Protein Structure of Fetal Antigen 1 (FA1)

Abstract

The present paper describes the primary structure, glycosylation and tissue localization of fetal antigen 1 (FA1) isolated from second-trimester human amniotic fluid. FA1 is a single-chained, heterogeneous glycoprotein of 225 -262 amino acid residues. FA1 has six well conserved epidermalgrowth-factor motifs and contains up to ten O-glycosylation and N-glycosylation sites, six of which are differentially glycosylated. Alignment to the translated sequences of Mus. musculus dlk and human dlk revealed 86% and 99% identity, respectively, to a 259-amino-acid residue overlap, and this high similarity extends with minor corrections to the human adrenal-specific mRNA, pG2 as well. Immunohistochemical analysis demonstrated the presence of FA1 in 10 out of 14 lung tumors containing neuroendocrine elements, and in the placental villi where FA1 was exclusively seen in stromal cells in close contact to the vascular structure. In the pancreas, FA1 co-localized with insulin in the insulin secretory granules of the p cells within the islets of Langerhans.Our findings suggest that FA1 is synthesized as a membrane anchored protein and released into the circulation after enzymic cleavage, and that circulating FA1 represents the post-translationally modified gene product of human dlk which, in turn, is identical to human adrenal-specific mRNA pG2.The circulating form of fetal antigen 1 (FA1) was originally isolated from normal human amniotic fluid during the second trimester of pregnancy (Fay et al., 1988). The fetal origin of FA1 was suggested in accordance with the quantitative distribution in fetal and maternal compartments and immunohistochemical analysis (Fay et al., 1988;Tornehave et al., 1989). High concentrations of FA1 were found in secondtrimester amniotic fluid and in fetal serum, whereas the concentration in normal human serum was found to be approximately 20ng/mL which is 1000-times less than in secondtrimester fetal serum (Jensen, 1992). In preliminary-tissue localization studies using the indirect immunoperoxidase technique on fetal tissues (week 7), FA1 immunoreactivity was detected in the hepatocytes of the fetal liver (Tomehave et Abbreviations. FA1, fetal antigen 1 ; EGF, epidermal growth factor; PDMS, plasma-desorption mass spectrometry ; MALDI-MS, matrix-assisted laser-desorptionionisation mass spectrometry; BCG, Bacille, Calmette, GuLrin ; PPD, purified protein derivative; PEG, poly(ethy1ene glycol) ; FITC, fluoroscein isothiocyanate ; NSE, neuron-specific enolase.

Regulation of growth hormone expression by Delta-like protein 1 (Dlk1)

“…Dlk1, also known as pref-1 [4], pG2 [5] and zona glomerulosa specific mRNA [6], contains 6 EGF-like repeat domains in the extracellular domain, a transmembrane domain and a short cytoplasmic tail. It is cleaved by a yet unidentified protease to release a soluble circulating peptide containing the EGF-like repeats [7][8][9][10]; but its function remains elusive. This circulating peptide was originally identified as fetal antigen 1 [7].…”

“…It is cleaved by a yet unidentified protease to release a soluble circulating peptide containing the EGF-like repeats [7][8][9][10]; but its function remains elusive. This circulating peptide was originally identified as fetal antigen 1 [7]. By virtue of its EGF-like motifs, Dlk1 is a member of the EGF-like repeat containing protein family that includes other members such as notch, delta, and serrate [6].…”

Adipogenesis

“…DLK-1 is the human homolog of Pref-1 and was originally identified by purification from fetal circulation as fetal antigen-1 (Jensen et al 1994). Pref-1 is a transmembrane protein that belongs to a family of EGF-repeat-containing proteins and shares structural homology with the Notch/ Delta/Serrate proteins (reviewed in Sul 2009).…”

Transit-Amplifying Ductular (Oval) Cells and Their Hepatocytic Progeny Are Characterized by a Novel and Distinctive Expression of Delta-Like Protein/Preadipocyte Factor 1/Fetal Antigen 1

“…30,31 Delta-like protein, also referred to as pG2 (a human adrenal-specific mRNA), Pref-1 (preadipocyte factor 1), SCP-1, ZOG, as well as the soluble product FA1 (fetal antigen 1) 32 are encoded by the gene DLK1. The function of dlk is not known but the protein is widely expressed in embryonic tissues, including liver, tongue, vertebrae, skeletal myotubes, chondroblasts, and pancreas, 33,34 whereas in adults it is confined to the zona glomerulosa of the adrenal gland, 32 the somatotrophs in the pituitary gland, 35 the monoaminergic neurons in the central nervous system, 36 the Leydig and hilus cells of the testes and ovaries, 37 and the ␤ cells in the endocrine pancreas. 33 The cysteine spacing and the amino acid sequence within the individual EGF-like repeats are similar to those of Delta, which contains nine EGF-like repeats and the cysteine-rich motif DSL important for binding to Notch in the extracellular region.…”