Early endosomes motility in filamentous fungi: How and why they move

Elongate hyphae of filamentous fungi grow predominantly at their tips, whereas organelles are positioned in the subapical parts of the cell. Organelle positioning and long-distance intracellular communication involves active, energy-dependent transport along microtubules (MTs). This is mediated by specialized molecular motors, named kinesins and dynein, which utilize ATP hydrolysis to "walk" along the tubulin polymers. Work in the basidiomycete Ustilago maydis and the ascomycete Aspergillus nidulans has shown that early endosomes (EEs) are one of the major cargos of MT-dependent motors in fungi. EEs are part of the early endocytic pathway, and their motility behavior and the underlying transport machinery is well understood. However, the physiological role of constant bi-directional EE motility remains elusive. Recent reports, conducted in the corn smut fungus U. maydis, have provided novel insights into the cellular function of EE motility. They show that EE motility is crucial for the distribution of the protein synthesis machinery, and also that EEs transmit signals during plant infection that trigger the production of fungal effector proteins, required for successful invasion into host plants.