Structures suggest a mechanism for energy coupling by a family of organic anion transporters

Abstract

Members of the solute carrier 17 (SLC17) family use divergent mechanisms to concentrate organic anions. Membrane potential drives uptake of the principal excitatory neurotransmitter glutamate into synaptic vesicles, whereas closely related proteins use proton cotransport to drive efflux from the lysosome. To delineate the divergent features of ionic coupling by the SLC17 family, we determined the structure of Escherichia coli D-galactonate/H + symporter D-galactonate transporter (DgoT) in 2 states: one open to the cytoplasmic side and the other open to the periplasmic side with substrate bound. The structures suggest a mechanism that couples H + flux to substrate recognition. A transition in the role of H + from flux coupling to allostery may confer regulation by trafficking to and from the plasma membrane.

Ion transport and regulation in a synaptic vesicle glutamate transporter

“…2B) and is conserved, which is consistent with a common role in anion recognition by the SLC17 family. The equivalent residue (R47) in the bacterial homolog D-galactonate transporter (DgoT) makes a salt bridge with the carboxyl group of its substrate D-galactonate (22) (Fig. 2C).…”

“…E191 is highly conserved in TM4 of the VGLUTs, and mutations of E191 reduce transport activity (24). The equivalent residue in DgoT (E133) is essential for H + symport (22) and is conserved in the H + symporter sialin ( fig. S1).…”

Molecular, Structural, Functional, and Pharmacological Sites for Vesicular Glutamate Transporter Regulation

“…VGLUTs show primary sequence homology with the major facilitator superfamily (MFS), the second major family of transmembrane transporters involved in the translocation of small solutes using the driving force of an electrochemical gradient [168]. The crystallographic 3D structures of the lactose bacterial permease, also known as glycerol-3-phosphate transporter (GlpT), as well as D-galactonate transporter (DgoT) led to the conclusion that these transporters consist of 12 α-helices organized into two groups of 6 (two halves) [169,170]. The two groups of 6 α-helices are connected by a cytoplasmic flexible loop, forming a hydrophilic cavity at their center deep in the transporter for translocation of hydrophilic substrates.…”

“…The two groups of 6 α-helices are connected by a cytoplasmic flexible loop, forming a hydrophilic cavity at their center deep in the transporter for translocation of hydrophilic substrates. The amino acid residues responsible for the specificity of the transporter are located on the walls of this polar pocket [170][171][172]. Because of the distant, yet distinct, homology between GlpT, DgoT, and VGLUTs, a putative 3D homology model of VGLUTs can be postulated (Fig.…”

“…For example, Shuldiner and colleagues examined SARs between various transmembrane embedded charged amino acids in the bacterial multiresistant transporters for the study of their mammalian counterparts, such as VMATs [178][179][180][181]. Recently, Edwards and colleagues used a similar approach with DgoT showing an interesting cooperation of H + transfer between transmembrane embedded glutamate (Glu133) and arginine (Arg47) residues that suggest a mechanism that couples H + flux to substrate recognition [170]. That is, protonation of this bacterial transmembrane glutamate residue effectively releases a transmembrane arginine residue to bind and translocate substrate.…”