Spatial Control of Protein Homeostasis by Microtubule-Based Motors in Filamentous Fungi

Abstract

Extreme cell polarity places unique demands on long-range transport mediated by the microtubule-based motors dynein and kinesin. Accordingly, deficient protein homeostasis in highly polarized neurons, a process to which these motors contribute through spatial organization of damaged proteins, is closely associated with neurodegenerative disease and cellular aging. Despite this relationship, relatively little is understood regarding how dynein and kinesin meet the challenges of extreme cell polarity to achieve proper protein homeostasis. Furthermore, mechanistic understanding of such processes is limited by a lack of a model system capable of addressing questions both in vivo and in vitro. In this thesis, I use the tractable filamentous fungus Aspergillus nidulans to show that microtubules and dynein mediate the assembly of periodic deposition sites of deleterious aggregated proteins in highly polarized fungal cells as a mechanism of cytoplasmic quarantine. Overwhelming this spatial sequestration pathway yields impaired cellular fitness and global impediments of microtubule-based transport processes. In addition, I demonstrate that the activity of purified A. nidulans dynein and kinesin motors can be observed in vitro, providing a foundation for complementary reconstitution and fine mechanistic study of processes such as aggregate transport. Collectively, these results demonstrate and expand the versatility of filamentous fungi in studying microtubule-based transport in highly polarized cells.