A novel RAB11-containing adaptor complex anchoring myosin-5 to secretory vesicles

Hyphal fungi grow rapidly by apical extension, providing a notorious example of polarized growth. The continuous supply of secretory vesicles necessary to meet the demands of the extending tip and the long intracellular distances existing between the tip and the basal septum, often localized > 100 µm away from the former, impose the need of efficient networks of intracellular traffic involving exquisite cooperation between microtubule- and actin-mediated transport. In Aspergillus nidulans kinesin-1 conveys secretory vesicles to the hyphal tip, where they are transferred to myosin-5, which focuses them at the growing apex, thereby determining cell shape. This relay mechanism and the central role played by myosin-5 in hyphal morphogenesis suggested that the mechanisms anchoring secretory vesicles to this motor should involve specific adaptor(s) ensuring the robustness of actomyosin-dependent transport.

Secretory vesicles are charged with RAB11, a regulatory GTPase that determines the Golgi to post-Golgi identity transition. By using a combination of shotgun proteomics, GST-RAB pull-down assays, in vitro reconstitution experiments, targeted reverse genetics and multidimensional fluorescence microscopy with endogenously tagged proteins we show that RAB11, the master regulator of fungal exocytosis, mediates myosin-5 engagement both by contacting the motor and by recruiting UDS1, a homologue of an as yet uncharacterized Schizosaccharomyces protein ‘upregulated during mitosis’, which we demonstrate to be a novel RAB11 effector. Analytical ultracentrifugation determined that UDS1 is an elongated dimer and negative-stain electron microscopy showed that, in agreement, UDS1 is rod-shaped. UDS1 does not contact myosin-5 directly, but rather recruits the coiled-coil HMSV, which bridges RAB11/UDS1 to myosin-5. An HMSV-scaffolded complex containing UDS1 and myosin-5 is present in cells, and a RAB11-UDS1-HMSV complex can be reconstituted in vitro in a RAB nucleotide state-dependent manner. In the absence of UDS1/HMSV the steady state levels of myosin-5 at the apical vesicle supply center diminish markedly, such that microtubule-dependent transport spreading vesicles across the apical dome predominates over apex-focused actin-mediated transport. As a consequence, RAB11 and chitin-synthase B (a cargo of the RAB11 pathway) are not focused at the apex, being distributed instead across the apical dome. Therefore, the RAB11 effector UDS1/HMSV cooperates with the GTPase to adapt secretory vesicles to myosin-5, which is required for the apical targeting of RAB11 cargoes and thus for the normal morphology of the hyphae.