Unconventional secretion and its biotechnological applications in corn smut
Our group is mainly interested in a novel mechanism of unconventional secretion in the corn smut fungus Ustilago maydis. In different projects we aim on uncovering the molecular details of this mechanism and on applying it for protein production.
Uncovering the mechanism of unconventional secretion
Protein secretion in eukaryotic cells mostly occurs via the endomembrane system including endoplasmic reticulum (ER) and Golgi apparatus. Proteins directed to this route carry an N-terminal secretion signal that targets them to the ER. Within the endomembrane system proteins are post-translationally modified e.g. by N-glycosylation. Besides this well-established mechanism unconventional secretion mechanisms exist. These include vesicular and non-vesicular routes which are independent of the canonical signal peptides for classical secretion. In U. maydis we discovered an unconventional secretory mechanism for chitinase Cts1. In a basic research project we study this mechanism in detail in order to understand why it is employed by the cell. To this end, we identify components of the secretion pathway followed by their functional characterization.
Establishment and application of a novel protein expression platform
Establishing and applying novel protein expression systems is crucial in producing the full repertoire of economically relevant proteins in the area of white biotechnology. Although a variety of prokaryotic and eukaryotic expression systems is already successfully applied in the industrial biotechnology, there is a great need for novel expression platforms that allow heterologous expression of proteins that are currently difficult to produce. To fill existing gaps, we use the basidiomycete fungus Ustilago maydis as novel cellular platform for protein production. This eukaryotic microorganism is very well suited for biotechnological applications as it is amenable to genetic manipulation and mutants can be generated rapidly. Furthermore, biochemical and proteomic approaches are well established and easily applicable.
In an applied project, we use an unconventional secretion mechanism based on chitinase Cts1 in U. maydis to export heterologous proteins of interest, i.e. therapeutic antibody formats (Fig. 1). The advantage is that proteins secreted via this route are not N-glycosylated. This can be advantageous for expression of bacterial or therapeutic proteins. Currently, we are optimizing the system on different levels. For example, knowledge gained on the mechanism of secretion (see above) is applied for improving the secretion of heterologous target proteins. In parallel, we are scaling-up the system by establishing optimal cultivation and fermentation conditions.
Biomass valorization – from biomass side-streams to valuable products
The conversion of biomass side-streams or waste to valuable substances is one main challenge in bioeconomy. As a plant pathogen, U. maydis contains a distinct set of hydrolytic enzymes that can be exploited for biomass degradation. These can be complemented by potent heterologous enzymes using conventional secretion for eukaryotic and unconventional secretion for bacterial enzymes. In addition, U. maydis is a natural producer of organic acids and glycolipids. Both are considered valuable substances with multiple biotechnological applications. Hence it is conceivable to use U. maydis for one-pot biomass conversion into valuable products. Together with cooperation partners (group of Dr. Nick Wieckx, RWTH Aachen) we provided first evidence, that U. maydis is able to convert simple biomass components into organic acid. Currently, this strategy is extended to more complex biomass sources.