Cloning and characterization of a human brain Na+-independent transporter for small neutral amino acids that transports D-serine with high affinity

We isolated a cDNA for the human homologue of system asc transporter Asc-1 from human brain. The encoded protein designated as hAsc-1 (human Asc-1) ex hibited 91 % sequence identity to mouse Asc-1. Consistent with mouse Asc-1, hAsc-1 required 4F2 heavy chain for its functional expression in Xenopus o ocytes. hAsc-1 exhibited the properties of amino acid transport system asc which transports small neutral amino acids in a Na+-independent manner. hAs c-1 transported D-serine at high affinity with a K-m value of 22.8 mu M. In brain, 2.0 kb mRNA was highly expressed. hAsc-1 gene was mapped to human c hromosome 19, region q12-q13.1. Because of the high-affinity transport with the K-m value close to the physiological concentration of D-serine, togeth er with the high levels of expression in brain, hAsc-1 is proposed to play significant roles in the D-serine mobilization in brain. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.