Anion transport systems in the mouse erythrocyte: kinetic studies in situ and after expression of mouse erythroid band 3 protein in oocytes of Xenopus laevis

Regulation of DRA and AE1 in rat colon by dietary Na depletion

Two distinct Cl/anion exchange activities (Cl/HCO(3) and Cl/OH) identified in apical membranes of rat distal colon are distributed in cell type-specific patterns. Cl/HCO(3) exchange is expressed only in surface cells, whereas Cl/OH exchange is localized in surface and crypt cells. Dietary Na depletion substantially inhibits Cl/HCO(3) but not Cl/OH exchange. We determined whether anion exchange isoforms (AE) and/or downregulated in adenoma (DRA) are expressed in and related to apical membrane anion exchanges by examining localization of AE isoform-specific and DRA mRNA expression in normal and Na-depleted rats. Amplification of AE cDNA fragments by RT-PCR with colonic mRNA as template indicates that AE1 and AE2 but not AE3 mRNAs are expressed. In situ hybridization study revealed that AE1 mRNA is expressed predominantly in surface but not crypt cells. In contrast, AE2 polypeptide is expressed in basolateral membranes and DRA protein is expressed in apical membranes of both surface and crypt cells. AE1 mRNA is only minimally present in proximal colon, and DRA mRNA abundance is similar in distal and proximal colon. Dietary Na depletion reduces AE1 mRNA abundance but did not alter DRA mRNA abundance. This indicates that AE1 encodes surface cell-specific aldosterone-regulated Cl/HCO(3) exchange, whereas DRA encodes aldosterone-insensitive Cl/OH exchange.

Characterization of the chloride carrier in the plasmalemma of the alga Valonia utricularis: the inhibition by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid

The effect of the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) on the Cl(-)-transport system located in the plasmalemma of cells of the giant marine alga Valonia utricularis was studied by using the charge pulse relaxation technique. Analysis of the biphasic relaxation patterns in terms of the kinetic model published previously (Wang, J., Wehner, G., Benz, R. and Zimmermann, U. (1991) Biophys. J. 59, 235-248) demonstrated that extracellular DIDS dramatically reduced the translocation rate, KAS, of the Cl(-)-carrier complex (maximal inhibition 79%). The translocation rate of the free carrier molecules, KS, as well as the total surface concentration of the carrier, No, were not affected. A Hill-plot of DIDS inhibition on KAS yielded an half-maximal inhibition concentration (IC50) of 3.9 x 10(-5) M and Hill-coefficient of 1.61, suggesting a co-operative binding of the inhibitors to the Cl(-)-carrier. The maximal inhibition of DIDS was dependent on the extracellular Cl(-)-concentration. This inhibition was not competitive to chloride, since it increased and did not decrease with increasing chloride concentration. The DIDS effect decreased with increasing pH-value (investigated pH range between 6.5 and 10). Intravascular DIDS or SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid) had no effect on the biphasic voltage relaxation pattern. These results showed that the binding sites of DIDS must be located on the outer surface of the plasmalemma of V. utricularis and, in turn, supported previous conclusions that the Cl(-)-carrier (which is assumed to be part of the turgor-pressure-sensing mechanism) is only located in the outer membrane.

Characterization of BADS-binding proteins in epithelial plasma membranes

When a fluorescent stilbene was added to epithelial plasma membrane suspension the emission spectrum showed a broad peak containing overlapping emissions resulting from different adducts. By focusing on a specific emission wavelength a common site having a dissociation constant of approximately 5 microM was calculated in the rat kidney, small intestine, pancreatic islets and shark rectal gland. This binding could be displaced by loop diuretics, (e.g., furosemide with an IC50 of 40 microM), DIDS (ki 1 microM) and thiocyanate. These results pose certain questions such as: (i) whether the evidence for multiple peaks are due to specific interactions representing multiple binding affinities and (ii) whether the binding of stilbene and the observed displacement can be identified on a specific protein. Separating the proteins present in the purified basolateral and brush-border membranes by SDS-PAGE, transfer of these proteins onto nitrocellulose paper and labeling of the nitrocellulose strips by radioactive BADS (4-benzamido-4'aminostilbene-2-2'disulphonic acid) and bumetanide could identify labeled proteins. These experiments showed that whereas some proteins bound either BADS or bumetanide, one protein with a molecular weight of approximately 100 or 130,000 D appeared to bind both. This protein was found on the basolateral membrane in the rat kidney cortex and medulla and the shark rectal gland and in the basolateral and brush-border membranes of the small intestine. Displacement of the protein-bound stilbene by loop diuretics could not be quantitated on the nitrocellulose transfer strips for this protein. Antibodies raised against the cytoplasmic fragment of band 3 reacted with the stilbene-labeled 100-130,000 D proteins indicating sufficient immuno-cross-reactivity between the separate species. These experiments involving binding of BADS and bumetanide and cross-reactivity with the human band 3 antibody suggest that these kilodalton proteins could structurally resemble human band 3.

Band 3 protein degradation by calpain is enhanced in erythrocytes of old people

Band 3 protein is a major erythrocyte transmembrane glycoprotein. We compared the degradation of band 3 protein by calpain I (a cytoplasmic, micromolar-Ca2(+)-requiring thiol proteinase) in the cells from old individuals (greater than 70 years old) to that in the cells from young ones (20-30 years old). In the young, little degradation of band 3 protein occurred in calpain-treated erythrocyte ghosts. In the old, significant band 3 protein degradation was found in erythrocyte ghosts treated similarly. The difference between young and old in the susceptibility of band 3 protein to calpain was retained in membrane vesicles (membranes stripped of peripheral proteins by NaOH) and in chymotrypsin-generated 60 kDa fragment (CH-60). The isolated N-terminal cytoplasmic 43 kDa fragment was degraded by calpain to a similar extent in old and in young. The separated 17 kDa membrane domain of the CH-60 and the trypsin-generated C-terminal 55 kDa membrane-spanning fragment were not degraded by calpain I in the young, nor in the old. Thus the N-terminal cytoplasmic domain is the domain degraded by calpain I. Enhanced sensitivity in the old is observed in intact band 3 protein and in CH-60, the isolated cytoplasmic domain being equally susceptible in young and old. The observed age-related enhanced sensitivity to calpain is consistent with the presence of modifications in band 3 protein and alterations in the association with the calpain-calpastatin system. Band 3 protein has several important functions, with modifications in the protein having implications for altered cell behaviour in the old individual.

Identification by site-directed mutagenesis of Lys-558 as the covalent attachment site of H2DIDS in the mouse erythroid band 3 protein

After functional expression of mouse erythroid band 3 by cRNA microinjection into Xenopus oocytes, 36Cl- efflux is irreversibly inhibited by H2DIDS. When a cRNA is injected that is derived from a cDNA in which the nucleotides encoding for lysine-558 were replaced by nucleotides encoding for asparagine, transport and inhibition of transport by H2DIDS still occur. However, when measured under conditions where no intramolecular crosslinking takes place the inhibition by H2DIDS is no longer irreversible. This indicates that thiourea bond formation between H2DIDS and band 3 takes place at Lys-558.