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Abstract:
The plant pathogenic fungus Ustilago maydis secretes effector proteins during the biotrophic interaction with its host plant Zea mays, with the aim being the suppression of the plant immune response virulence. All effector proteins virulence characterized to date were shown to be conventionally secreted proteins. However, over the last decade it could be established that proteins without a signal peptide can also be targeted to the outside of the cell in an ER/Golgi independent manner. A proteomic analysis on the apoplastic fluid of U. maydis infected maize plants identified 65 fungal proteins which did not contain a signal peptide. One of these proteins is the Acyl-CoA binding protein (ACBP) of U. maydis, Acb1, which will be investigated in this thesis. Over the years many studies have been dedicated to the functional analysis of Acyl-CoA binding proteins, in which ACBPs were shown to have many distinct intracellular functions, such as control over the intracellular Acyl-CoA pool size, roles in vesicular trafficking and their relevance for fatty acid biosynthesis. Further, in addition to their metabolic roles, ACBPs were shown to be involved in gene regulation, and also signaling by receptor binding, as was shown for the ACBP of the amoeba Dictyostelium discoideum (AcbA). In this organism it was shown that AcbA is secreted under nutrient limitation and extracellularly processed into the Spore Differentiation Factor 2 (SDF-2), which induces terminal spore differentiation. In this thesis it could be shown that the Acb1 protein of U. maydis is unconventionally secreted, subsequently processed into a SDF-2 like peptide and that this peptide is capable of inducing the sporulation of D. discoideum cells. The study of the protein’s extracellular function in U. maydis, as a full length secreted protein or in its processed form as the SDF-2 like peptide was hampered by the important intracellular functions of Acb1. Deletion of the acb1 gene resulted in strong growth defects and an accumulation of suppressor mutations, which were able to restore growth in a mutation dependent manner. Ultimately it could be shown that a virulence related function of the SDF-2 like peptide in both the solopathogenic strain SG200, as well as the compatible wild type strains FB1 and FB2, has to be considered unlikely. A potential extracellular role of the full length secreted protein has to date been neither confirmed nor ruled out.
