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Ion translocation via the transmembrane domain 5 in the sodium/bicarbonate transporter NBCe1
Author(s) -
LEE SOOJUNG,
CHOI INYEONG
Publication year - 2010
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.24.1_supplement.1025.21
Subject(s) - chemistry , sodium , transporter , bicarbonate , voltage clamp , transmembrane domain , biophysics , membrane potential , biochemistry , membrane , biology , organic chemistry , gene
We previously demonstrated that the charge‐conserved substitution of Asp at position 555 with a Glu in NBCe1 (D555E) alters the anion selectivity of the transporter. This mutant protein normally produces HCO 3 − currents, but can elicit Cl − currents in CO 2 /HCO 3 − ‐free solution. Thus, we hypothesized that Cl − binds to the mutant transporter at the HCO 3 − binding site if HCO 3 − is not present. To test the hypothesis, we expressed D555E in Xenopus oocytes and assessed Cl − currents and HCO 3 − currents using two‐electrode voltage clamp. Under Cl − ‐free conditions, the mutant transporter produced an outward current upon exposure to Cl − (104 ± 28 nA, n = 5). The Cl − current was markedly reduced in CO 2 /HCO 3 − solution (28 ± 5, n = 5). However, the Cl − current was smaller than Cl − currents in Na + ‐free CO 2 /HCO 3 − (48 ± 20, n = 5). Current‐voltage relationships showed that the slope in CO 2 /HCO 3 − approached to the slope in Na + ‐free CO 2 /HCO 3 − (p < 0.05, n = 6 for each), indicating the current in Na + ‐free CO 2 /HCO 3 − is mediated by Cl − . Replacing transmembrane domain 5 (TM5) of NBCe1 with the corresponding TM5 of the electroneutral Na/HCO 3 transporter NBCn1 produced outwardly rectifying currents that are progressively smaller at positive voltages, where electrogenic Na/HCO 3 transport should be favorable. We conclude that TM5 of NBCe1 containing Asp 555 is responsible for electrogenicity of the transporter.