The position of an arginine residue influences substrate affinity and K+ coupling in the human glutamate transporter, EAAT1.
Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system and extracellular glutamate levels are controlled by a family of transporters known as excitatory amino acid transporters (EAATs). The EAATs transport glutamate and aspartate with similar micromolar affinities and this transport is coupled to the movement of Na(+), K(+), and H(+). The crystal structure of a prokaryotic homologue of the EAATs, aspartate transporter from Pyrococcus horokoshii (Glt(Ph)), has yielded important insights into the architecture of this transporter family. Glt(Ph) is a Na(+)-dependent transporter that has significantly higher affinity for aspartate over glutamate and is not coupled to H(+) or K(+). The highly conserved carboxy-terminal domains of the EAATs and Glt(Ph) contain the substrate and ion binding sites, however, there are a couple of striking differences in this region that we have investigated to better understand the transport mechanism. An arginine residue is in close proximity to the substrate binding site of both Glt(Ph) and the EAATs, but is located in transmembrane domain (TM) 8 in the EAATs and hairpin loop 1 (HP1) of Glt(Ph). Here we report that the position of this arginine residue can explain some of the functional differences observed between the EAATs and Glt(Ph). Moving the arginine residue from TM8 to HP1 in EAAT1 results in a transporter that has significantly increased affinity for both glutamate and aspartate and is K(+) independent. Conversely, moving the arginine residue from HP1 to TM8 in Glt(Ph) results in a transporter that has reduced affinity for aspartate.
Ryan
Renae M
RM
Department of Pharmacology, University of Sydney, Sydney, New South Wales, Australia. renae.ryan@sydney.edu.au
Kortt
Nicholas C
NC
Sirivanta
Tan
T
Vandenberg
Robert J
RJ
D000818:Animals; D019843:Archaeal Proteins / Q000378:metabolism; D001120:Arginine / Q000235:genetics*; D001224:Aspartic Acid / Q000378:metabolism; D001665:Binding Sites; D001693:Biological Transport, Active; D002414:Cations, Monovalent; D027341:Excitatory Amino Acid Transporter 1 / Q000235:genetics / Q000378:metabolism*; D005260:Female; D018698:Glutamic Acid / Q000378:metabolism; D006801:Humans; D016297:Mutagenesis, Site-Directed; D009865:Oocytes / Q000378:metabolism; D011188:Potassium / Q000378:metabolism*; D011485:Protein Binding; D043902:Pyrococcus horikoshii; D014982:Xenopus laevis
D019843:Archaeal Proteins; D002414:Cations, Monovalent; D027341:Excitatory Amino Acid Transporter 1; C493394:SLC1A3 protein, human; D001224:Aspartic Acid; D018698:Glutamic Acid; D001120:Arginine; D011188:Potassium