Abstract
Plant sulfate transporters (SULTR) mediate absorption and distribution of sulfate (SO42−) and are essential for plant growth; however, our understanding of their structures and functions remains inadequate. Here we present the structure of a SULTR from Arabidopsis thaliana, AtSULTR4;1, in complex with SO42− at an overall resolution of 2.8 Å. AtSULTR4;1 forms a homodimer and has a structural fold typical of the SLC26 family of anion transporters. The bound SO42− is coordinated by side-chain hydroxyls and backbone amides, and further stabilized electrostatically by the conserved Arg393 and two helix dipoles. Proton and SO42− are co-transported by AtSULTR4;1 and a proton gradient significantly enhances SO42− transport. Glu347, which is ~7 Å from the bound SO42−, is required for H+-driven transport. The cytosolic STAS domain interacts with transmembrane domains, and deletion of the STAS domain or mutations to the interface compromises dimer formation and reduces SO42− transport, suggesting a regulatory function of the STAS domain.
Highlights
Plant sulfate transporters (SULTR) mediate absorption and distribution of sulfate (SO42−) and are essential for plant growth; our understanding of their structures and functions remains inadequate
Previous studies of SULTRs used cell-based assays and showed that SO42− uptake is significantly enhanced in lower pH and SULTRs were defined as H+/SO42− symporters[2,4,20,21]
In the absence of a pH gradient, SO42− uptake is similar to blank control in symmetrical pH 7.5 or 5.5, indicating that a pH gradient is required for SO42− transport and that a higher H+ concentration alone is not sufficient to sustain SO42− uptake
Summary
Plant sulfate transporters (SULTR) mediate absorption and distribution of sulfate (SO42−) and are essential for plant growth; our understanding of their structures and functions remains inadequate. Structures of a mammalian chloride channel (SLC26A9) were solved recently[16,17], and so were the structures of two bacterial homologs of SLC26, a bicarbonate transporter from cyanobacteria (BicA)[18], and a fumarate transporter from the bacterium Deinococcus geothermalis (SLC26Dg)[19]. These structures show a common architecture of a homodimeric assembly and that each monomer is composed of 14 transmembrane helices and a Cterminal cytosolic domain named Sulfate Transporter and AntiSigma factor antagonist (STAS) domain. We show that H+ transport is likely mediated by a glutamate residue (Glu347) highly conserved among the plant SULTR family and that the cytosolic STAS domain modulates the transport process
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