Off the wall: The rhyme and reason of

Highlights • Chitin and β-1,3-glucan synthases are transported separately in chitosomes and macrovesicles. • Chitin synthases occupy the core of the SPK; β-1,3-glucan synthases the outer layer. • CHS-4 arrival to the SPK and septa is CSE-7 dependent. • Rabs YPT-1 and YPT-31 localization at the SPK mimics that of chitosomes and macrovesicles. • The exocyst acts as a tether between the SPK outer layer vesicles and the apical PM.
The fungal cell wall building processes are the ultimate determinants of hyphal shape. In Neurospora crassa the main cell wall components, β-1,3-glucan and chitin, are synthesized by enzymes conveyed by specialized vesicles to the hyphal tip. These vesicles follow different secretory routes, which are delicately coordinated by cargo-specific Rab GTPases until their accumulation at the Spitzenkörper. From there, the exocyst mediates the docking of secretory vesicles to the plasma membrane, where they ultimately get fused. Although significant progress has been done on the cellular mechanisms that carry cell wall synthesizing enzymes from the endoplasmic reticulum to hyphal tips, a lot of information is still missing. Here, the current knowledge on N. crassa cell wall composition and biosynthesis is presented with an emphasis on the underlying molecular and cellular secretory processes.