Flotillin-containing lipid rafts in Aspergillus nidulans are linked to calcium

Lipid rafts are tight assemblies of proteins and lipids in a biological membrane and are thought to be involved in many physiological processes such as immune signalling and host-pathogen interactions. However, due to their small sizes exceeding the resolution limit of conventional light microscopy, direct measurement and characterisation of lipid rafts in living membranes remains to be a challenge. While most studies on lipid rafts have been carried out on mammalian cells, here we use the genetic model fungus Aspergillus nidulans to allow a more versatile characterisation of lipid rafts at both molecular and organismic levels. In particular we investigate the role of flotillin (FloA), a lipid raft marker conserved across many organisms. Using a nanoluciferase reporter strain, we demonstrate that the A. nidulans flotillin FloA is highly expressed when the fungus is confronted with high calcium stress. Transcriptomic analysis further reveals that repression of FloA under these conditions leads to the upregulation of numerous mitochondrial genes, suggesting a functional connection between FloA and mitochondrial activity. In addition, in vivo protein-proximity labelling is performed to assess the physical interaction partner proteins of FloA. We also explore the role of FloA in microbial communication through co-cultivation experiments with the soil bacterium Streptomyces iranensis and show that the bacterium is able to induce the high expression of FloA, most likely by secreting a natural product. These findings contribute to a deeper understanding of lipid raft dynamics and the organisation of eukaryotic membranes.