Evidence of differential pH regulation of the Arabidopsis vacuolar Ca2+/H+ antiporters CAX1 and CAX2

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Abstract
The Arabidopsis Ca 2+ /H + antiporters cation exchanger (CAX) 1 and 2 utilise an electrochemical gradient to transport Ca 2+ into the vacuole to help mediate Ca 2+ homeostasis. Previous whole plant studies indicate that activity of Ca 2+ /H + antiporters is regulated by pH. However, the pH regulation of individual Ca 2+ /H + antiporters has not been examined. To determine whether CAX1 and CAX2 activity is affected by pH, Ca 2+ /H + antiport activity was measured in vacuolar membrane vesicles isolated from yeast heterologously expressing either transporter. Ca 2+ transport by CAX1 and CAX2 was regulated by cytosolic pH and each transporter had a distinct cytosolic pH profile. Screening of CAX1/CAX2 chimeras identified an amino acid domain within CAX2 that altered the pH-dependent Ca 2+ transport profile so that it was almost identical to the pH profile of CAX1. Results from mutagenesis of a specific His residue within this domain suggests a role for this residue in pH regulation.