An Atypical Aquaporin 1 Derived From Paramecium multimicronucleatum Functions as a Multifunctional Channel That...

Although it had been reported that the translation product of the functional aquaporin gene, aqp1, is localized to the contractile vacuole complex of Paramecium multimicronucleatum, the molecules that pass through this channel protein had not been identified. In the present study, we introduced and expressed this protein in Xenopus oocytes and identified the molecules that pass through this channel protein using a swelling assay. The aqp1 mRNA injected into Xenopus oocytes was found to be sufficiently expressed and functional within the oocytes. As a result, P. multimicronucleatum AQP1 (PmAQP1) allowed water to pass through at a relatively slow rate of approximately 50 × 10^-4cm/s, a rate comparable to that of aquaporins present in the contractile vacuoles of other protists. This water permeability was almost completely inhibited by tetraethylammonium, an orthodox aquaporin inhibitor, and phloretin, an aquaglyceroporin inhibitor. PmAQP1 also had the ability to pass glycerol or urea through, but the rates of these processes could not be accurately measured in this study. These findings demonstrate that PmAQP1 is an atypical multifunctional channel, suggesting that the contractile vacuole of free-living protists plays a crucial role not only in osmoregulation but also in the clearance of metabolic wastes.