In the collecting ducts from Aqp2F204V/+ mice, the wild type may rescue the mutant protein as suggested by the subcellular distribution of AQP2 protein. To test this idea, we first looked for an interaction between mutant and wild-type proteins in transfected MDCK cells (Figure 5B). MDCK cells stably expressing wild-type AQP2 were transiently transfected with GFP expression constructs encoding GFP-tagged wild-type AQP2, AQP2-F204V, or GFP alone. Antibodies against GFP coimmunoprecipitated wild-type AQP2 when AQP2-GFP or AQP2-F204V-GFP was transiently transfected, but not when GFP by itself was transiently transfected into MDCK cells stably expressing wild-type AQP2 (Figure 5B, upper blots). Western blot of total membranes showed that wild-type AQP2 is equivalently expressed in all three cases (Figure 5B, lower blots).