Filamentous fungi have a long history in biotechnology for the production of food ingredients, pharmaceuticals and enzymes. The advancements made in recent years have earned filamentous fungi such as Aspergillus species a dominant place among microbial cell factories. Although the model fungus A. nidulans has been extensively studied, the genetic and regulatory networks that underlie morphogenesis and development have yet to be fully characterized. The Rho GTPases (Cdc42 and RacA) are one of the most important regulators of the morphogenetic processes among diverse eukaryotic organisms. Although the functions of these GTPases are relatively well-characterized, little is known about their downstream effectors. One likely effector is the formin SepA, which also belongs to a complex known as the polarisome that helps to stabilize and support hyphal growth. The uncharacterized gene ANID_05595.1 (ModB) possesses sequence features that suggest it also belongs to the polarisome. My genetic and functional characterization of ModB reveals some overlap with SepA, but also shows that ModB possesses distinct roles in the maintenance of hyphal polarity.
In filamentous fungi, hyphal morphology requires the localized delivery of exocytic vesicles to the hyphal tip as well as to septation sites. The mechanisms that regulate vesicle trafficking to these locations are not yet well understood. In addition, because fungal hyphae presumably extend through a nutritionally variable environment, these mechanisms must be extremely sensitive to growth conditions. To begin to address these issues, I have investigated the effects of nutrient conditions on localization patterns of two different proteins that are trafficked to the cell surface; a glucose transporter (HxtB) and the enzyme β-glucosidase (BglA). Although the final localization of each protein differs, they display similar localization dynamics upon release from glucose repression. In parallel, I also used fluorescence microscopy to determine how shifts from glucose to a non-preferred carbon source (e.g., cellulose) affect hyphal extension and morphology. My results suggest that relief from glucose repression leads to the production of thinner hyphae after a transient delay in hyphal extension. Use of variety of signaling mutants further demonstrates that this response requires a functional protein kinase A (PKA) as well as proper down-regulation of heterotrimeric G protein signals. Collectively, these observations provide valuable new insight into how vesicle trafficking responds to variable growth conditions.
Advisor: Steven D. Harri
