The
nutrient amino acid transporter (NAT) subfamily of the
neurotransmitter sodium symporter family (NSS, also known as the
solute carrier family 6, SLC6) represents transport mechanisms with putative synergistic roles in the absorption of essential and conditionally essential neutral
amino acids. It includes a large
paralogous expansion of insect-specific genes, with seven genes from the genome of the
malaria mosquito,
Anopheles gambiae. One of the An. gambiae NATs, AgNAT8, was
cloned, functionally expressed and characterized in X. laevis
oocytes as a cation-coupled symporter of
aromatic amino acids, preferably l-phenylalanine, l-tyrosine and l-DOPA. To explore an evolutionary trend of NAT-SLC6
phenotypes, we have
cloned and characterized AgNAT6, which represents a counterpart of AgNAT8 descending from a recent
gene duplication (53.1% pairwise
sequence identity). In contrast to AgNAT8, which preferably mediates the absorption of phenol-branched
substrates, AgNAT6 mediates the absorption of indole-branched
substrates with highest apparent affinity to
tryptophan (K(0.5)(Trp)=1.3 mumol l(-1) vs K(0.5)(
Phe)=430 mumol l(-1)) and [2 or 1 Na(+) or K(+)]:[
aromatic substrate] stoichiometry. AgNAT6 is highly transcribed in absorptive and secretory regions of the
alimentary canal and specific
neuronal structures, including the neuropile of
ventral ganglia and sensory afferents. The alignment of AgNATs and LeuT(Aa), a
bacterial NAT with a resolved 3D structure, reveals three
amino acid differences in the substrate-binding pocket that may be responsible for the indole- vs phenol-branch selectivity of AgNAT6 vs AgNAT8. The identification of transporters with a narrow selectivity for
essential amino acids suggests that basal expansions in the SLC6 family involved duplication and retention of NATs, improving the absorption and distribution of under-represented
essential amino acids and related
metabolites. The identified physiological and expression profiles suggest unique roles of AgNAT6 in the active absorption of indole-branched
substrates that are used in the synthesis of the
neurotransmitter serotonin as well as the key
circadian hormone and potent
free-radical scavenger
melatonin.
