Submitted to: Plant Molecular Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/25/2002
Publication Date: 10/15/2002
Citation: SHIGAKI,T., SREEVIDYA,C., HIRSCHI,K. ., ANALYSIS OF THE CA2+ DOMAIN IN THE ARABIDOPSIS H+/CA2+ ANTIPORTERS CAX1 AND CAX3, PLANT MOLECULAR BIOLOGY, 2002. v. 50. p. 475-483. Interpretive Summary: The long term goal of our research is to improve the calcium content in fruits and vegetables to improve human nutrition. Our short term goal is to understand the transporters which move calcium into plant cells. In this study we have analyzed specific regions of a plant protein to improve calcium transport. The finds will lead to the ability to boost the calcium content in foods through increased expression of these transporters.
Technical Abstract: Ca2+ levels in plants are controlled in part by H+/Ca2+ exchangers. Structure/function analysis of the Arabidopsis H+/cation exchanger, CAX1, revealed that a nine amino acid region (87-95) is involved in CAX1-mediated Ca2+ specificity. CAX3 is 77% identical (93% similar) to CAX1, and when expressed in yeast, localizes to the vacuole but does not suppress yeast mutants defective in vacuolar Ca2+ transport. Transgenic tobacco plants expressing CAX3 containing the 9 amino acid Ca2+ domain (Cad) from CAX1 (CAX3-9) displayed altered stress sensitivities similar to CAX1-expressing plants, whereas CAX3-9-expressing plants did not have any altered stress sensitivities. A single leucine-to-isoleucine change at position 87 (CAX3-I) within the Cad of CAX3 allows this protein to weakly transport Ca2+ in yeast (less than 10% of CAX1). Site-directed mutagenesis of the leucine in the CAX3 Cad demonstrated that no amino acid change tested could confer more activity than CAX3-I. Transport studies in yeast demonstrated that the first three amino acids of the CAX1 Cad could confer twice the Ca2+ transport capability compared to CAX3-I. The entire Cad of CAX3 (87-95) inserted into CAX1 abolishes CAX1 mediated Ca2+ transport. However, single, double, or triple amino acid replacements within the native CAX1 Cad did not block CAX1 mediated Ca2+ transport. Together these findings suggest that other domains within CAX1 and CAX3 influence Ca2+ transport. This study has implications for the ability to engineer CAX-mediated transport in plants by manipulating Cad residues.