Split personality of Aluminum Activated Malate Transporter family proteins: facilitation of both GABA and malate transport

Plant aluminum activated malate transporters (ALMTs) are currently classified as anion channels; they are also known to be regulated by diverse signals leading to a range of physiological responses. Gamma-aminobutyric acid (GABA) regulation of anion flux through ALMT proteins requires the presence of a specific amino acid motif in ALMTs that shares similarity with a GABA-binding site in mammalian GABAA receptors. Here, we explore why TaALMT1-activation leads to a negative correlation between malate efflux and endogenous GABA concentrations ([GABA]i) in both wheat root tips and in heterologous expression systems. We show that TaALMT1 activation reduces [GABA]i because TaALMT1 facilitates GABA efflux. TaALMT1-expression also leads to GABA transport into cells, demonstrated by a yeast complementation assay and via 14CGABA uptake into TaALMT1-expressing Xenopus laevis oocytes; this was found to be a general feature of all ALMTs we examined. Mutation of the GABA motif (TaALMT1F213C) prevented both GABA influx and efflux, and uncoupled the relationship between malate efflux and [GABA]i. We conclude that ALMTs are likely to act as both GABA and anion transporters in planta. GABA and malate appear to interact with ALMTs in a complex manner regulating each others transport, suggestive of a role for ALMTs in communicating metabolic status.