Import peroxisomaler Matrixproteine über die Ustilago maydis Rezeptoren Pex5a, Pex5b und Pex7

Peroxisomes are organelles surrounded by a single membrane and are near-ubiquitous to eukaryotic cells. These organelles are essential for degradation of fatty acids and detoxify the in this process produced hydrogen peroxide. Peroxisomes can adapt their enzymatic content to the environment and the life cycle. Specialised peroxisomes can adopt very specific functions and are only found in a few cell types or organisms. Despite their diversity, peroxisomes share the same import system to transport proteins from the cytosol into the peroxisomal matrix. Peroxisomal proteins are translated in the cytosol and are imported as fully folded and co-factor bound proteins and even as oligomers. The vast majority of peroxisomal proteins contain a short conserved C-terminal targeting signal (PTS1). These proteins are imported by a conserved cytosolic receptor protein, Pex5. Only a few peroxisomal proteins carry an N-terminal targeting signal (PTS2), which is recognized by the receptor protein Pex7. After transferring the matrix protein across the peroxisomal membrane, the import receptors are recycled for further rounds of import. The plant pathogenic fungus Ustilago maydis encodes two Pex5 receptors (Pex5a, Pex5b) and one Pex7 receptor. Deletion of pex5a abolished growth on oleic acid but has nearly no effect on pathogenic development. In contrast, Pex5b was found to be important not only for growth on fatty acids, but also for filament formation and virulence. While both Pex5a and Pex5b are essential for growth on oleic acid the PTS2 receptor is not requiredunder these conditions. Many PTS2 proteins found in a bioinformatic analysis are of unknown function and some of them harbour additonal targeting signals, leading to the conclusion that most matrix proteins are imported by the PTS1 receptors, especially during growth on oleic acid. Deletion of pex5b disrupted the peroxisomal targeting of all PTS1 proteins. Despite their colocalisation with a peroxisomal marker protein in wild type cells, a few were found to additionally reside in the cytosol after deletion of pex5a. Thus, the peroxisomal targeting of some PTS1 proteins depends on Pex5a as well as Pex5b. In this work it could be shown, that for import functions Pex7 depends on the presence of Pex5b. As a conclusion, Pex5b serves as co-receptor for both Pex5a and Pex7. Pex5a and Pex5b do not only interact with their cargos by binding of the PTS1. It is feasible that additional interactions may serve to discriminate between different cargos. Additionally, transcription of pex5a and pex5b is differentially regulated during the life cycle of U. maydis suggesting modulation of the peroxisomal proteome in adaption to the environment.