Deficient HCO3- Transport in an AE1 Mutant with Normal Cl- Transport Can be Rescued by Carbonic Anhydrase II Presented on an Adjacent AE1 Protomer

Abstract

؊ exchange by an AE1 missense mutant devoid of CA2 binding, with activity further enhanced by CA2 co-expression. The same transport-incompetent AE1 mutants failed to rescue Cl ؊ /HCO 3 ؊ exchange by the AE1 truncation mutant 896X, despite preservation of the latter's core CA2 binding site. These data increase the minimal extent of a functionally defined CA2 binding site in AE1. The inter-protomeric rescue of HCO 3 ؊ transport within the AE1 dimer shows functional proximity of the C-terminal cytoplasmic tail of one protomer to the anion translocation pathway in the adjacent protomer within the AE1 heterodimer. The data strongly support the hypothesis that an intact transbilayer anion translocation pathway is completely contained within an AE1 monomer.

Distal Renal Tubular Acidosis in Mice Lacking the AE1 (Band3) Cl−/HCO3 − Exchanger (slc4a1)

“…kAE1 forms functional homodimers or hetero-oligomers, and oligomerization of a dominant negative mutant AE1 with a wild-type polypeptide in the heterozygote A-IC likely explains some dominant disease. The mutant/ wild-type heterodimer either fails to traffic to the cell surface or, in some cases, traffics normally but may not function normally at the cell surface (18,61,62). Another subset of kAE1 mutant polypeptides seems to cause dominant dRTA by inappropriate targeting to A-IC apical membrane, with resultant apical bicarbonate secretion likely short-circuiting luminal acid secretion (19,20,63).…”

Molecular physiology and genetics of Na+-independent SLC4 anion exchangers

“…conformations is anion substrate dependent in symmetrical anion solutions, but this conformational regulation is not thought to be important in physiological solutions (Jennings, 2005;Knauf and Pal, 2003 -exchange activity was rescued completely by coexpression of the binding site carried on either of two surface expression-competent AE1 polypeptides devoid of their own transport activity (Dahl et al, 2003). If indeed the oocyte lacks CAII (Nakhoul et al, 1998), then the C-terminal cytoplasmic tail may play a direct role in maintaining HCO 3 -selectivity, and can do so in trans (from one protomer to another within a dimer).…”

“…If indeed the oocyte lacks CAII (Nakhoul et al, 1998), then the C-terminal cytoplasmic tail may play a direct role in maintaining HCO 3 -selectivity, and can do so in trans (from one protomer to another within a dimer). Alternatively, bound CAII (apparently endogenous to the oocyte) might be crucial for HCO 3 -transport, and binding to an adjacent protomer within a dimer brings CAII into proximity sufficient to play this role (Dahl et al, 2003 Trout AE1 expressed in Xenopus oocytes increased constitutively active Cl -conductance with properties similar to that of the intact trout red cell. The expression of Cl -conductance has been mapped to two discreet regions of the trout AE1 transmembrane domain, and in engineered mutants need not be tightly linked to the anion exchange mechanism (Borgese et al, 2004).…”

“…Homozygotes have not been found, and are presumed to be embryonic lethal. The stable mutant polypeptide is present at normal abundance in the membrane, where it heterodimerizes at apparent normal affinity with wild-type polypeptide (Jennings and Gosselink, 1995 (Dahl et al, 2003), the minimal impact of its dominant effects upon the wild-type monomer within the SAO/wt heterodimer Kuma et al, 2002) explain its lack of renal phenotype. Heterozygote red cell membranes exhibit increased rigidity and cold-induced cation permeability, and the allele seems to confer protection against cerebral complications of malaria.…”

Evidence from simultaneous intracellular- and surface-pH transients that carbonic anhydrase II enhances CO2 fluxes across Xenopus oocyte plasma membranes

“…5A), even though this pretreatment completely blocks the sizeable effects of injected CA II protein. Note that Dahl et al (17) report the presence of endogenous CA II protein in Xenopus oocytes and demonstrate that injected acetazolamide (ACZ) causes a slight but statistically significant decrease in the rate of pH i rise elicited by removing extracellular Cl Ϫ . Thus, it is possible that modest, endogenous CA II activity-sensitive to ACZ but not EZA-contributed to CO 2 -induced pH i (and to a lesser extent, pH S ) transients in our experiments.…”