caxes

Caxes, commonly referred to as CAX transporters, are a family of membrane proteins that mediate the exchange of divalent cations, primarily Ca2+, with protons across membranes, most prominently at the vacuolar membrane (tonoplast). By moving Ca2+ into the vacuole, CAX transporters help regulate cytosolic calcium levels and contribute to calcium storage and detoxification of excess metals. Some family members can transport other divalent cations such as Mn2+.

Most characterized CAX proteins contain the conserved CaCA superfamily domain and multiple transmembrane segments that facilitate proton motive force-driven antiport. In many plants, several CAX genes exist (for example, Arabidopsis thaliana encodes multiple CAX genes including CAX1, CAX2, CAX3), with tissue- and condition-specific expression patterns that reflect their roles in development and stress responses.

Functional studies in model species show that CAX proteins influence cytosolic Ca2+ signaling and vacuolar Ca2+ storage. Loss-of-function mutants and overexpression lines generally reveal altered metal homeostasis, supporting a role in calcium storage and in tolerance to metal stress. Regulation of CAX activity involves the proton gradient generated by H+-ATPases, and interactions with other proteins and signaling molecules modulating Ca2+ transport.

Understanding CAX transporters has practical implications for agriculture, including strategies to manipulate mineral storage in seeds and improve tolerance to abiotic stresses, though the precise roles of individual genes can vary among species.