Vesicle-driven endomembrane systems in fungi

SUMMARYIn fungi, the endomembrane system is a pleiomorphic, dynamic network of organelles, driven by vesicle trafficking pathways, which maintain cellular homeostasis, hyphal polar growth, and the secretion of proteins and metabolites. In syncytial hyphae, spatial specialization of organelles and other cellular components of the endomembrane system is evident to support growth and adaptation. Young, apical regions of hyphae contain a Golgi-Spitzenkörper-exocyst triad for rapid polar expansion, whereas distal, older hyphal regions employ unconventional secretion via multivesicular bodies (MVBs), septal vesicle fusion, and extracellular vesicles (EVs) to enhance nutrient acquisition for the entirety of the mycelium. Vesicular trafficking integrates distinct endomembrane compartments into specialized pathways that involve vesicle biogenesis, transport, and fusion to sustain polarized growth and secretion. Actin and microtubules provide tracks for vesicle motility, while Rab GTPases regulate vesicle localization and fusion events. The ESCRT machinery governs MVB formation and scission, COPI/II regulate bidirectional endoplasmic reticulum-Golgi transport, SNARE proteins allow for vesicle and target membrane fusion, and the exocyst complex tethers vesicles to exocytic regions of the plasma membrane. Together, these components form dynamic endomembrane assembly lines that coordinate many cellular processes. The "distance hypothesis" predicts that extracellular vesicle-mediated secretion predominates in subapical regions as tip growth slows. This mechanism extends the secretory capabilities of hyphae and promotes broader distribution of secreted enzymes along hyphae. Having a better understanding of spatially regulated secretion pathways will advance our understanding of fungal cell biology and provide strategies to optimize fungi for industrial protein production.