Serotransferrin receptors

Serotransferrin receptor genes are encoded on the same chromosome (chromosomal region 3q21-3q29) in humans [131,132] as those for sero- and melanotransferrins (for a review, see ref [88]). 

Receptors of numerous cells have been identified by the use of monoclonal antibodies and appear to have similar structures to that represented in Fig. 5. They are all disulfide-linked dimers consisting of two identical transmembrane glycosylated units (MM 95 kDa) of 760 amino acids with the N-terminus facing the cytoplasm. The cytoplasmic domain corresponds to the first 61 amino acid residues and is followed by a single hydrophobic 

The sites of 7/-glycosylation of serotransferrin receptors are highly conserved in evolution. In fact, all of the receptors cloned up to date, including human, chicken, Chinese hamster, mouse and rat, possess three sites of A -glycosylation [131,132,139]. The first two sites are equivalent to human sites Asn-251 and Asn-317 [139,140]. Rodents have two glycosylation sites [139] and chicken one glycosylation site [141]. 

The presence of one O-glycan linked through an 7 -acetylgalactosamine residue to threonine-104 near the transmembrane domain has been established by Do and Cummings [137]. 

The type of glycan is not randomly distributed along the peptide chain. Asn-251 site contains glycans of the iV-acetyllactosaminic type while Asn-727 is entirely of the oligomannosidic type [142,143]. Glycans linked to Asn-317 are a mixture of glycans of the 7/-acetyllactosaminic, oligomannosidic and hybrid type [144]. 

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Fig. 5. Schematic representation of the human serotransferrin receptor. C, cysteine residues positions 89 and 98, disulfide bridges position 62, fatty acid chain. From Trowbridge et al. [122].

Contrary to serotransferrin receptors, little is known about the structure, physicochemical properties and gene expression of lactotransferrin receptors. Their main characteristics, which are totally different from those of serotransferrin receptors, can be summarized as follows ... 

Pregnancy. Serotransferrin is involved in the process of iron transfer to the foetus by a placental receptor-mediated mechanism. The three glycovariants of serotransferrin and their binding to human syncytiotrophoblast microvillar membranes have been studied by Leger et al. [108]. The results obtained by these authors and others [243,244] showed that in the serum of pregnant women, especially in the last... 

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