Aluminium-activated malate transporters can facilitate GABA transport

Abstract

Plant aluminium-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 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 (Titicum aestivum L) root tips and in heterologous expression systems. We show that TaALMT1 activation reduces [GABA]i because TaALMT1 facilitates GABA efflux but GABA does not complex Al³⁺. TaALMT1 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 resulted in no correlation 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 to regulate each other's transport, suggestive of a role for ALMTs in communicating metabolic status.