Polarisation, key to good localisation.

Polarisation of cells is crucial for vectorial transport of ions and solutes. In literature, however, proteins specifically targeted to the apical or basolateral membrane are often studied in non-polarised cells. To investigate whether these data can be extrapolated to expression in polarised cells, we studied several membrane-specific proteins. In polarised MDCK cells, the Aquaporin-2 water channel resides in intracellular vesicles and apical membrane, while the vasopressin-type 2 receptor, anion-exchanger 1 (AE1) protein and E-Cadherin mainly localise to the basolateral membrane. In non-polarised MDCK cells, however, Aquaporin-2 localises, besides plasma membrane, mainly in the Golgi complex, while the others show a dispersed staining throughout the cell. Moreover, while AQP2 mutants in dominant nephrogenic diabetes insipidus are missorted to different organelles in polarised cells, they all predominantly localise to the Golgi complex in non-polarised MDCK cells. Additionally, the maturation of V2R, and likely its missorting, is affected in transiently-transfected compared to stably-transfected cells. In conclusion, we show that the use of stably-transfected polarised cells is crucial in interpreting the processing and the localisation of membrane targeted proteins.