Ehnes C et al. (2004), Structure-function relations of the first and f...

XB-ART-2834

J Gen Physiol 2004 Nov 01;1245:489-503. doi: 10.1085/jgp.200409061.

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Structure-function relations of the first and fourth extracellular linkers of the type IIa Na+/Pi cotransporter: II. Substrate interaction and voltage dependency of two functionally important sites.

Ehnes C , Forster IC , Bacconi A , Kohler K , Biber J , Murer H .

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Functionally important sites in the predicted first and fourth extracellular linkers of the type IIa Na+/Pi cotransporter (NaPi-IIa) were identified by cysteine scanning mutagenesis (Ehnes et al., 2004). Cysteine substitution or modification with impermeant and permeant methanethiosulfonate (MTS) reagents at certain sites resulted in changes to the steady-state voltage dependency of the cotransport mode (1 mM Pi, 100 mM Na+ at pH 7.4) of the mutants. At Gly-134 (ECL-1) and Met-533 (ECL-4), complementary behavior of the voltage dependency was documented with respect to the effect of cys-substitution and modification. G134C had a weak voltage dependency that became even stronger than that of the wild type (WT) after MTS incubation. M533C showed a WT-like voltage dependency that became markedly weaker after MTS incubation. To elucidate the underlying mechanism, the steady-state and presteady-state kinetics of these mutants were studied in detail. The apparent affinity constants for Pi and Na+ did not show large changes after MTS exposure. However, the dependency on external protons was changed in a complementary manner for each mutant. This suggested that cys substitution at Gly-134 or modification of Cys-533 had induced similar conformational changes to alter the proton modulation of transport kinetics. The changes in steady-state voltage dependency correlated with changes in the kinetics of presteady-state charge movements determined in the absence of Pi, which suggested that voltage-dependent transitions in the transport cycle were altered. The steady-state and presteady-state behavior was simulated using an eight-state kinetic model in which the transition rate constants of the empty carrier and translocation of the fully loaded carrier were found to be critical determinants of the transport kinetics. The simulations predict that cys substitution at Gly-134 or cys modification of Cys-533 alters the preferred orientation of the empty carrier from an inward to outward-facing conformation for hyperpolarizing voltages.

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Species referenced: Xenopus laevis
Genes referenced: krt12.4 krt18.1 tbx2

