ABC transporters required for endocytosis and endosomal pH regulation in Dictyostelium

In Dictyostelium, the RtoA protein links both initial cell-type choice and physiological state to cell-cycle phase. rtoA^– cells (containing a disruption of the rtoA gene) generally do not develop past the mound stage, and have an abnormal ratio of prestalk and prespore cells. RtoA is also involved in fusion of endocytic/exocytic vesicles. Cells lacking RtoA, although having a normal endocytosis rate, have a decreased exocytosis rate and endosomes with abnormally low pHs. RtoA levels vary during the cell cycle, causing a cell-cycle-dependent modulation of parameters such as cytosolic pH (Brazill et al., 2000). To uncover other genes involved in the RtoA-mediated differentiation, we identified genetic suppressors of rtoA. One of these suppressors disrupted two genes, mdrA1 and mdrA2, a tandem duplication encoding two members of the ATP binding cassette (ABC) transporter superfamily. Disruption of mdrA1/mdrA2 results in release from the developmental block and suppression of the defect in initial cell type choice caused by loss of the rtoA gene. However, this is not accomplished by re-establishing the link between cell type choice and cell cycle phase. MdrA1 protein is localized to the endosome. mdrA1^–/mdrA2^– cells (containing a disruption of these genes) have an endocytosis rate roughly 70% that of wild-type or rtoA^– cells, whereas mdrA1^–/mdrA2^–/rtoA^– cells have an endocytosis rate roughly 20% that of wild-type. The exocytosis rates of mdrA1^–/mdrA2^– and mdrA1^–/mdrA2^–/rtoA^– are roughly that of wild-type. mdrA1^–/mdrA2^– endosomes have an unusually high pH, whereas mdrA1^–/mdrA2^–/rtoA^– endosomes have an almost normal pH. The ability of mdrA1/mdrA2 disruption to rescue the cell-type proportion, developmental defects, and endosomal pH defects caused by rtoA disruption, and the ability of rtoA disruption to exacerbate the endocytosis defects caused by mdrA1/mdrA2 disruption, suggest a genetic interaction between rtoA, mdrA1 and mdrA2.