Hyperbranching in the filamentous fungus Aspergillus niger following deletion of the GTPase RacA leads to altered glucoamylase secretion upon overexpression of the enzyme

Filamentous fungi secrete hydrolytic enzymes to degrade polymeric substances into smaller molecules which are then taken up as to sustain growth and metabolism. The accepted paradigm in fungal biology is that the tips of fungal hyphae are the highly active regions of a fungal colony, where polarised growth and secretion are coupled processes. However, it is currently debated if the amount of growing hyphal tips in filamentous fungi correlates with an increase in secretion, with previous studies showing either a positive or no correlation. In this study, we investigated the previously described hyperbranching strainof the industrial cell factory Aspergillus niger, which is deleted in the GTPase RacA and builds more hyphal tips but shows otherwise identical growth rate and total protein secretion as the wildtype.

Here, we use a v-SNARE reporter strain (SncA-GFP) to show that the hyperbranching strain exhibits an increased level in secretory vesicles at the hyphal tip upon overexpression of glucoamylase driven by the Tet-on system. Thus, we establish for the first time a link between level of transcript/secretory cargo load with the gradient of secretory vesicles at hyphal tip. We show that ΔracA secretes altered amounts of glucoamylase upon Tet-on driven overexpression of the enzyme in comparison to the parental strain despite unaltered biomass yields, total secretory vesicles, or total protein secretion. Our results contribute to the understanding of fungal protein secretion at the hyphal tip, and have profound implications for biotechnology and applied mycology.