High impact information on CAX1

• Mutations in the Ca2+/H+ transporter CAX1 increase CBF/DREB1 expression and the cold-acclimation response in Arabidopsis [1].
• The Arabidopsis Ca(2+)/H(+) transporter CAX1 (Cation Exchanger1) may be an important regulator of intracellular Ca(2+) levels [2].
• To determine the function of CAX1 in Arabidopsis stress tolerance, we identified two T-DNA insertion mutants, cax1-3 and cax1-4, that display reduced tonoplast Ca2+/H+ antiport activity [1].
• We propose that CAX1 regulates myriad plant processes and discuss the observed phenotypes with regard to the compensatory alterations in other transporters [2].
• However, they exhibited increased freezing tolerance after cold acclimation, demonstrating that CAX1 plays an important role in this adaptive response [1].

Biological context of CAX1

• Functional association of Arabidopsis CAX1 and CAX3 is required for normal growth and ion homeostasis [3].
• Together with other recent studies, these results suggest that CAX1 is regulated by several signaling molecules that converge on the N-terminus of CAX1 to regulate H+/Ca2+ antiport [4].
• Previously, we made a chimeric Arabidopsis thaliana vacuolar transporter CAX2B [a variant of N-terminus truncated form of CAX2 (sCAX2) containing the "B" domain from CAX1] that has enhanced calcium (Ca(2+)) substrate specificity and lost the manganese (Mn(2+)) transport capability of sCAX2 [5].
• Mutations in CAX1 produce phenotypes characteristic of plants tolerant to serpentine soils [6].
• The CAX1 N-terminal regulatory region was shown to physically interact with this 7-amino acid region by yeast two-hybrid analysis [7].

Anatomical context of CAX1

• 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 [8].
• Ca(2+)/H(+) antiport activity measured from vacuolar-enriched membranes of Arabidopsis root was also inhibited by the CAX1 peptide [7].

Associations of CAX1 with chemical compounds

• The Arabidopsis H(+)/Ca(2+) exchanger CAX1 contains an N-terminal autoinhibitory domain that prevents Ca(2+) transport when CAX1 is heterologously expressed in yeast [9].
• Identification of a crucial histidine involved in metal transport activity in the Arabidopsis cation/H+ exchanger CAX1 [10].
• 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) [8].
• The expression of CAX1 was induced in response to low temperature through an abscisic acid-independent pathway [1].
• To determine the domains within CAX2 that mediate Mn(2+) specificity, six CAX2 mutants were constructed that contained different regions of the CAX1 transporter [11].

Physical interactions of CAX1

• Using a yeast two-hybrid assay, SOS2 interacted with the N terminus of CAX1 [12].
• CAX1 has a much higher capacity for Ca(2+) transport than CAX2 [13].

Regulatory relationships of CAX1

• Chimeric constructs and site-directed mutagenesis showed that CAX3 could suppress yeast vacuolar Ca(2+) transport mutants if a nine-amino acid region of CAX1 was inserted into CAX3 (CAX3-9) [13].
• Here we demonstrate that SOS2 regulates the vacuolar H(+)/Ca(2+) antiporter CAX1 [12].

Other interactions of CAX1

• We show that CAX1 is predominately expressed in leaves, while CAX3 is highly expressed in roots [3].
• Characterization of CXIP4, a novel Arabidopsis protein that activates the H+/Ca2+ antiporter, CAX1 [4].
• Experiments on vacuolar membrane-enriched vesicles isolated from yeast expressing CAX1 or CAX2 demonstrate that these genes encode high efficiency and low efficiency H+/Ca2+ exchangers, respectively [14].
• CAX1-like chimeric transporters were activated by SOS2 if the chimeric proteins contained the N terminus of CAX1 [12].
• Using a yeast growth assay, co-expression of SOS2 specifically activated CAX1, whereas SOS3 did not [12].

Analytical, diagnostic and therapeutic context of CAX1

• In a yeast two-hybrid assay, CXIP1 interacted with the N terminus of CAX1 [9].
• We have used site-directed mutagenesis to assess the impact of altering the seven histidine residues to alanine within Arabidopsis CAX1 [10].
• Stable integration and expression of the CAX1 gene in T0 plants and T1 progeny were confirmed by DNA hybridization, Northern blot analysis, and luminescent analysis [15].
• The introduction of the CAX1 gene was also proven by PCR using CAX1-specific oligonucleotide primers in regenerated plants [15].

References