ABSTRACT Aspergillus fumigatus is the main etiological agent of invasive aspergillosis (IA). IA primarily affects immunocompromised patients and carries a mortality rate as high as 60%. Due to the significant increase in the immunocompromised patient population and the emergence of azole-resistant A. fumigatus, a critical understanding of A. fumigatus biology is needed to improve patient outcomes. Caspofungin, which targets fungal cell wall synthesis, is a second-line therapy for invasive aspergillosis. However, this antifungal is fungistatic rather than fungicidal. Comprehending how A. fumigatus responds to caspofungin can lead to a much-needed breakthrough, improving the caspofungin treatment success. Septins are a conserved family of GTP-binding proteins. Septins interact with each other to form higher- order structures and recruit other proteins. Septins play roles in recognizing micron-scale plasma membrane curvature, cytokinesis, cell cycle progression, and response to cell wall stress. The overall aim of our R01 application is to determine the molecular mechanism that contributes to the septin-dependent fungal response to caspofungin. We hypothesize that septin AspB mediates Aspergillus fumigatus fungistatic response to caspofungin. Our hypothesis is supported by 1) an increase in susceptibility to caspofungin after deletion of core septin genes in various fungal pathogens, 2) the increase in the number of visible septin structures after exposure to caspofungin, 3) protein pulldowns that show septin complex together with components of the cell wall integrity pathway, and 4) preliminary proteomics analysis that shows differential septin-protein interactions after exposure to caspofungin. We will test our central hypothesis in three aims: Aim 1. Determine the role of the septin cytoskeleton in response to caspofungin, Aim 2. Define septin-protein interactions after caspofungin exposure and Aim 3. Elucidate the role of the septin cytoskeleton in the cell wall integrity pathway. Completing this grant will close a critical gap in understanding how fungal pathogens respond to caspofungin and how septins facilitate this. With this knowledge, novel therapeutic approaches can be designed to ultimately improve the outcome of caspofungin treatment.