Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/27/2001
Publication Date: 4/4/2002
Interpretive Summary: As scientists working in the field of nutrition, we are always interested in finding out new ways of boosting the nutrient content of food obtained from our agricultural crops. To do that, we have to go to the genetic level. We have already done a lot of productive work with the CAX gene, which is involved with the movement of nutrients through plants. In this case, we figured that a good way to analyze the situation was to look at a particular plant transporter, figure out how it modulates its activities, and then determine how to alter that in order to increase the nutrient quality of the food produced by the plant. Here, we isolated a new transporter, CAX4, and went about the process of finding out what it does. We did this successfully in a small mustard plant called Arabidopsis and also in brewer's yeast, using these as models representative of agricultural crops. With this new genetic tool and an enlightened understanding of how it works, we can move forward another step toward achieving our nutrition-oriented goals for the benefit of increasingly healthy generations of Americans.
Technical Abstract: Ion compartmentalization is essential for plant growth and development. The Arabidopsis open reading frames for CAX1, CAX2, and CAX3 (CAtion eXchanger) were previously identified as transporters which may modulate ion fluxes around the vacuole. To understand the diversity and role of H+/Cation transporters in controlling plant ion levels, another homolog of the CAX genes, CAX4, was cloned from an Arabidopsis thaliana cDNA library. CAX4, is 53% identical to CAX1 at the amino acid level, 42% identical to CAX2 and 54% identical to CAX3. CAX4 transcripts appeared to be expressed at low levels in all tissues and levels of CAX4 RNA increased after Na+ and Ni+ treatment. A CAX4 fusion appeared to localize to both yeast and plant vacuoles. When expressed in yeast, CAX4, like CAX3, failed to suppress the Ca2+ sensitivity of yeast strains deficient in vacuolar Ca2+ transport. Several modifications to CAX4 allowed the protein to transport Ca2+. Addition of amino acids to the N-terminus of CAX4, and CAX3 caused both transporters to strongly suppress the sensitivity of yeast strains deficient in vacuolar Ca2+ transport. These findings suggest that all CAX transporters may modulate their ion transport properties through alterations at the N-terminus.