Instability of aquaglyceroporin (AQP) 2 contributes to drug resistance in Trypanosoma brucei

Defining mode of action is vital for both developing new drugs and predicting potential resistance mechanisms. African trypanosome pentamidine and melarsoprol sensitivity is predominantly mediated by aquaglyceroporin 2 (TbAQP2), a channel associated with water/glycerol transport. TbAQP2 is expressed at the flagellar pocket membrane and chimerisation with TbAQP3 renders parasites resistant to both drugs. Two models for how TbAQP2 mediates pentamidine sensitivity have emerged; that TbAQP2 mediates pentamidine translocation or via binding to TbAQP2, with subsequent endocytosis, but trafficking and regulation of TbAQPs is uncharacterised. We demonstrate that TbAQP2 is organised as a high order complex, is ubiquitylated and transported to the lysosome. Unexpectedly, mutation of potential ubiquitin conjugation sites, i.e. cytoplasmic lysine residues, reduced folding and tetramerization efficiency and triggered ER retention. Moreover, TbAQP2/TbAQP3 chimerisation also leads to impaired oligomerisation, mislocalisation, and increased turnover. These data suggest that TbAQP2 stability is highly sensitive to mutation and contributes towards emergence of drug resistance.