title: Activity and distribution of intracellular carbonic anhydrase II and their effects on the transport activity of anion exchanger AE1/SLC4A1.
abstract: We have investigated the previously published 'metabolon hypothesis' postulating that a close association of the anion exchanger 1 (AE1) and cytosolic carbonic anhydrase II (CAII) exists that greatly increases the transport activity of AE1. We study whether there is a physical association of and direct functional interaction between CAII and AE1 in the native human red cell and in tsA201 cells coexpressing heterologous fluorescent fusion proteins CAII-CyPet and YPet-AE1. In these doubly transfected tsA201 cells, YPet-AE1 is clearly associated with the cell membrane, whereas CAII-CyPet is homogeneously distributed throughout the cell in a cytoplasmic pattern. Förster resonance energy transfer measurements fail to detect close proximity of YPet-AE1 and CAII-CyPet. The absence of an association of AE1 and CAII is supported by immunoprecipitation experiments using Flag-antibody against Flag-tagged AE1 expressed in tsA201 cells, which does not co-precipitate native CAII but co-precipitates coexpressed ankyrin. Both the CAII and the AE1 fusion proteins are fully functional in tsA201 cells as judged by CA activity and by cellular HCO3(-) permeability (P(HCO3(-))) sensitive to inhibition by 4,4-Diisothiocyano-2,2-stilbenedisulfonic acid. Expression of the non-catalytic CAII mutant V143Y leads to a drastic reduction of endogenous CAII and to a corresponding reduction of total intracellular CA activity. Overexpression of an N-terminally truncated CAII lacking the proposed site of interaction with the C-terminal cytoplasmic tail of AE1 substantially increases intracellular CA activity, as does overexpression of wild-type CAII. These variously co-transfected tsA201 cells exhibit a positive correlation between cellular P(HCO3(-)) and intracellular CA activity. The relationship reflects that expected from changes in cytoplasmic CA activity improving substrate supply to or removal from AE1, without requirement for a CAII-AE1 metabolon involving physical interaction. A functional contribution of the hypothesized CAII-AE1 metabolon to erythroid AE1-mediated HCO3(-) transport was further tested in normal red cells and red cells from CAII-deficient patients that retain substantial CA activity associated with the erythroid CAI protein lacking the proposed AE1-binding sequence. Erythroid P(HCO3(-)) was indistinguishable in these two cell types, providing no support for the proposed functional importance of the physical interaction of CAII and AE1. A theoretical model predicts that homogeneous cytoplasmic distribution of CAII is more favourable for cellular transport of HCO3(-) and CO2 than is association of CAII with the cytoplasmic surface of the plasma membrane. This is due to the fact that the relatively slow intracellular transport of H(+) makes it most efficient to place the CA in the vicinity of the haemoglobin molecules, which are homogeneously distributed over the cytoplasm.
authors.affiliation: G. Gros: Zentrum Physiologie, Vegetative Physiologie 4220, Medizinische Hochschule Hannover, Carl-Neuberg-Str. 1, 30625 Hannover, Germany. Gros.Gerolf@MH-Hannover.de; V. Endeward: Zentrum Physiologie 4220, Medizinische Hochschule Hannover, Germany.  Endeward.Volker@MH-Hannover.de.
authors.forename: Samer
authors.initials: S
authors.lastname: Al-Samir
authors.forename: Symeon
authors.initials: S
authors.lastname: Papadopoulos
authors.forename: Renate J
authors.initials: RJ
authors.lastname: Scheibe
authors.forename: Joachim D
authors.initials: JD
authors.lastname: Meißner
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Cartron
authors.forename: William S
authors.initials: WS
authors.lastname: Sly
authors.forename: Seth L
authors.initials: SL
authors.lastname: Alper
authors.forename: Gerolf
authors.initials: G
authors.lastname: Gros
authors.forename: Volker
authors.initials: V
authors.lastname: Endeward
mesh_terms: D001457:Anion Exchange Protein 1, Erythrocyte / Q000235:genetics / Q000378:metabolism*; D017487:Ankyrins / Q000378:metabolism; D024402:Carbonic Anhydrase II / Q000235:genetics / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D017136:Ion Transport; D008954:Models, Biological; D011485:Protein Binding; D021381:Protein Transport
chemical_list: D001457:Anion Exchange Protein 1, Erythrocyte; D017487:Ankyrins; C454201:SLC4A1 protein, human; D024402:Carbonic Anhydrase II