The secretory pathway of cell wall building enzymes in Neurospora crassa hyphae

Neurospora crassa has been at the vanguard of biochemical and genetics research for over a century. Remarkably, this filamentous fungus has become as well a magnificent model system to study polarized cell growth. We have used N. crassa to identify and analyze by high-resolution live fluorescence imaging key players of the secretory processes leading to a polarized delivery of vesicles at sites of cell wall growth, i. e. tips and septa. While chitin synthases (CHSs) are contained in microvesicles (chitosomes) that concentrate at the core of the Spitzenkörper (Spk) at hyphal apices, and β-1,3-glucan synthases are contained in macrovesicles that occupy the outer layer of the Spk, the mechanisms of traffic and sorting of these enzymes from synthesis to delivery sites remain largely unexplored. We have shown that distinct Rab GTPases are the likely candidates that mediate the vesicular journey along the hyphae towards the apex, where the octameric exocyst complex mediates vesicle tethering at the apical plasma membrane. Our current efforts are oriented towards understanding further the secretory pathway/s followed by the cell wall biosynthetic nanomachineries. We have focused on CHS-4; its localization at the apex and septa depends on CSE-7, a putative ER-based chaperone, but is independent of the exocyst. We are characterizing an extensive endomembranous network of tubules and sheets, presumably the secretory compartments associated with the biogenesis of CHS-4.