The invertebrate transporters of SLC6 family are able to transport D-amino acid

Neutral amino acids, which predominantly represent essential substrates in animals, are absorbed via a B0 transport system (the Broad neutral amino acid system). Molecular and phylogenetic analysis of insect SLC6 members revealed a Nutrient Amino Acid Transporters (NATs) cluster that is phylogenetically close to mammalian B0 transporters and that can be classified in SLC6 Family. Differently from the mammalian and fish orthologs, these insect proteins can transport D-amino acids. Since their cloning, the Manduca sexta neutral amino acid cotransporter KAAT1 (K+-coupled amino acid transporter 1) and its homologous CAATCH1 (cation anion activated amino acid transporter/channel 1) have represented a useful tool for investigation in structure function because of unique physiological features. In presence of D- isomers of leucine, serine and proline, the msKAAT1, when heterologously expressed in Xenopus laevis oocytes, generates inward, transport-associated, currents with variable relative potencies, depending on the driving ion Na+ or K+. msCAATCH1 is also able to transport the D-amino acids tested, including D-leucine, while L-leucine acts as a blocker. A similar behavior is exhibited by the KAAT1 mutant S308T, confirming the relevance of this residue in L-leucine binding and the different interaction of D-leucine. Instead, the residue T67 that it is present as threonine only in KAAT1 and the paralogue CAATCH1, when mutated in tyrosine increased the stereoselectivity for L-isomers in KAAT1. T67 it is not conserved among SLC6 family members but according to LeuT structure organization, coordinates the organic substrate. D-leucine and D-serine on various vertebrate orthologs B0AT1 (SLC6A19) elicited only a very small current, indicating that transport of D isomers it is specific of the insect neutral amino acid transporters. These findings highlight the relevance of D-amino acid absorption in the insect nutrition and metabolism and may provide new evidences in the molecular transport mechanism of SLC6 family.