Accurate membrane targeting is essential for the organisation of membrane traffic. This step is particularly crucial at the Golgi apparatus, a central sorting hub of membrane traffic in the cell. Golgins, a class of highly conserved coiled-coil vesicle tethers, have been shown to mediate specificity of membrane traffic to the Golgi via the mitochondrial relocation assay in mammalian tissue culture cells (Wong and Munro, 2014). However, little is known about the role of golgins in polarised cells and the exact subclasses of vesicles they tether in different cell types. Hence, a tissue-specific mitochondrial relocation assay was devised in Drosophila to investigate the function of seven different golgins (Golgin-97, Golgin-245, GCC88, GCC185, TMF, GMAP, and Golgin-84) in different polarised cell types and identify cargoes, Rabs, and adaptor proteins that these golgins relocate. By doing so, I have shown that the broad specificities of the golgins in tissue culture cells perdure in the physiological context: trans-Golgi localized Golgin-97 and Golgin-245 capture endosome-to-Golgi vesicles, while the intra-Golgi golgins TMF, Golgin-84, and GMAP tether Golgi-derived retrograde transport carriers. Furthermore, ultrastructural data evidence the function of these golgins as bona fide vesicle tethers in Drosophila tissues.

More importantly, adding a tissue-specific dimension to the relocation assay elicited novel insights into golgin function that were not yet discovered in cultured cells. Golgins acquired or lost specific vesicle capture activities depending on the cell type, and GCC185, an inert golgin in COS and HeLa cells, unambiguously captured vesicles in Drosophila larval salivary glands (Wong and Munro, 2014). Furthermore, Golgin-97 and Golgin-245 exhibited functional differences in Drosophila tissues despite reported functional redundancy in mammalian tissue culture cells (Shin et al., 2017). This finding led to the generation and study of Drosophila knock-out and knock-in lines of Golgin-97/245 molecular receptor, TBC1D23. Finally, the combined cargo, adaptor protein, and Rab capture data evidenced partial functional redundancies between golgins, perhaps suggestive of collective vesicle tethering by the golgins. This hypothesis was partly confirmed by evidence of functional compensation between TMF and GMAP in experiments relocating ectopic golgins in golgin mutant backgrounds. Taken together, this work reveals physiologically relevant insights into the function of golgin vesicle tethers and illuminates the utility of Drosophila for understanding the complex pathways of traffic in polarised cells.