title: Transinhibition and voltage-gating in a fungal nitrate transporter.
abstract: We have applied enzyme kinetic analysis to electrophysiological steady-state data of Zhou et al. (Zhou, J.J., Trueman, L.J., Boorer, K.J., Theodoulou, F.L., Forde, B.G., Miller, A.J. 2000. A high-affinity fungal nitrate carrier with two transport mechanisms. J. Biol. Chem. 275:39894-9) and to new current-voltage-time records from Xenopus oocytes with functionally expressed NrtA (crnA) 2H+-NO3- symporter from Emericella (Aspergillus) nidulans. Zhou et al. stressed two Michaelis-Menten (MM) mechanisms to mediate the observed nitrate-induced currents, I(NO3-). We show that a single straightforward reaction cycle describes the data well, pointing out that during exposure to external substrate, S = (2H+ + NO3-)o, the product concentration inside, [P] = [H+]i(2) x [NO3-]i, may rise substantially near the plasma membrane, violating the condition [P] << [S] for MM kinetics. Here, [P] and its changes during experimentation are treated explicitly. K(1/2) approximately 20 microM for I(NO3-) at pHo from Zhou et al. is confirmed. According to our analysis, NrtA operates between about 0.2 and 0.6 of the electrical distance in the membrane (outside 0, inside 1). In absence of thermodynamic gradients, the predominant orientation of the binding site(s) is probably inwards. The activity of the enzyme is sensitive to the transmembrane voltage, V, with an apparent gating charge of +1.0 +/- 0.5 for inactivation, and transition probabilities of 0.3-1.3 s(-1) at V = 0. This gating mode impedes loss of cellular NO3- during depolarization.
authors.affiliation: Institute for Marine Bioscience, National Research Council, Halifax, NS, B3H 3Z1 Canada.
authors.forename: J
authors.initials: J
authors.lastname: Boyd
authors.forename: D
authors.initials: D
authors.lastname: Gradmann
authors.forename: C M
authors.initials: CM
authors.lastname: Boyd
mesh_terms: D000818:Animals; D027321:Anion Transport Proteins / Q000235:genetics / Q000378:metabolism*; D001233:Aspergillus nidulans / Q000378:metabolism*; D001692:Biological Transport; D004594:Electrophysiology; D005260:Female; D015640:Ion Channel Gating / Q000502:physiology*; D007700:Kinetics; D008956:Models, Chemical; D000091285:Nitrate Transporters; D009566:Nitrates / Q000378:metabolism*; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D014982:Xenopus laevis
chemical_list: D027321:Anion Transport Proteins; D000091285:Nitrate Transporters; D009566:Nitrates