Development of a label-free assay for sodium-dependent phosphate transporter NaPi-IIb

The most widely used assay format for characterizing plasma membrane transporter activity measures accumulation of radiolabeled substrates in tissues or cells expressing the transporters. This assay format had limitations and disadvantages; therefore, there was an unmet need for development of a homogeneous, nonradioactive assay for membrane transporter proteins. In this report, the authors describe the development of a label-free homogeneous assay for the sodium-dependent phosphate transporter NaPi-IIb using the Epic system. The addition of phosphate stimulated a dynamic mass redistribution (DMR) profile unique to cells expressing NaPi-IIb but not on parental cells. This DMR profile was phosphate specific because sulfate or buffer alone did not elicit the same response. Furthermore, the DMR response observed was phosphate and sodium dependent, with Km values in the micromolar and millimolar range, respectively. A known NaPi-IIb noncompetitive inhibitor was shown to completely inhibit the phosphate-stimulated DMR response, suggesting that this observed DMR response is an NaPi-IIb-mediated cellular event. The results demonstrate that a novel label-free assay was developed for studying transporter-mediated cellular activity, and this DMR assay platform could be applicable to other membrane transporter proteins.