Correlating Charge Movements with Local Conformational Changes of a Na+-Coupled Cotransporter

Abstract

To gain insight into the steady-state and dynamic characteristics of structural rearrangements of an electrogenic secondary-active cotransporter during its transport cycle, two measures of conformational change (pre-steady-state current relaxations and intensity of fluorescence emitted from reporter fluorophores) were investigated as a function of membrane potential and external substrate. Cysteines were substituted at three believed-new sites in the type IIb Na+-coupled inorganic phosphate cotransporter (SLC34A2 flounder isoform) that were predicted to be involved in conformational changes. Labeling at one site resulted in substantial suppression of transport activity, whereas for the other sites, function remained comparable to the wild-type. For these mutants, the properties of the pre-steady-state charge relaxations were similar for each, whereas fluorescence intensity changes differed significantly. Fluorescence changes could be accounted for by simulations using a five-state model with a unique set of apparent fluorescence intensities assigned to each state according to the site of labeling. Fluorescence reported from one site was associated with inward and outward conformations, whereas for the other sites, including four previously indentified sites, emissions were associated principally with one or the other orientation of the transporter. The same membrane potential change induced complementary changes in fluorescence at some sites, which suggested that the microenvironments of the respective fluorophores experience concomitant changes in polarity. In response to step changes in voltage, the pre-steady-state current relaxation and the time course of change in fluorescence intensity were described by single exponentials. For one mutant the time constants matched well with and without external Na+, providing direct evidence that this label reports conformational changes accompanying intrinsic charge movement and cation interactions.