Functional and regulatory analysis of the Arabidopsis thaliana CAX2 cation transporter

The vacuolar sequestration of metals is an important metal tolerance mechanism in plants. The Arabidopsis thaliana vacuolar transporters CAX1 and CAX2 were originally identified in a Saccharomyces cerevisiae suppression screen as Ca2+/H+ antiporters. CAX2 has a low affinity for Ca2+ but can transport other metals including Mn2+ and Cd2+. Here we demonstrate that unlike cax1 mutants, CAX2 insertional mutants caused no discernable morphological phenotypes or alterations in Ca2+/H+ antiport activity. However, cax2 lines exhibited a reduction in vacuolar Mn2+/H+ antiport and, like cax1 mutants, reduced V-type H+-ATPase (V-ATPase) activity. Analysis of a CAX2 promoter β-glucoronidase (GUS) reporter gene fusion confirmed that CAX2 was expressed throughout the plant and strongly expressed in flower tissue, vascular tissue and in the apical meristem of young plants. Heterologous expression in yeast identified an N-terminal regulatory region in CAX2, suggesting that Arabidopsis contains multiple cation/H+ antiporters with shared regulatory features. Furthermore, despite significant variations in morphological and biochemical phenotypes, cax1 and cax2 lines both significantly alter V-ATPase activity, hinting at coordinate regulation among transporters driven by H+ gradients and the V-ATPase.