Exploring the autoinhibitory domain of the electrogenic Na+/HCO3- transporter NBCe1-B, from residues 28 to 62

Variant B of the electrogenic Na+/HCO3- cotransporter (NBCe1-B) contributes to the vectorial transport of HCO3- in epithelia (e.g. pancreatic ducts) and to the maintenance of intracellular pH in the central nervous systems (e.g. astrocytes). NBCe1-B has very low basal activity due to an autoinhibitory domain (AID) located, at least in part, in the unique portion (residues 1-85) of the cytosolic NH2-terminus. Previous work has shown that removing 23 amino acids (residues 40-62) stimulates NBCe1-B. Here, we test the hypothesis that a cationic cluster of nine consecutive positively charged amino acids (residues 40-48) is a necessary part of the AID. Using two-electrode voltage clamping of Xenopus oocytes, we assess the activity of human NBCe1-B constructs in which we systematically replace or delete residues 28-62, which includes the cationic cluster. We find that replacing or deleting all residues within the cationic cluster markedly increases NBCe1-B activity (i.e. eliminates autoinhibition). On the background of a cationic clusterless construct, systematically restoring Arg residues restores autoinhibition in two distinct quanta, with one to three Arg residues restoring approximate to 50%, and four or more Arg residues restoring virtually all autoinhibition. Systematically deleting residues before the cluster reduces autoinhibition by, at most, a small amount. Replacing or deleting residues after the cluster has no effect. For constructs with low NBCe1 activity (but good surface expression, as assessed by biotinylation), co-expression with super-IRBIT (lacking PP1-binding site) restores full activity (i.e. relieves autoinhibition). In summary, the cationic cluster is a necessary component of the AID of NBCe1-B.