The Rab GTPase YPT-1 associates with Golgi cisternae and Spitzenkörper microvesicles in Neurospora crassa.

Vesicle traffic involves budding, transport, tethering and fusion of vesicles with acceptor membranes. GTP-bound small Rab GTPases interact with the membrane of vesicles, promoting their association with other factors before their subsequent fusion. Filamentous fungi contain at their hyphal apex the Spitzenkörper (Spk), a multivesicular structure to which vesicles concentrate before being redirected to specific cell sites. The regulatory mechanisms ensuring the directionality of the vesicles that travel to the Spk are still unknown. Hence, we analyzed YPT-1, the Neurospora crassa homologue of Saccharomyces cerevisiae Ypt1p (Rab1), which regulates different secretory pathway events. Laser scanning confocal microscopy revealed fluorescently tagged YPT-1 at the Spk and putative Golgi cisternae. Co-expression of YPT-1 and predicted post-Golgi Rab GTPases showed YPT-1 confined to the Spk microvesicular core, while SEC-4 (Rab8) and YPT-31 (Rab11) occupied the Spk macrovesicular peripheral layer, suggesting that trafficking and organization of macro and microvesicles at the Spk are regulated by distinct Rabs. Partial colocalization of YPT-1 with USO-1 (p115) and SEC-7 indicated the additional participation of YPT-1 at early and late Golgi trafficking steps.