Primary structure of the human melanoma-associated antigen p97 (melanotransferrin) deduced from the mRNA sequence.

Abstract

ABSTRACTp97 is a cell-surface glycoprotein that is present in most human melanomas but only in trace amounts in normal adult tissues. To determine the structure of this tumor-associated antigen and to identify its functional domains, we have purified and cloned p97 mRNA and determined its nucleotide sequence. The mRNA encodes a 738-residue precursor, which contains the previously determined N-terminal amino acid sequence of p97. After removal of a 19-residue signal peptide, the mature p97 molecule comprises extracellular domains of 342 and 352 residues and a C-terminal 25-residue stretch of predominantly uncharged and hydrophobic amino acids, which we believe acts as a membrane anchor. Each extracellular domain contains 14 cysteine residues, which form seven intradomain disulfide bridges, and one or two potential N-glycosylation sites. Protease digestion studies show that the three major antigenic determinants of p97 are present on the N-terminal domain. The domains are strikingly homologous to each other (46% amino acid sequence homology) and to the corresponding domains of human serum transferrin (39% homology). Conservation of disulfide bridges and of amino acids thought to compose the iron binding pockets suggests that p97 is also related to transferrin in tertiary structure and function. We propose that p97 be renamed melanotransferrin to denote its original identification in melanoma cells and its evolutionary relationship to serotransferrin and lactotransferrin, the other members of the transferrin superfamily.p97 is a tumor-associated antigen that was first identified in human melanoma by using monoclonal antibodies (1-3). It has been studied extensively with regard to its expression in normal and neoplastic tissues and is present in most human melanomas and in certain fetal tissues, but it is found only in trace amounts in normal adult tissues (4-6). p97 has been used as a target for diagnostic imaging of melanomas in human clinical trials (7).p97 is a monomeric cell surface sialoglycoprotein, with an apparent molecular weight as determined by NaDodSO4/ polyacrylamide gel electrophoresis of slightly less than 97,000 (4). Use of monoclonal antibodies has defined three major antigenic sites, which are present on a stable Mr 40,000 tryptic fragment (4). Subsequent work has shown that at least two other independently characterized human melanomaassociated antigens, gp95 (3) and gp87 (8), are identical to p97.The N-terminal amino acid sequence of p97 is homologous to transferrin and, like transferrin, p97 binds iron (9). Analysis of somatic cell hybrids and by in situ hybridization has shown that the p97 gene, like the genes for transferrin and the transferrin receptor, is located on chromosomal region 3q21-3q29 (10, 11). These observations suggest that p97 plays a role in iron metabolism. To determine the structure of p97 and identify functional and antigenic domains, we have cloned and sequenced p97 mRNA. The availability of cloned p97 cDNA will allow us to study the regulation of the expression of p97 and...

Directing adenovirus across the blood–brain barrier via melanotransferrin (P97) transcytosis pathway in an in vitro model

“…21,22,[24][25][26] Currently, no specific receptor has been identified for MTf. Although human MTf shares 39% homology with human serum transferrin, 13 TfR is found not to be responsible for MTf transcytosis. Instead, a member of LDL receptor family, LRP, may play an essential role in this regard.…”

“…13 The exact function of both forms remains largely unknown. The membrane bound MTf is composed of 719 amino-acid residues, which comprises two homologous extracellular domains of 342 and 352 amino-acid residues, respectively, and a C-terminal 25-residue stretch of predominantly uncharged and hydrophobic amino-acid residues, which is believed to form a glycosylphosphatidylinositol (GPI) membrane anchor.…”

“…The membrane bound MTf is composed of 719 amino-acid residues, which comprises two homologous extracellular domains of 342 and 352 amino-acid residues, respectively, and a C-terminal 25-residue stretch of predominantly uncharged and hydrophobic amino-acid residues, which is believed to form a glycosylphosphatidylinositol (GPI) membrane anchor. 13 It has been demonstrated that transendothelial transport of MTf occurs via receptor-mediated endocytosis, and low-density lipoprotein (LDL) receptor-related protein (LRP) appears to be involved in MTf transendothelial transport. 11 In this study, we attempted to re-direct Ad5 vectors to the MTf transcytosis pathway so that they can deliver transgenes across the BBB in an in vitro model system.…”

Human melanotransferrin (p97) has only one functional iron‐binding site

“…Its amino acid sequence [S] shows a high level of identity with proteins of the transferrin family (-40% identity with human transferrin or lactoferrin), it binds iron [6] and it has been proposed to play a role in iron translocation [5].…”

The melanoma tumor antigen, melanotransferrin (p97): a 25-year hallmark – from iron metabolism to tumorigenesis

“…MTf shares many important characteristics with serum Tf, including: (i) a sequence homology of 37-39% with human serum Tf, human lactoferrin (Lf) and chicken Tf; (ii) the MTf gene is located on chromosome 3, as are those for Tf and the transferrin receptor 1 (TfR1); (iii) the presence of many disulfide bonds in MTf that are also present in Tf and Lf; (iv) MTf has an N-terminal Febinding site that is identical to the N-lobe Fe-binding site found in serum Tf; and (v) isolated and purified MTf can bind Fe from Fe(III) citrate complexes (Brown et al, 1981a(Brown et al, , 1982Rose et al, 1986;Richardson and Baker, 1991a;Food et al, 2002). These observations suggested that MTf played a role in Fe transport.…”