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	Figure 1. . Voltage dependency of Pi activation in 100 mM Na+. (A) Pi activation characteristics for WT-expressing oocytes determined at six membrane potentials from −100 mV to 0 mV as indicated by different symbols. Each data point is the difference between current recorded in ND100 + Pi and ND100 shown as mean ± SEM (n = 3). Currents were normalized to the magnitude of IPi at 1 mM Pi, V = −100 mV for each cell. Currents for these cells ranged from −76 to −88 nA at 1 mM, V = −100 mV. Arrow indicates direction of depolarization. Continuous lines are fits to the data points using Eq. 1. (B) Pi activation characteristics for G134C-expressing oocytes (n = 4) determined at six membrane potentials as in A, before (top) and after (bottom) incubation in 1 mM MTSEA for 3–5 min. Note the different ordinate scales for the −MTS and +MTS cases. Currents were normalized to the magnitude of IPi for G134C + MTS at 1 mM Pi, V = −100 mV. Currents for these cells ranged from −22 to −69 nA at 1 mM, V = −100 mV. (C) Pi activation characteristics for M533C-expressing oocytes (n = 4) determined at six membrane potentials as in A before (top) and after (bottom) incubation in 1 mM MTSEA for 3–5 min. Note the different ordinate scales for the −MTS and +MTS cases. Currents were normalized to the magnitude of IPi for M533C − MTS at 1 mM Pi, V = −100 mV. Currents for these cells ranged from −61 to −123 nA at 1 mM, V = −100 mV. (D) Voltage dependency of apparent Pi affinity (KmPi) for G134C (left) and M533C (right). Data points indicate mean ± SEM of KmPi reported from fits to Pi activation data for the individual oocytes pooled in A–C. WT, filled circles; mutant − MTS, filled squares; mutant + MTS, empty squares. (E) Voltage dependency of predicted maximum Pi-dependent current (ImaxPi) for G134C (left) and M533C (right) with WT data superimposed. ImaxPi is shown as mean ± SEM predicted from fits to data for the individual oocytes pooled in A–C, with WT data superimposed. WT, filled circles; mutant − MTS, filled squares; mutant + MTS, empty squares. Maximum cotransport activity determined using the leak correction procedure (see text) is also shown. Mutant − MTS, filled triangles; mutant + MTS, empty triangles, with points joined by dashed lines.
	Figure 2. . Voltage dependency of Na+ activation in 1 mM Pi. (A) Na+ activation characteristics for WT-expressing oocytes determined at six membrane potentials from −100 to 0 mV as indicated by different symbols. Each data point is the difference between current recorded in NDX + Pi and NDX, shown as mean ± SEM (n = 4), where X is the respective Na+ concentration (in mM) and Pi = 1 mM. Currents were normalized to the magnitude of IPi at 1 mM Pi, V = −100 mV, and ND100. Arrow indicates direction of depolarization. Continuous lines are fits to the data points using Eq. 2 with nH as a free parameter. (B) Normalized Na+ activation data for G134C-expressing oocytes determined at six membrane potentials as in A before (top) and after (bottom) incubation in 1 mM MTSEA for 3–5 min. Continuous lines are fits to the data points using Eq. 2 with nH as a free parameter for G134C + MTS, and nH = 2 for G134C − MTS. Note the different ordinate scales for the −MTS and +MTS cases. Currents were normalized to the magnitude of IPi for G134C + MTS, ND100, V = −100 mV. (C) Normalized Na+ activation data for M533C-expressing oocytes (n = 4) determined at six membrane potentials as in A before (top) and after (bottom) incubation in 1 mM MTSEA for 3–5 min. Continuous lines are fits to the data points using Eq. 2 with nH as a free parameter for M533C − MTS, and nH = 2 for M533C + MTS. Note the different ordinate scales for the −MTS and +MTS cases. Currents were normalized to the magnitude of IPi for M533C − MTS, ND100, V = −100 mV. (D) Voltage dependency of apparent Na+ affinity (KmNa) for G134C (left) and M533C (right). Data points indicate mean ± SEM of KmPi reported from fits to Na+ activation data for the individual oocytes pooled in A–C. WT, filled circles; mutant − MTS, filled squares; mutant + MTS, empty squares.
