Albumin Gene Family Research Articles

Nucleotide Sequences of cDNAs Encoding Two Members of the Brazil Nut Methionine-Rich 2S Albumin Gene Family

The 2S albumins of Brazil nut (Bertholletia excelsa H.B.K.) are a group of abundant seed proteins containing high concentrations of the sulfur amino acids. Because these proteins contain about 18% methionine, the albumins have been used in gene transfer experiments to improve the methionine content of seed proteins from other crop plants (1, 3). The Brazil nut albumins consist of two subunits, a small subunit of 3 kD and a large subunit of 9 kD, which are joined by disulfide linkages. Both subunits are derived from a precursor polypeptide in a series of proteolytic processing events (5, 8). Like many other seed storage proteins, the Brazil nut albumins are encoded by a multigene family. Amino acid sequences of five isoforms of the large subunit (2, 4) and one isoform of the small subunit (4) have been determined. In addition, the nucleotide sequences of cDNAs encoding two other isoforms have been reported (2, 5). Considerable homology is apparent among proteins encoded by members of the Brazil nut 2S albumin family. The Brazil nut protein also exhibits strong structural homology with the 2S albumins of other seeds, including those from rapeseed, Arabidopsis, sunflower, castor bean, barley, and lupin (2, 4, 6, 7). Indeed, the positions of seven to eight cysteine residues have been conserved in albumins from these species. These homologies have been important in defining regions of 2S seed proteins that might tolerate modifications without adversely affecting processing, transport, or stability of the proteins in seeds. In this paper, the nucleotide sequences of cDNAs encoding two new isoforms of the Brazil nut 2S albumin are reported (Fig. 1, Table I). These sequences are compared with the sequence of cDNA clone pHS-3, which encodes the entire protein precursor of a Brazil nut albumin (2). One cDNA, pHS-9, shows 99% homology to pHS-3 at the nucleotide level and 96% homology at the protein level. The other cDNA, pHS-6, has 95% homology to pHS-3 at the DNA level and 88% homology at the protein level. The protein encoded by pHS-9 contains 16.8% methionine, and the protein encoded by pHS-6 contains 20.8% methionine.

Vitamin D binding protein: Genomic structure, functional domains, and mRNA expression in tissues

The vitamin D binding protein (DBP), alternatively known as Gc-globulin, is a member of the albumin (ALB) and α-fetoprotein (AFP) gene family. The rat DBP gene is expressed at high levels in liver and moderate levels in kidney, testis, abdominal fat, and yolk sac. Very low levels of DBP as well as ALB and AFP transcripts can be detected in all other tissues studied by the reverse transcriptase/polymerase chain reaction technique. During development, liver DBP gene transcripts are detectable at 14 days of gestation and levels rise gradually until adulthood in parallel with ALB. DBP present on the surface of U937 monocyte-derived cells is acquired from serum, suggesting cell surface binding sites for DBP. The rat DBP gene has been cloned and characterized. It spans 35 kb and contains 13 exons and 12 introns. The DBP gene contains two fewer exons than the ALB or AFP genes, accounting for the shortest size of its mRNA and protein product. Its 5′-flanking region contains a high degree of structural similarity to both ALB and AFP promoters.

Fibulin is an extracellular matrix and plasma glycoprotein with repeated domain structure.

We have studied the expression of fibulin in cultured fibroblasts and determined its primary structure by cDNA cloning. Our results show that fibulin is a secreted glycoprotein that becomes incorporated into a fibrillar extracellular matrix when expressed by cultured cells or added exogenously to cell monolayers. In addition, we find that fibulin is present in plasma at a level of 33 +/- 3 micrograms/ml. Sequencing of multiple fibulin cDNAs indicates that a process of alternative splicing results in the expression of three fibulin transcripts. The transcripts encode overlapping polypeptides differing only in carboxy-terminal segments. Common to the three predicted forms of fibulin is a unique 537-amino acid-long cysteine-rich polypeptide and a 29-residue signal peptide. The amino-terminal portion of fibulin contains a repeated element with potential disulfide loop structure resembling that of the complement component anaphylatoxins C3a, C4a, and C5a as well as proteins of the albumin gene family. The bulk of the remaining portion of the molecule is a series of nine EGF-like repeats.

Serum vitamin D-binding protein is a third member of the albumin and alpha fetoprotein gene family.

A near full-length cDNA encoding the human vitamin D-binding protein (hDBP) was isolated from a human liver mRNA expression library. Complete sequence analysis of this clone predicts the full-length amino acid sequence of the pre-hDBP. Comparison of the sequence of the hDBP mRNA and protein to existing protein and nucleic acid data banks demonstrates a strong and highly characteristic homology of the hDBP with human albumin (hALB) and human alpha-fetoprotein (hAFP). Based upon this structural comparison, we establish that DBP is a member of the ALB and AFP gene family.