Molecular cloning and characterization of the human folate-binding protein cDNA from placenta and malignant tissue culture (KB) cells.

Human folate-binding proteins (FBPs) are single chain glycoproteins that contain a high affinity binding site for folates and methotrexate and occur in a soluble or membrane-associated form. The membrane-associated FBP is involved in the uptake of physiologic folates and methotrexate. In this study, human FBP cDNA clones were isolated from human malignant nasopharyngeal carcinoma (KB) cell and placental cDNA libraries by means of oligonucleotide probes derived from determined internal amino acid sequences. The longest cDNA nucleotide sequence is 1126 base pairs and encodes a polypeptide that contains 257 amino acid residues (calculated molecular mass = 29,817). The deduced amino acid sequence is 80% homologous to a bovine soluble FBP, is greater than 99% homologous to the reported partial amino acid sequence of the human soluble FBP, contains three potential N-linked glycosylation sites, and has hydrophobic amino- and carboxylterminal regions which are consistent with a signal peptide and a potential membrane-anchoring domain, respectively. On Northern blot analysis, radiolabeled cDNA probes hybridize to a single 1100-base pair mRNA species that is expressed to a variable degree in human KB cells, placenta, brain, and epithelial mRNA but is not detectable in human liver mRNA. In vitro translation of RNA transcripts from the FBP cDNA inserts yields a 30-kDa and a 42-kDa polypeptide in the absence and presence of microsomal membranes, respectively.