	Figure 3. . Effect of MTS exposure on pH and voltage dependency. (A) Steady-state pH–V profile of WT showing IPi (1 mM total Pi) as a function of external pH and V. Currents were normalized to the magnitude of IPi at −100 mV, pH 7.4, and pooled (n = 4). Error bars omitted for clarity. Legend indicates color and corresponding IPi (norm). (B) Normalized, pooled (n = 5) I–V data for WT that compares IPi at pH 5.0 (open squares) with PFA-dependent current at pH 7.4 (filled triangles), normalized to the magnitude of IPi at −100 mV, pH 7.4 (filled squares). (C) Steady-state pH–V profiles of mutant G134C (left) and mutant M533C (right) showing IPi (1 mM total Pi) as a function of external pH and V before (−MTS) and after (+MTS) exposure to 1 mM MTSEA. Currents were normalized to the magnitude of IPi at −100 mV, pH 7.4, for G134C + MTS and M533C − MTS. Each data point is the mean of four cells from the same donor frog. Error bars omitted for clarity. The assay was repeated after exposure for 5 min to 1 mM MTSEA. Colored contour planes are as in A. (D) Voltage dependency of apparent inhibition constant for protons, KiH, (expressed in pH units) for G134C (left) and M533C (right) before (filled) and after (open) exposure to MTSEA. pKiH for WT is shown superimposed in both cases (filled circles). Data points were fitted with the modified Hill equation (Eq. 2) with nH as a free parameter and external proton concentration expressed logarithmically. Error bars indicate SEM of fit estimate for pKiH.
	Figure 4. . Effect of MTS exposure on presteady-state relaxations of mutants. (A) Representative presteady-state relaxations recorded from an oocyte expressing the WT NaPi-IIa protein superfused in 100 mM Na+ (ND100) and 0 mM Na+ (ND0) for the voltage step protocol shown in the inset. (B) Main component of presteady-state relaxation for a noninjected oocyte (NI) and the same WT-expressing cell in A, resolved by applying a two-exponential fitting procedure (see materials and methods). The ON (step from −60 mV to test potential) and OFF (step from test potential to −60 mV) relaxations are shown for superfusion in ND100 (top and middle traces) and ND0 (bottom traces). (C) Main ON and OFF relaxations for a representative G134C-expressing oocyte before (−MTS) and after (+MTS) exposure to 1 mM MTSEA (1 mM) for 3 min, superfused in ND100 and ND0. (D) Main ON and OFF relaxations for a representative M533C-expressing oocyte before (−MTS) and after (+MTS) exposure to 1 mM MTSEA (1 mM) for 3 min, superfused in ND100 and ND0.
	Figure 5. . Analysis of presteady-state relaxations; ON time constants (τON) for mutant G134C (A) and M533C (B). Voltage dependencies of time constants (τON) were obtained by fitting presteady-state relaxations in response to voltage steps from −60 mV to the indicated voltage with a double exponential (see materials and methods). Each point represents mean ± SEM of four cells. Fast τON (triangles), slow τON (squares); before MTSEA (filled symbols); after MTSEA (empty symbols) incubation (1 mM, 3 min). Continuous lines were obtained by fitting Eq. 4 to the data (Table I). Data for two superfusion conditions: ND100 (left) and ND0 (right). Data points at the holding potential (−60 mV) were determined from fits to the OFF relaxations.
	Figure 6. . Analysis of presteady-state relaxations: charge–voltage (Q–V) data for a representative oocyte expressing G134C (A) and M533C (B). Each point is given by (QON–QOFF)/2, where QON and QOFF are the charges moved for the ON and OFF voltage steps from and to −60 mV, respectively. Errors smaller than symbol size are not shown. Left, superfusion in ND100; right, superfusion in ND0 for the same oocyte. Filled symbols, before MTSEA exposure; empty symbols, after MTSEA exposure. Dashed lines have been drawn to indicate the apparent equality of charge movement at hyperpolarizing potentials for ND100 and ND0 superfusion for G134C − MTS and M533C + MTS. Continuous lines were obtained by fitting Eq. 3 to the data. For G134C, the fit parameters were as follows: in ND100 (±MTS), Qmax = 5.1/5.9 nC, Qhyp = −0.9/−3.4 nC; V0.5 = −7/−77 mV; z = 0.5/0.4; and in ND0 (±MTS), Qmax = 4.0/4.3 nC, Qhyp = −1.0/−2.1 nC; V0.5 = −5/−59 mV; z = 0.5/0.4. For M533C, the fit parameters were as follows: in ND100 (±MTS), Qmax = 5.8/3.7 nC, Qhyp = −2.0/−0.7 nC; V0.5 = −32/+11 mV; z = 0.6/0.6; and in ND0 (±MTS), Qmax = 4.8/2.9 nC, Qhyp = −2.1/−0.6 nC; V0.5 = −48/−5 mV; z = 0.5/0.5.
