Homologous membrane folate binding proteins in human placenta: cloning and sequence of a cDNA

Abstract

A preparation of folate binding protein purified from human placental membranes in the presence of a variety of protease inhibitors followed by deglycosylation with N-glycanase gave a sharp band at Mr approximately 28,000 following SDS-polyacrylamide gel electrophoresis. The deglycosylated protein bound [3H]folic acid as tightly as the native protein. Peptides obtained following digestion of the purified protein with staphylococcal V8 protease and HPLC purification were sequenced. Polyclonal antibodies against the protein preparation were affinity purified and used to screen a placental cDNA expression library. A full-length cDNA for a placental folate binding protein was thus obtained and the corresponding protein sequence deduced. This result, taken together with the peptide sequence data, indicates the expression of at least two homologous folate binding proteins in placenta, one of which appears to be identical with the folate binding protein from human milk and nasopharyngeal epidermoid carcinoma (KB) cells; the cDNA sequence obtained corresponds to the other protein. The deduced protein sequence is characterized by a putative 16-residue amino-terminal signal peptide that is cleaved, resulting in a 239-residue polypeptide. The mature protein exhibits two potential sites for N-linked glycosylation at Asn-99 and Asn-179, eight potential intramolecular disulfide bonds, and a stretch of hydrophobic residues at the carboxyl terminus that could form a transmembrane domain. The protein bears a 68% sequence homology with the KB cell folate binding protein and may represent a fetal folate transport protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Folate binding protein: molecular characterization and transcript distribution in pig liver, kidney and jejunum

“…Dideoxynucleotide sequencing of the 1.2 kb inserts from three clones resulted in a cDNA of 1230 bases that contained an identical 759-base open reading frame between bases 90 and 849 that encodes a peptide sequence of 253 amino acids ( Figure 1). The similarity between the cDNA sequences in the open reading frame of pig liver FBP and the published sequence for human placenta FBP [3] was 80 %. Back-translation of the open reading frame revealed 67-73 % similarity of the deduced pig liver FBP sequence to previously published amino acid sequences [2,3,[6][7][8]20] (Figure 2).…”

“…The similarity between the cDNA sequences in the open reading frame of pig liver FBP and the published sequence for human placenta FBP [3] was 80 %. Back-translation of the open reading frame revealed 67-73 % similarity of the deduced pig liver FBP sequence to previously published amino acid sequences [2,3,[6][7][8]20] (Figure 2). The deduced amino acid sequence revealed cross-species conservation of 16 cysteine sites, 11 tryp-…”

“…Folate-binding proteins (FBPs) have a high affinity for monoglutamyl folates [1] and have been identified in human placenta [2,3], rat [4] and pig kidney [5], bovine milk [6], and human cancer cell lines including KB [2,7] and Caco-2 cells [8]. The nucleotide and amino acid similarities of FBPs from placenta and several different mammalian cells lines [1][2][3]7,8] suggest that they may play a role in folate homeostasis through regulation of transport across intestinal, liver and kidney membranes.…”

“…The nucleotide and amino acid similarities of FBPs from placenta and several different mammalian cells lines [1][2][3]7,8] suggest that they may play a role in folate homeostasis through regulation of transport across intestinal, liver and kidney membranes. In this context, previous investigators demonstrated FBPs by gel filtration of rat and human liver plasma membranes [9,10], affinity chromatography of rat [4] and pig [5] renal tubular membranes and by autoradiography of affinity-labelled pig jejunal mucosal proteins separated by gel electrophoresis [11].…”

Spatial and temporal expression of folate‐binding protein 1 (Fbp1) is closely associated with anterior neural tube closure in mice

“…Folates bound to FBP are incorporated into cells by means of endocytosis (Kamen et al, 1988;Anderson et al, 1992). Four isoforms of folate receptors (␣ ␤, ␥, and ␦) have been cloned in human (Elwood, 1989;Ratnam et al, 1989;Shen et al, 1994;Spiegelstein et al, 2000), whereas three isoforms of FBPs (1, 2, and 3) have been identified in mice (Brigle et al, 1991;Spiegelstein et al, 2000). RFC has a relatively low affinity for folate, whereas a larger amount of folate is transported into cells by this protein (Sirotnak and Tolner, 1999).…”