Novel Properties of the Wheat Aluminum Tolerance Organic Acid Transporter (TaALMT1) Revealed by Electrophysiological Characterization in Xenopus Oocytes: Functional and Structural Implications

Abstract

Many plant species avoid the phytotoxic effects of aluminum (Al) by exuding dicarboxylic and tricarboxylic acids that chelate and immobilize Al 31 at the root surface, thus preventing it from entering root cells. Several novel genes that encode membrane transporters from the ALMT and MATE families recently were cloned and implicated in mediating the organic acid transport underlying this Al tolerance response. Given our limited understanding of the functional properties of ALMTs, in this study a detailed characterization of the transport properties of TaALMT1 (formerly named ALMT1) from wheat (Triticum aestivum) expressed in Xenopus laevis oocytes was conducted. The electrophysiological findings are as follows. Although the activity of TaALMT1 is highly dependent on the presence of extracellular Al 31 (K m1/2 of approximately 5 mM Al 31 activity), TaALMT1 is functionally active and can mediate ion transport in the absence of extracellular Al 31 . The lack of change in the reversal potential (E rev ) upon exposure to Al 31 suggests that the ''enhancement'' of TaALMT1 malate transport by Al is not due to alteration in the transporter's selectivity properties but is solely due to increases in its anion permeability. The consistent shift in the direction of the E rev as the intracellular malate activity increases indicates that TaALMT1 is selective for the transport of malate over other anions. The estimated permeability ratio between malate and chloride varied between 1 and 30. However, the complex behavior of the E rev as the extracellular Cl 2 activity was varied indicates that this estimate can only be used as a general guide to understanding the relative affinity of TaALMT1 for malate, representing only an approximation of those expected under physiologically relevant ionic conditions. TaALMT1 can also mediate a large anion influx (i.e. outward currents). TaALMT1 is permeable not only to malate but also to other physiologically relevant anions such as Cl 2 , NO 3 2, and SO 4 22 (to a lesser degree).

Activity of guard cell anion channel SLAC1 is controlled by drought-stress signaling kinase-phosphatase pair

“…Furthermore SLAC1 shares neither structural nor functional similarities to AtALMT1, the Arabidopsis orthologue to the wheat aluminum-induced malate channel (48). Thus SLAC1 represents a novel type of inorganic anion channel with unknown tertiary structure, but kinetic properties very similar to the slow anion channel first described with Xanthium strumarium and Vicia faba guard cells (49,50).…”

“…3B). The putative elevated relative malate permeability in planta likely results from malate transporters expressed in addition to SLAC1 in guard cells such as AtALMT-type carriers or AtABCB14 (48,51,52).…”

Malate. Jack of all trades or master of a few?

“…Detailed analysis of the wheat ALMT1 revealed that the channel is activated with a Km1/2 of 5µM and is strongly rectifying (Pineros et al 2008). However, the exact binding site and mechanism of Al 3+ channel activation awaits elucidation.…”

Channel-Like Characteristics of the Low-Affinity Barley Phosphate Transporter PHT1;6 When Expressed inXenopusOocytes

“…Limited information is available for comparison of anion selectivity in oocytes expressing plant transporters. It has been shown that GmN70, an anion transporter on the symbiosome membrane, favors NO 3 2 transport (Vincill et al, 2005), and TaALMT1 favors malate transport but can be selective for Cl 2 under certain conditions (Pineros et al, 2008 ) and has the same charge and oxyanion characteristics, which seem to be required for selective transport by HvPHT1;6. NO 3 2 is also an oxyanion, but is smaller in size, and has only a single negative charge, whereas Cl 2 is much smaller and does not have the oxygen.…”