Filamentous fungi like Aspergillus nidulans form long tubular cells, called hyphae, which grow rapidly by apical extension, enabling them to explore substrates and facilitating tissue invasion by pathogenic species. Because the shape of the hyphae is determined by the rigid external cell wall, apical growth requires that cell-wall sculpting enzymes polarize to the tips.
Endocytosis is essential for Aspergillus. It has long been suspected that its crucial physiological role reflects its involvement in a recycling pathway that takes up the excess of lipids as well as cell wall-modifying enzymes that diffuse across the plasma membrane and re-delivers them to the apex, thereby facilitating rapid apical extension.
In a recent study of the Aspergillus Molecular Genetics Laboratory published in PLOS Genetics, Dr. Miguel Peñalva’s group investigated endocytic recycling using chitin synthase ChsB, a cell wall-modifying enzyme that is an integral membrane protein. Miguel Hernández-González et al demonstrate that endocytic recycling indeed polarizes chitin synthase ChsB, which is delivered to the apex by exocytosis, reaches the plasma membrane and diffuses away from the apex in basipetal direction until it is captured by a subapical collar of endocytic patches. The enzyme internalized with endocytic vesicles arrives at a sorting endosome from which it is transported to the nearest trans-Golgi cisternae, where it is incorporated into secretory vesicles that re-deliver it to the apex.
Impairing this recycling pathway compromises apical extension and hyphal morphology markedly, suggesting that the pathway could be manipulated to prevent fungal invasion of plants and humans, an enormous burden on human welfare.
Reference: Endocytic recycling via the TGN underlies the polarized hyphal mode of life. Miguel Hernández-González, Ignacio Bravo-Plaza, Mario Pinar, Vivian de los Ríos, Herbert N. Arst Jr., Miguel A. Peñalva. PLOS Genetics (2018). https://doi.org/10.1371/journal.pgen.1007291