networks within the cell in unicellular microalgae. In this work, heterologous localization studies of candidate proteins from G. theta with a putative C-terminal peroxisomal targeting signal type 1 (PTS1) were performed in the diatom Phaeodactylum tricornutum. This allowed the colocalization with the peroxisomal marker (mRuby3-SKL) of putative PTS1-proteins that might be involved in important metabolic processes such as carbohydrate, ether phospholipid, nucleotide, vitamin K, ROS, amino acid, and amine metabolism. In addition, G. theta homologs to factors of peroxisomal β-oxidation, the glyoxylate cycle, and photorespiration were analyzed in silico, and candidates with PTS1-like sequences were localized in P. tricornutum. Colocalization of these G. theta homologs with mRuby3-SKL was not observed heterologously and it is likely that the glyoxylate cycle in G. theta does not localize peroxisomally and that G. theta peroxisomes are not involved in photorespiration. Furthermore, the β-oxidative degradation of fatty acids in G. theta is probably exclusively mitochondrial. In the second part of this work, comparative genomic analyses of selected chlorophytes, rhodophytes, and glaucophytes identified putative peroxin sequences, indicating the presence of functional peroxisomes in most organisms analyzed in this study. Furthermore, in silico analyses indicate that in chlorophytes and glaucophytes PTS2-containing proteins might be present and imported into peroxisomes in addition to PTS1-proteins. The PTS2 is an N-terminal nonapeptide and is less abundant compared to PTS1.