ATP6AP2 bearing a Leu98Ser substitution in the extracellular domain, which is associated with a glycosylation disorder in the human ortholog, is expressed under the control of UAS. (Mutagenesis was done on the pUASg-HAattB plasmid containing the coding sequence of ATP6AP2 optimised by GeneArt (Invitrogen) with a final stop codon that impedes the expression of the HA tag.)