A novel gene encoding a small neutral amino acid transporter was cloned from the genome of the hyperthermophilic archaeon
Thermococcus sp. KS-1 by functional cloning using
Escherichia coli strain AK430, which is defective in transporting glycine and D-alanine. The cloned gene,
snatA, encoded a protein of 216 amino acid residues, SnatA, and was predicted to be a membrane protein with six membrane-spanning segments.
E. coli AK430 cells transformed with
snatA transported glycine with an apparent K
t value of 24μM, which was one order of magnitude higher than that of other known glycine/alanine transporters, including
cycA of
E. coli and
aep of thermophilic bacterium PS3. Competition studies revealed that SnatA transported various L-type neutral amino acids, but its substrate specificity was different from that of CycA or ACP. The glycine transport was inhibited by a protonophore, FCCP, or valinomycin
plus nigericin, indicating that the process is dependent on an electrochemical potential of H
+. Homology searches revealed no homology with any transporters known to date. However, several hypothetical genes in prokaryote cells enrolled in the gene bank showed significantly high homology scores, indicating that
snatA and its homologues form a family of prokaryotes. To our knowledge, this is the first report on the cloning of a gene of an amino acid transporter from a hyperther-mophilic archaeon.
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