Figure 1. Predicted bicarbonate transporter superfamily relationships. The a) panel is a dendrogram for representative member of the bicarbonate transporter superfamily (BTS) [54]. The open circle indicates the electrogenic Na+/HCO3 - cotransporter isoforms collectively known as NBC or NBC1. AE1-3 are the mammalian anion exchangers. Other BTS members have been identified and cloned: ceNBC (AF004926), NDAE1 (AF47468), NBC3 (AF47033), NBCn1-D (AF80106), NCBE (AB033759), NDCBE1 (AF069512), AE4a and AE4b (AB038264), NBC4 (AF207661), and yeast yn1275w1. A second Drosophila protein (CG8177) and three Arabidopsis proteins (Arab-Chr1, Arab-Chr2, and Arab-Chr3) have been predicted from genome sequencing projects but not functionally evaluated. The b) panel is a working membrane topology model of NBC based largely on structural results of AE1 (see [55]). Both N- and C-termini are predicted as intracellular. A large, central extracellular loop uses one N-linked glycosylation site N617 [56] though seven are predicted. Three predicted DIDS-binding motifs are indicated by boxes.