Loss of TBC1D5 expression would be expected to lead to increased activation of Rab7a and this is what we observe. This, in turn, results in increased association of the retromer CSC (a Rab7a effector), with Rab7a and most likely accounts for the elevated levels of endosomally localised retromer CSC proteins. Increased active Rab7a would be predicted to also lead to changes in lysosome function and/or morphology as Rab7a is a key regulator of lysosomes (Basuray et al., 2012; Guerra and Bucci, 2016). We did observe changes in the morphology of lysosomes (using the protein Lamp1 as a marker) after TBC1D5 knockdown in line with changes elicited by knockdown of Rab7a itself (see Fig. S4). Studies in yeast have shown that the membrane-associated SNX-BAR dimer of Vps5p-Vps17p can displace yeast Rab7 (Ypt7p) from the retromer CSC and thereby coordinate the cargo selection activity of the retromer CSC with tubule formation (Purushothaman et al., 2017). Yeast do not have an obvious TBC1D5 homologue, however, and the yeast retromer complex is a much tighter association of the SNX-BAR dimer with the retromer CSC
