ARS | Publication request: Functional analysis of "CAX2" and "CAX2"-like genes, which encode Arabidopsis cation/proton transporters with broad metal specificity

Submitted to: Joint Meeting of the American Society of Plant Biologists and the Canadian Society of Plant Physiologists
Publication Type: Abstract Only
Publication Acceptance Date: July 1, 2006
Publication Date: August 5, 2006
Citation: Pittman, J.K., Edmond, C., Morris, J., Shigaki, T., Chalova, V., Hirschi, K.D. 2006. Functional analysis of "CAX2" and "CAX2"-like genes, which encode Arabidopsis cation/proton transporters with broad metal specificity [abstract]. The Joint Annual Meeting of the American Society of Plant Biologists and the Canadian Society of Plant Physiologist, August 5-9, 2006, Boston, Massachusetts. Session P11: Membrane Transport, Abstract P11014, p. 153-154.

Technical Abstract: The vacuolar sequestration of metals is an important metal tolerance mechanism in plants. Cation transport into the vacuolar lumen can be mediated by cation/H(+) exchanger (CAX) type transporters. Arabidopsis CAX2 was originally identified as a Ca(2+)/H(+) antiporter, but it can also transport other metals, including Mn(2+) and Cd(2+). Analysis of a "cax2" knockout found no alteration in Ca(2+)/H(+) antiport activity but a reduction in vacuolar Mn(2+)/H(+) antiport activity. Two closely related homologues of "CAX2", termed "CAX5" and "CAX6" have been cloned. Several CAX transporters appear to be regulated by an N-terminal regulatory mechanism. Heterologous expression of CAX2 and CAX5 in yeast indicates that both of these transporters possess an N-terminal regulatory region, as only N-terminally truncated constructs are able to suppress the Ca(2+) hypersensitivity phenotype of a Ca(2+) sensitive yeast mutant. Both CAX2 and CAX5 function in Ca(2+) and Mn(2+) transport when expressed in yeast. Transport competition analysis was used to compare the substrate characteristics of CAX2 and CAX5 and shows that despite significant sequence similarity between these genes, there are significant differences in transport activity. Like CAX2, a CAX5-CFP fusion protein is localized to the vacuole. Promoter-GUS analysis shows that like "CAX2", "CAX5", and "CAX6" are expressed in vascular tissue, although expression of "CAX6" is significantly weaker, and is mostly restricted to the leaf petiole and main leaf vein. A knockout mutant of "CAX6" has alterations in metal tolerance, indicating that like CAX2 and CAX5, it may also play a role in metal homeostasis. Further characterisation of CAX2, CAX5, and CAX6 aims to elucidate the relative contributions of these transporters in cation transport at the cellular and whole plant level.