Figure 5.

Effect of site-specific mutations on transport and substrate interactions in human NaPi-IIa. (A) Tracer uptake (³²P) data normalized to WT for the mutants and noninjected (NI) oocytes as control. Each data set is the mean ± SE of data from two batches of oocytes (4–5 oocytes per batch). (B) Presteady-state relaxations recorded from representative oocytes expressing selected constructs in RU1 and RU2, together with WT and NI as controls superfused in 100 mM Na⁺. Bold label indicates active P_i transport was also measured. Transient currents shown in response to voltage steps as indicated, from a holding potential = –60 mV to two test potentials: –160 (blue) and +60 mV (red). Traces were baseline corrected and are shown with the same ±500 nA range. Each relaxation was fit with a double exponential function. Dotted traces is the fitted fast component (relaxation time constant <1ms) corresponding to endogenous membrane charging and are comparable to the NI data in all cases. For all mutants shown there was charge movement in excess of the endogenous component. To see this figure in color, go online.