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

The Arabidopsis Ca2+/H+ antiporters cation exchanger (CAX) 1 and 2 utilise an electrochemical gradient to transport Ca2+ into the vacuole to help mediate Ca2+ homeostasis. Previous whole plant studies indicate that activity of Ca2+/H+ antiporters is regulated by pH. However, the pH regulation of individual Ca2+/H+ antiporters has not been examined. To determine whether CAX1 and CAX2 activity is affected by pH, Ca2+/H+ antiport activity was measured in vacuolar membrane vesicles isolated from yeast heterologously expressing either transporter. Ca2+ 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 Ca2+ 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.