Identification And Functional Characterization Of A Novel Cationic Amino Acid Transporter Cat5 Specifically Expressed In Placenta

We identified a novel Na+-independent cationic amino cid transporter (CAT) with distinct characteristics in substrate selectivity and transport property. The encoded protein designated CAT5 that belongs to SLC7 family shows highest amino acid sequence similarity to the CAT3 with 59% identity. When expressed in Xenopus oocytes, CAT5 transported cationic amino acids (CAA) at high affinity, consistent with the properties of system y+. We found that an artificial CAA (H-L-Dab-OH) was the shortest chain CAA recognized by CAT5. CAT5 showed particularly high affinity to arginine (km=2.7 microM). Distinct from already known system y+ isoforms, CAT5 also mediate Na+- independent transport of large neutral amino acids (NAA) at low affinity. CAT5-mediated [14C]arginine uptake was also inhibited by large NAA and the artificial NAA (H-Nva-OH and H-Nle-OH). NAA uptake but not CAA uptake was affected by H+ and fit to the two-component kinetics. Therefore it was proposed that there are two distinct substrate recognition modes in CAT5. CAT5-mediated uptake was inhibited by the pretreatment with N-ethylmaleimide (NEM). Cyteine391 was found to be the main target of inactivation by NEM. Northern blot and Western blot show that CAT5 specifically expressed in the mouse placenta. Immunofluorescence analysis revealed that CAT5 is co-localized with eNOS. CAT5 is proposed to be involved in the placental barrier permeation and NO production in placenta.