title: Electrogenic Na/HCO3 cotransporter (NBCe1) variants expressed in Xenopus oocytes: functional comparison and roles of the amino and carboxy termini.
abstract: Using pH- and voltage-sensitive microelectrodes, as well as the two-electrode voltage-clamp and macropatch techniques, we compared the functional properties of the three NBCe1 variants (NBCe1-A, -B, and -C) with different amino and/or carboxy termini expressed in Xenopus laevis oocytes. Oocytes expressing rat brain NBCe1-B and exposed to a CO(2)/HCO(3)(-) solution displayed all the hallmarks of an electrogenic Na(+)/HCO(3)(-) cotransporter: (a) a DIDS-sensitive pH(i) recovery following the initial CO(2)-induced acidification, (b) an instantaneous hyperpolarization, and (c) an instantaneous Na(+)-dependent outward current under voltage-clamp conditions (-60 mV). All three variants had similar external HCO(3)(-) dependencies (apparent K(M) of 4-6 mM) and external Na(+) dependencies (apparent K(M) of 21-36 mM), as well as similar voltage dependencies. However, voltage-clamped oocytes (-60 mV) expressing NBCe1-A exhibited peak HCO(3)(-)-stimulated NBC currents that were 4.3-fold larger than the currents seen in oocytes expressing the most dissimilar C variant. Larger NBCe1-A currents were also observed in current-voltage relationships. Plasma membrane expression levels as assessed by single oocyte chemiluminescence with hemagglutinin-tagged NBCs were similar for the three variants. In whole-cell experiments (V(m) = -60 mV), removing the unique amino terminus of NBCe1-A reduced the mean HCO(3)(-)-induced NBC current 55%, whereas removing the different amino terminus of NBCe1-C increased the mean NBC current 2.7-fold. A similar pattern was observed in macropatch experiments. Thus, the unique amino terminus of NBCe1-A stimulates transporter activity, whereas the different amino terminus of the B and C variants inhibits activity. One or more cytosolic factors may also contribute to NBCe1 activity based on discrepancies between macropatch and whole-cell currents. While the amino termini influence transporter function, the carboxy termini influence plasma membrane expression. Removing the entire cytosolic carboxy terminus of NBCe1-C, or the different carboxy terminus of the A/B variants, causes a loss of NBC activity due to low expression at the plasma membrane.
authors.affiliation: Department of Physiology and Biophysics, University of Alabama at Birmingham, 35294, USA.
authors.forename: Suzanne D
authors.initials: SD
authors.lastname: McAlear
authors.forename: Xiaofen
authors.initials: X
authors.lastname: Liu
authors.forename: Jennifer B
authors.initials: JB
authors.lastname: Williams
authors.forename: Carmel M
authors.initials: CM
authors.lastname: McNicholas-Bevensee
authors.forename: Mark O
authors.initials: MO
authors.lastname: Bevensee
mesh_terms: D000818:Animals; D005260:Female; D005786:Gene Expression Regulation / Q000235:genetics / Q000502:physiology*; D014644:Genetic Variation / Q000235:genetics / Q000502:physiology*; D016297:Mutagenesis, Site-Directed; D009865:Oocytes; D018408:Patch-Clamp Techniques; D017434:Protein Structure, Tertiary / Q000235:genetics; D051381:Rats; D027982:Sodium-Bicarbonate Symporters / Q000096:biosynthesis / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D014162:Transfection; D014982:Xenopus laevis
chemical_list: D027982:Sodium-Bicarbonate Symporters