Structural and biochemical characterisation of enzymes involved in mannan biosynthesis

Systemic infections caused by fungal pathogens pose a threat to immunocompromised patients worldwide. The structural integrity of fungi is mainly attributed to their rigid cell wall. The three layers of the fungal cell wall - chitin, glucan, and mannan - are mainly formed by carbohydrates. Mannan consists of proteins, mannoproteins, that carry N-linked glycans with a prominent mannose decoration. Mannoproteins have been shown to be involved in the detection of fungal pathogens by the immune system as well as adhesion factors of the pathogen to initiate invasion. The biosynthesis of mannoproteins occurs in the Golgi apparatus. The mannan polymerase complex M-Pol I, containing the glycosyltransferases Mnn9 and Van1, forms an α-1,6-linked mannose backbone that is the base for the extensive decoration of mannoproteins. The mechanisms by which M-Pol I identifies its substrates and its molecular mechanism are not known. Initially, mannoproteins can be trapped in the fungal cell membrane by a GPI-anchor. The anchor can be cleaved and the mannoproteins will become loosely attached or covalently linked to the glucan in the cell wall. The enzymes involved in these processes are unknown or poorly characterised. The two extracellular fungal proteins Dfg5 and Dcw1 are homologs to the bacterial mannosidase Aman6. The enzymatic function of Dfg5 and Dcw1 is unknown. However, both proteins may be involved in the transglycosylation of GPI-anchored mannoproteins. Dfg5 and Dcw1 are essential in yeasts, making them excellent drug targets against fungal pathogens. The aim of the work presented here was to structurally and enzymatically characterise the enzymes of the M-Pol I complex, Mnn9 and Van1, as well as the proteins Dfg5 and Dcw1 or their bacterial homolog Aman6. This would serve as a basis for the identification of potent inhibitors and their optimisation to lead compounds as antifungal drugs.