Functional characterization of NBC4: a new electrogenic sodium-bicarbonate cotransporter.

Sodium-bicarbonate cotransporters are homologous membrane proteins mediating the electrogenic or electroneutral transport of sodium and bicarbonate. Of the functionally characterized sodium-bicarbonate cotransporters (NBC), NBC1 proteins are known to be electrogenic. Here we report the cloning and functional characterization of NBC4c, a new splice variant of the NBC4 gene. At the amino acid level, NBC4c is 56% identical to NBC1 protein variants and 40% identical to electroneutral NBC3. When expressed in mammalian cells, NBC4c mediates electrogenic sodium-bicarbonate cotransport. The transport of sodium and bicarbonate is chloride independent and is completely inhibited by DIDS. NBC4c transcripts were detected in several tissues including brain, heart, kidney, testis, pancreas, muscle, and peripheral blood leukocytes. The data indicate that NBC4c is an electrogenic sodium-bicarbonate cotransporter. The finding that both NBC1 and NBC4c proteins function as electrogenic sodium-bicarbonate cotransporters will aid in determining the structural motifs responsible for this unique functional property, which distinguishes these transporters from other members of the bicarbonate transporter superfamily.