Targeting of the AE2 anion exchanger to the Golgi apparatus is cell type‐dependent and correlates with the expression of Ank₁₉₅, a Golgi membrane skeletal protein

Abstract

Sodium-independent anion exchangers (AE1^4) show remarkable variability in their tissue-speci¢c expression and subcellular localization. Currently, isoform-speci¢c targeting mechanisms are considered to be responsible for this variable localization. Here, we report that targeting can also be cell type-speci¢c. We show that the full-length AE2 protein and its green £uorescent protein-or DsRed-tagged variants localize predominantly either to the Golgi apparatus in COS-7 cells, or to the plasma membrane in HeLa cells. This alternative targeting did not seem to result from either translational or posttranslational di¡erences, but rather from di¡erential expression of at least one of the Golgi membrane skeletal proteins, ankyrin 195 (Ank 195 ), between the two cell types. Comparative studies with several di¡erent cell lines revealed that the Golgi localization of the AE2 protein correlated strictly with the expression of Ank 195 in the cells. The two Golgi-associated proteins also co-localized well and similarly resisted detergent extraction in the cold, whereas the plasma membrane-localized AE2 in Ank 195 -de¢cient cells was mostly detergent-soluble. Collectively, our results suggest that Ank 195 expression is a key determinant for the variable and cell type-dependent localization of the AE2 protein in the Golgi apparatus in mammalian cells.

Ionic determinants of pH of acidic compartments under hypertonic conditions in trout hepatocytes

“…pH of acid compartments combine with translocated H + , preventing the formation of a large chemical gradient. Apart from this study, and while anion exchangers have already been detected in acidic compartments (Holappa et al, 2001;Holappa and Kellokumpu, 2003), evidence for a contribution of HCO 3 -to organelle pH regulation is lacking (Paroutis et al, 2004). Actually, in the present study, removal of extracellular HCO 3 -appeared to have a slight effect on steady-state pH L while no effect on the hypertonicity-induced acidification or the subsequent restoration of isotonicity was seen.…”

Actin filaments are involved in the maintenance of Golgi cisternae morphology and intra‐Golgi pH

“…This interaction is essential for the maintenance of the characteristic flattened shape of the cell and its mechanical stability [Marchesi, 1985;Nishi and Forgac, 2002]. Hence, in the Golgi complex, the potential ankyrin-interacting Golgi membrane protein anion exchanger 2 (AE2) [Holappa et al, 2001;Holappa and Kellokumpu, 2003] could provide a functional membrane anchorage site for the Golgi-associated spectrin/ ankyrin/actin cytoskeleton [De Matteis et al, 2000;Valderrama et al, 2000]. The structural role of the AE2 protein in the Golgi complex would be linked to its function (the reversible electroneutral exchange of Cl À for HCO 3 À ), thus contributing to the regulation of intracellular pH, organelle volume, and chloride concentration.…”

Shared apical sorting of anion exchanger isoforms AE2a, AE2b1, and AE2b2 in primary hepatocytes

“…The parallel and cell-type-specific sorting of these isoforms (all three isoforms are apical in hepatocytes and basolateral in kidney cells) suggests that no sorting signaling motifs are associated with the short isoform-specific N-terminal regions. Recent experiments in fibroblast-like cells suggest a role for the trans-Golgiassociated skeletal protein Ank 195 in the cell-type-specific sorting of AE2 to the Golgi in these non-polarized cells [34]. Which particular proteins may be involved in the cell-specific targeting of AE2 in polarized liver and renal cells deserves further investigation.…”