Interestingly, although levels of endosomally localised Fam21 are enhanced after loss of TBC1D5 expression, trafficking of the Glut1 protein to the cell surface, which is dependent upon Fam21 function, is not wholly rescued, possibly indicating that either the D620N mutation has effects on retromer function not confined to the loss of interaction with the WASH complex or that TBC1D5 function may be required for proper localisation of membrane proteins that pass through the endosome. There is some evidence to suggest that loss of TBC1D5 can cause mislocalisation of integrin proteins (Jia et al., 2016) but in our experiments we have not observed any pronounced deleterious effects on protein trafficking after loss of TBC1D5 function. When we investigated the localisation of the classical retromer cargo protein, the CIMPR, we found that there was a modest increase in the colocalisation of the CIMPR with the TGN marker protein, TGN46. It is not likely, however, that enhanced retromer function through TBC1D5 inhibition would markedly increase the levels of TGN-localised retromer cargo proteins as this would require increased production of transport intermediates and elevated