	Figure 7. . Modeling voltage-dependent presteady-state and steady-state kinetics for G134C − MTS and G134C + MTS. (A) Eight-state kinetic scheme for NaPi-IIa. Voltage-dependent transitions are shown bold. All other transitions are electroneutral. Critical transitions for simulating G134C − MTS (−) and G134C + MTS (+) are 1–8, 8–1, and 4–5. The corresponding voltage-independent rates used in the simulations are indicated. Dashed transition indicates a proposed leak pathway. (B) Simulations of presteady-state relaxations before (−) and after (+) MTS modification of Cys-134 using the model scheme of A for steps to potentials in the range −180 to +80 mV from Vh = −60 mV and two superfusion conditions. The vertical gray bars indicate the initial 2.5-ms interval after which presteady-state currents were resolved experimentally. Differential equations describing the transitions were solved for the state occupancies as a function of time. The presteady-state current per cotransporter molecule can be expressed as −e(α′(k12X1 − k21X2) − δ* (k18X1 − k81X8) + α′′(k78X7 − k87X8)) (e.g., Parent et al., 1992), where Xn is the occupancy of state n and kij is the transition rate from state i to state j. The empty carrier (1–8) carries an equivalent charge of −1 that moves an equivalent electrical distance δ through the membrane. The Na+ binding transitions (1–2 and 8–7) also involve movement of +1 charge through equivalent electrical distances α′ and α′′, respectively. To simplify the simulations, α′′ = 0.3, α′ = 0.3, and δ = 0.4 and the corresponding energy barriers were symmetrical. The rate constants for voltage-dependent transitions are given by k18 = k18o exp(δeV/2kT), k81 = k81o exp(−δeV/2kT), k12 = Na k12o exp(−α′eV/2kT), k21 = k21o exp(α′eV/2kT), where kijo is the rate constant for transition ij at V = 0, and Na is the concentration of Na+ (mM). Other fixed rate constants for the simulations were k12o = 2000 s−1M−1, k21o = 600 s−1, k23 = 7.5 × 106 s−1M−1, k32 = 1000 s−1, k34 = 106 s−1M−2, k43 = 1000 s−1, k54 = 25 s−1, k78 = 100, k87 = 106 s−1M−1, k67 = 1000 s−1, k76 = 106 M−1s−1, k65 = 105 s−1M−2, k56 = 1000, k23= 0.005 s−1. To satisfy microscopic reversibility, k32 and k76 were defined in terms of the other rate constants. (C) Predicted τ–V data was obtained by applying a single exponential fit to the ON presteady-state currents for voltages in the range −180 to +60 mV, commencing 2.5 ms after the step onset. Filled symbols, −MTS; open symbols, +MTS; circles, ND100; squares, ND0. Model parameters are as given in A and B above for the −MTS and +MTS conditions with Pi = 0. (D) Predicted Q–V data obtained by numerical integration of the presteady-state currents, commencing 2.5 ms after the step onset. Filled symbols, −MTS; open symbols, +MTS; circles, ND100; squares, ND0. Model parameters are as given in A and B above for the −MTS and +MTS conditions with Pi = 0. (E) Predicted steady-state I–V data for the −MTS (filled squares) and +MTS (open squares) conditions, using rate constants given in A and assuming Pi = 1 mM and external Na+ = 100 mM. The steady-state current is proportional to k14X1 − k41X4, i.e., the net flux for the only transition that involves transmembrane charge movement. Filled circles represent the simulated I–V data without changing k45 (see text). (F) Apparent substrate affinities for Pi activation (KmPi) (left) and Na+ activation (KmNa) (right) as a function of membrane potential. KmPi was obtained from a fit using the Michaelis-Menten equation (Eq. 1) to the steady-state activation data simulated with Pi in the range 0–3 mM and Na+ = 100 mM. KmNa was obtained from a fit using the modified Hill equation (Eq. 2) with Na+ in the range 0–125 mM and Pi = 1 mM. Filled symbols, −MTS; open symbols, +MTS. Error bars indicate ±SEM reported by fitting algorithm.
	Figure 8. . Simulations of occupancy of states 1, 2, 4, and 8 of the model scheme depicted in Fig. 7 A, as a function of membrane potential in zero external Na+ (ND0), 100 mM external Na+ (ND100), and 100 mM external Na+ with 1 mM Pi (ND100 + 1 mM Pi), before (−) and after (+) modification of Cys-134.

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