Mf. Romero et al., CLONING AND FUNCTIONAL EXPRESSION OF RNBC, AN ELECTROGENIC NA-HCO3- COTRANSPORTER FROM RAT-KIDNEY(), American journal of physiology. Renal, fluid and electrolyte physiology, 43(2), 1998, pp. 425-432
We have recently cloned the renal electrogenic Na+-bicarbonate cotrans
porter of the salamander Ambystoma tigrinum (aNBC) (M. F. Romero, M. A
. Hediger, E. L. Boulpaep, and W. F. Boron. FASEB J. 10: 89, 1996; and
Nature 387: 409-413, 1997). Here we report the cloning of a mammalian
homolog of aNBC, named rNBC for rat Na+-bicarbonate cotransporter. NB
C constitutes the major route for HCO3- reabsorption and assists in Na
+ reabsorption across the basolateral membrane of the renal proximal t
ubule (PT). We used aNBC as a probe to screen a rat kidney cortex cDNA
library in lambda gt10 and identified several clones. Each has an ini
tiator Met and a large open-reading frame followed by a 3'-untranslate
d region of similar to 500 bp. The 7.5-kb mRNA for rNBC is present in
kidney, liver, lung, brain, and heart. In situ hybridization with the
rNBC probe in the rat kidney revealed staining in the S2 segment of PT
. rNBC encodes a protein of 1,035 amino acids, with a predicted molecu
lar mass of 116 kDa. Its deduced amino acid sequence is 86% identical
to that of aNBC. Comparison of both the aNBC and rNBC sequences to the
GenBank database reveals a low level of amino acid identity (similar
to 30%) to the AE family of Cl-/HCO3- exchangers. Injection of rNBC cR
NA into Xenopus oocytes leads to expression of an electrogenic Na+-HCO
3- cotransporter that is qualitatively similar to that of aNBC but at
a much lower level. Placement of the rNBC cDNA into the context of a X
enopus expression vector produces a substantial increase in rNBC expre
ssion. Addition of 1.5% CO2/10 mM HCO3- elicits a hyperpolarization of
>50 mV and a rapid decrease of intracellular pH (pH(i)), followed by
an increase in pH(i). Subsequent removal of Na+ in the presence of CO2
/HCO3- causes a depolarization of >50 mV and a concomitant decrease of
pH(i). Thus rNBC is in the same newly identified family of Na+-linked
HCO3- transporters as is aNBC.