Glutamate 59 is critical for transport function of the amino acid cotransporter KAAT1

The Lepidopteran KAAT1 and CAATCH1: Orthologs to Understand Structure-Function Relationships in Mammalian SLC6 Transporters

To the SLC6 family belong 20 human transporters that utilize the sodium electrochemical gradient to move biogenic amines, osmolytes, amino acids and related compounds into cells. They are classified into two functional groups, the Neurotransmitter transporters (NTT) and Nutrient amino acid transporters (NAT). Here we summarize how since their first cloning in 1998, the insect (Lepidopteran) Orthologs of the SLC6 family transporters have represented very important tools for investigating functional–structural relationships, mechanism of transport, ion and pH dependence and substate interaction of the mammalian (and human) counterparts.

Molecular physiology of the insect K-activated amino acid transporter 1 (KAAT1) and cation-anion activated amino acid transporter/channel 1 (CAATCH1) in the light of the structure of the homologous protein LeuT

K-activated amino acid transporter 1 (KAAT1) and cation-anion-activated amino acid transporter/channel 1 (CAATCH1) are amino acid cotransporters, belonging to the Na/Cl-dependent neurotransmitter transporter family (also called SLC6/NSS), that have been cloned from Manduca sexta midgut. They have been thoroughly studied by expression in Xenopus laevis oocytes, and structure/function analyses have made it possible to identify the structural determinants of their cation and amino acid selectivity. About 40 mutants of these proteins have been studied by measuring amino acid uptake and current/voltage relationships. The results obtained since the cloning of KAAT1 and CAATCH1 are here discussed in the light of the 3D model of the first crystallized member of the family, the leucine transporter LeuT.

Interaction between lysine 102 and aspartate 338 in the insect amino acid cotransporter KAAT1

KAAT1 is a lepidopteran neutral amino acid transporter belonging to the NSS super family (SLC6), which has an unusual cation selectivity, being activated by K(+) and Li(+) in addition to Na(+). We have previously demonstrated that Asp338 is essential for KAAT1 activation by K(+) and for the coupling of amino acid and driver ion fluxes. By comparing sequences of NSS family members, site-directed mutagenesis, and expression in Xenopus laevis oocytes, we identified Lys102 as a residue likely to interact with Asp338. Compared with wild type, the single mutants K102V and D338E each showed altered leucine uptake and transport-associated currents in the presence of both Na(+) and K(+). However, in K102V/D338E double mutant, the K102V mutation reversed both the inhibition of Na(+)-dependent transport and the block in K(+)-dependent transport that characterize the D338E mutant. K(+)-dependent leucine currents were not observed in any mutants with D338E. In the presence of the oxidant Cu(II) (1,10-phenanthroline)(3), we observed specific and reversible inhibition of K102C/D338C mutant, but not of the corresponding single cysteine mutants, suggesting that these residues are sufficiently close to form a disulfide bond. Thus both structural and functional evidence suggests that these two residues interact. Similar results have been obtained mutating the bacterial transporter homolog TnaT. Asp338 corresponds to Asn286, a residue located in the Na(+) binding site in the recently solved crystal structure of the NSS transporter LeuT(Aa) (41). Our results suggest that Lys102, interacting with Asp338, could contribute to the spatial organization of KAAT1 cation binding site and permeation pathway.