Maturation of late Golgi cisternae into RabE^RAB11 exocytic post-Golgi carriers visualized in vivo

The mechanism(s) by which proteins traverse and exit the Golgi are incompletely understood. Using Aspergillus nidulans hyphae, we show that late Golgi cisternae undergo changes in composition to gradually lose Golgi identity while acquiring post-Golgi RabE^RAB11 identity. This behavior of late Golgi cisternae is consistent with the cisternal maturation model. Post-Golgi RabE^RAB11 carriers travel to, and accumulate at, the apex, indicating that fusion is rate limiting for exocytosis. These carriers, which are loaded with kinesin, dynein, and MyoE^MYO5, move on a microtubule-based bidirectional conveyor belt relaying them to actin, which ultimately focuses exocytosis at the apex. Dynein drags RabE^RAB11 carriers away if engagement of MyoE^MYO5 to actin cables fails. Microtubules seemingly cooperating with F-actin capture can sustain secretion if MyoE^MYO5 is absent. Thus, filamentous fungal secretion involving post-Golgi carriers is remarkably similar, mechanistically, to the transport of melanosomes in melanocyte dendrites, even though melanosome biogenesis involves lysosomes rather than Golgi.