The invertebrate B0 system transporter, D. melanogaster NAT1, has unique d-amino acid affinity and mediates gut and brain functions

Abstract

The CG3252 gene product, DmNAT1, represents the first Nutrient Amino acid Transporter cloned from Drosophila. It absorbs a broader set of neutral amino acids versus earlier characterized insect NATs and mammalian NATs-B 0 system transporters from the Sodium Neurotransmitter symporter Family (SNF, a.k.a. solute carrier family 6, SLC6). In addition to B 0 -specific L-substrates, DmNAT1 equally or more effectively transports D-amino acids with sub millimolar affinities and 1:1 sodium:amino acid transport stoichiometry. DmNAT1 is strongly transcribed in the absorptive and secretory regions of the larval alimentary canal and larval brain, revealing its roles in the primary absorption and redistribution of large neutral L-amino acids as well as corresponding D-isomers. The absorption of D-amino acids via DmNAT1 may benefit the acquisition of fermented and symbiotic products, and may support the unique capacity of fruit fly larvae to utilize a diet with substitution of essential amino acids by D-isomers. It also suggests a remarkable adaptive plasticity of NAT-SLC6 mechanisms via alterations of a few identifiable sites in the substrate-binding pocket. The strong transcription in the brain suggests roles for DmNAT1 in neuronal nutrition and clearance of L-neutral amino acids from the fly brain. In addition, neuronal DmNAT1 may absorb synaptic D-serine and modulate NMDA receptor-coupled signal transduction. The characterization of the first invertebrate B 0 -like transporter extends the biological roles of the SLC6 family, revealing adaptations for the absorption of D-isomers of the essential amino acids. These findings suggest that some members of the NAT-SLC6 subfamily evolving specific properties which contribute to nutrient symbiotic relationships and neuronal functions.

Cloning and functional expression of the first eukaryotic Na+–tryptophan symporter, AgNAT6

“…All currently characterized NATs encode transporters with similar electrochemical mechanisms utilizing the most ubiquitous monovalent cation (primarily Na + ) electrochemical gradient to symport (co-transport) neutral amino acids. The NATs population includes broad substrate spectrum, neutral amino acid transporters, which were identified in mammals (Broer et al, 2004) and insects [see figure 1s, B 0 AT1 and DmNAT1, in Miller et al (Miller et al, 2008)]. In insects the NAT subfamily has an additional expansion, which is absent in mammals.…”

The Digestive Tract of Drosophila melanogaster

“…Many of the mammalian transporter families are present in Drosophila, including the facilitative glucose transporters of the GLUT family (60), cationic amino acid transporters (38), ion-dependent and -independent amino acid transporters for neutral amino acids (70,108,117,149), and dipeptide transporters (154). However, intestinal expression has only been reported for the amino acid transporters pathetic, minidiscs, and NAT1 and the dipeptide transporter yin (opt1) (70,117,154), and the roles of these transport systems in the intestine remain to be established. Although many of them may handle dietary nutrients, some may be involved in absorption of fermented and symbiotic products.…”

“…Although many of them may handle dietary nutrients, some may be involved in absorption of fermented and symbiotic products. A case in point is the broad neutral amino acid transporter NAT1, which is expressed in the larval posterior midgut and is able to transport both L-and D-isomers of several amino acids (117). D-isomers are particularly abundant in the cell walls of bacteria and can substitute essential L-amino acids in the Drosophila diet (68), suggesting that NAT1 may normally function to absorb substrates from bacteriaenriched fermented diets (16).…”

A SLC6 transporter of the novel B0,- system aids in absorption and detection of nutrient amino acids in Caenorhabditis elegans

“…The substrate narrowing specializations are far more pronounced for some insect-specific NATs. For example, NAT expansion in mosquitoes is larger than that of mammalian NATs and, in addition to B 0 -like transporters (Boudko et al, 2005a;Miller et al, 2008), includes currently identified Phe-, Trp-and Met-selective members (Meleshkevitch et al, 2006;Meleshkevitch et al, 2009a;Meleshkevitch et al, 2009b). The functional characterization of SNF-5 showed that B 0 -like transporters exist, and are structurally conserved, in free-living nematodes, making them one of the most universal and phylogenetically ancient types of metazoan NATs.…”