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CAX Transporters
What is CAX Transporter?
Plant cells have a large internal compartment (organelle) called vacuole, and the contents of this organelle is separated from the cytosol, where life’s essential biochemical reactions take place, by the vacuolar membrane. The vacuole is the place where the plant cell stores various materials, including toxic molecules and ions.
The membrane that defines the vacuole is called the tonoplast, and a steep pH gradient exists across this membrane. Usually, the pH of the cytosolic side of tonoplast is maintained at the physiological level, and the pH inside the vacuole is significantly lower (pH=4-5). CAX transporters take advantage of this pH gradient to move cations into the vacuole in exchange for protons that are abundantly present in the vacuole.
Discovery of CAX
Plant CAX was originally discovered from Arabidopsis thaliana by Dr. Kendal D. Hirschi (Baylor College of Medicine), and so named because he was searching for the transporters that can function as CAlcium eXchangers by yeast suppression assay. The two genes that he found were named CAX1 and CAX2.There is an interesting story behind it, which is rarely written in research publications. The CAX1 and CAX2 cDNAs that he used in the yeast assay were not full-length, but a short segment of their 5’ was missing, which, on translation, will truncate the first 36 amino acids of the transporter proteins. It was later found that this 36 amino acid region acts as an autoinhibitory domain, and the proteins are active only when this region is topped off. If he had used unbroken full-length cDNAs, he would never have discovered his CAXs.
CAX1 and CAX2 are calcium transporters, but it became clear later that the substrates of CAXs are not limited to calcium. The potential substrates of CAXs include cadmium, mercury, manganese, zinc, and nickel. Now CAX stands for CAtion eXchanger. Fortunately, the abbreviation did not change. This property of CAXs is important in practical applications, which I will discuss later.
Since the discovery of the first two CAXs in Arabidopsis, CAXs from other plants have been characterized. Significant discoveries have been made in Dr. Masayoshi Maeshima’s lab in Nagoya University, Japan. His group has identified new CAXs from mung bean and rice. Detailed mutagenesis studies identified domains that form an ion filter in the pore structure of a CAX from rice. The completion of the Arabidopsis genome project revealed that there are six CAXs in its genome. It appears that in most plant species, CAXs form a multigene family.
As of 2007, there are approximately 200 entries of CAXs in the GenBank database. CAX open reading frames appear to be present in all taxa, with a notable exception of higher vertebrates and other animals including insects and C. elegans. It is a mystery why CAX is dispensable in these organisms.
Practical applications of CAX transporters
The vacuole’s protected environment makes CAXs an ideal candidate for two practical applications. First, the vacuole can store high levels of calcium. For example, if we engineer a tomato plant to express a high number of CAX proteins, the tomato will accumulate more calcium than an ordinary tomato. Such calcium-enriched vegetables have been already developed and it is expected that more crops will be transformed with CAX genes for the future commercialization.
Second, when plants express CAXs that are capable of transporting heavy metals, such as cadmium and mercury, the plant accumulates copious amounts of these metals without suffering an adverse effect in growth. If these plants are grown in the soil contaminated with heavy metals, they help to clean up the site (phytoremediation). It is a low-cost way of removing environmental pollutants from soil, and the optimization of the properties of CAX proteins and the selection of right plant species will lead to a much needed decontamination technology.
