Long-Distance Movement of Aspergillus nidulans Early Endosomes on Microtubule Tracks

In fungal hyphal cells, intracellular membrane trafficking is constrained by the relatively long intracellular distances and the mode of growth, exclusively by apical extension. Endocytosis plays a key role in hyphal tip growth, which involves the coupling of secretory membrane delivery to the apical region with subapical compensatory endocytosis. However, the identity, dynamics and function of filamentous fungal endosomal compartments remain largely unexplored. Aspergillus nidulans RabA(Rab5) localizes to a population of endosomes that show long range bidirectional movement on microtubule (MT) tracks and are labelled with FM4-64 shortly after dye internalization. RabA(Rab5) membranes do not overlap with largely static mature endosomes/vacuoles. Impaired delivery of dynein to the MT plus ends or downregulation of cytoplasmic dynein using the dynein heavy chain nudA1(ts) mutation results in accumulation of RabA(Rab5) endosomal membranes in an abnormal NudA1 compartment at the tip, strongly supporting the existence in A. nidulans hyphal tips of a dynein loading region. We show that the SynA synaptobrevin endocytic recycling cargo traffics through this region, which strongly supports the contention that polarized hyphal growth involves the association of endocytic recycling with the plus ends of MTs located at the tip, near the endocytic internalization collar.