Distribution of mRNA of a Na+-independent neutral amino acid transporter cloned from rat kidney and its expression in mammalian tissues and Xenopus laevis oocytes
The Na(+)-independent neutral amino acid transporter (NAA-Tr) that we had previously cloned from rat kidney has been investigated with respect to its distribution in mammalian tissues and cells. By Northern blot analysis and RNase protection assay, a 2.4-kilobase (kb) mRNA in rat intestine was found to be identical to that in rat kidney. Of the other rat tissues examined, only brain and heart were found to contain mRNAs related to kidney NAA-Tr by Northern assay. However, these were larger (approximately 5 and approximately 7 kb). Mouse and rabbit kidney also contain mRNAs of 2.4 kb that exhibited a high degree of homology with rat kidney NAA-Tr. Of the several cultured cells investigated that demonstrated considerable Na(+)-independent neutral amino acid transport activity, only human colon carcinoma (Caco) cells were positive by Northern assay. The failure to detect NAA-Tr mRNA in many cells and tissues that carry out Na(+)-independent transport indicates that unrelated transporters must also exist. Cells and tissues that were negative with respect to rat kidney NAA-Tr as well as those that were positive transported leucine and tryptophan equally well. However, when mRNA from the same cells and tissues was expressed in oocytes, in all cases tryptophan was transported far less efficiently than leucine. This defect in tryptophan transport is apparently due to aberrant expression of neutral amino acid transporters in general in Xenopus oocytes.