Comparative analysis of CAX2-like cation transporters indicates functional and regulatory diversity

Abstract

Internal compartmentalization of metals is an important metal tolerance mechanism in many organisms. In plants and fungi, sequestration into the vacuole is a major detoxification mechanism for metals. Cation transport into the vacuole can be mediated by CAX (cation exchanger) transporters. The Arabidopsis thaliana AtCAX2 transporter was shown previously to transport Ca(2+), Cd(2+) and Mn(2+). To assess the conservation of the functional and regulatory characteristics of CAX2-like transporters in higher plants, we have characterized AtCAX2 orthologues from Arabidopsis (AtCAX5), tomato (LeCAX2) and barley (HvCAX2). Substrate specificity and regulatory activity were assessed using a yeast heterologous-expression assay. Each CAX could transport Ca(2+) and Mn(2+) into the yeast vacuole, but they each had different cation transport kinetics. Most notably, there was variation in the regulation of the transporters. As found with AtCAX2 previously, only expression of an N-terminally truncated form of AtCAX5 in yeast was able to mediate Ca(2+) and Mn(2+) transport, indicating that activity may be controlled by an autoregulatory region at the N-terminus. In contrast, either full-length or truncated LeCAX2 could efficiently transport Ca(2+), although Mn(2+) transport was controlled by the N-terminus. HvCAX2 did not appear to possess an N-terminal regulatory domain. Expression of AtCAX2 was not significantly modulated by metal stress; however, AtCAX5 and HvCAX2 were transcriptionally up-regulated by high Mn(2+) treatment, and by Ca(2+) and Na(+) stress respectively. It is therefore apparent that, despite the high sequence identity between plant CAX2 orthologues, there is significant diversity in their functional characteristics, particularly with regard to regulatory mechanisms.

CAX‐ing a wide net: Cation/H+ transporters in metal remediation and abiotic stress signalling

“…2007; Edmond et al . 2009; Migocka et al . 2011), it is unclear whether all examples of CAX involvement are due to direct metal transport rather than a Ca 2+ signalling role.…”

Influence of Soil Chemistry and Plant Physiology in the Phytoremediation of Cu, Mn, and Zn

“…The cation/H exchanger (CAX) transporters AtCAX2 and AtCAX4, which were originally identified as Ca transporters, also have the ability to transport Mn into the vacuole (Connorton et al, 2012;Korenkov et al, 2007). Likewise, vacuolar Mn/H antiporter activity in the Arabidopsis cax2 knockout mutant is significantly reduced compared with wild-type, although it is not completely absent, suggesting the presence of additional vacuolar transporters (e.g., AtCAX5) that contribute to Mn transport (Edmond et al, 2009). AtMTP11, which belongs to the cation diffusion facilitator (CDF) family, is implicated in the pre-vacuolar compartmentation of Mn as well as in the Mn homeostasis mechanisms (Delhaize et al, 2007).…”

The Evolutionarily Conserved Protein PHOTOSYNTHESIS AFFECTED MUTANT71 is Required for Efficient Manganese Uptake at the Thylakoid Membrane in Arabidopsis

“…A number of putative Mn 2+ transporters in plants have been described and can be grouped based on the direction of manganese transport either into or out of the cytosol (Socha and Guerinot, 2014). Manganese transport from the cytosol into the vacuole mediated by CATION EXCHANGER2 (CAX2), CAX4, and CAX5 (Hirschi et al, 2000;Cheng et al, 2002;Edmond et al, 2009) or into the prevacuolar and Golgi-like compartment mediated by the Mn 2+ transporter MTP11 (Delhaize et al, 2007;Peiter et al, 2007) presumably also involves a Mn 2+ /H + antiport mechanism in Arabidopsis. Most plant Mn 2+ transporters characterized to date have a broader specificity for other divalent cations (Socha and Guerinot, 2014), for instance, CAX2 (Hirschi et al, 2000;Schaaf et al, 2002;Shigaki et al, 2003).…”