Receptor-induced switch in site-site cooperativity during iron release by transferrin

Bali, Pawan K.; Aisen, Philip

doi:10.1021/bi00131a011

Cited by 56 publications

(62 citation statements)

References 13 publications

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“…The cycle requires a much shorter time than allowed by the natural dissociation rate of Fe # TF(CO $ ) at this pH. Further studies have established that the TFR itself is an active participant in increasing this dissociation rate ; the TFR appears to increase the rate of dissociation of iron from the C-lobe and decrease the rate from the N-lobe [7]. This converse effect is reminiscent of that caused by ' inert ' salt [8] but is independent of it [9].…”

Section: Introductionmentioning

confidence: 89%

Mutagenesis of the aspartic acid ligands in human serum transferrin: lobe–lobe interaction and conformation as revealed by antibody, receptor-binding and iron-release studies

Mason

Tam

et al. 1998

Biochemical Journal

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Recombinant non-glycosylated human serum transferrin and mutants in which the liganding aspartic acid (D) in one or both lobes was changed to a serine residue (S) were produced in a mammalian cell system and purified from the tissue culture media. Significant downfield shifts of 20, 30, and 45 nm in the absorption maxima were found for the D63S-hTF, D392S-hTF and the double mutant, D63S/D392S-hTF when compared to wild-type hTF. A monoclonal antibody to a sequential epitope in the C-lobe of hTF reported affinity differences between the apo- and iron-forms of each mutant and the control. Cell-binding studies performed under the same buffer conditions used for the antibody work clearly showed that the mutated lobe(s) had an open cleft. It is not clear whether the receptor itself may play a role in promoting the open conformation or whether the iron remains in the cleft.

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Section: Introductionmentioning

confidence: 89%

Mutagenesis of the aspartic acid ligands in human serum transferrin: lobe–lobe interaction and conformation as revealed by antibody, receptor-binding and iron-release studies

Mason

Tam

et al. 1998

Biochemical Journal

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“…However, this helix undergoes no discernable movement between the apo, monoferric, or diferric states of TF (using pig serum TF as a model for the diferric form). The absence of a significant change in the position of this helix between the conformations may simply reflect the absence of the TFR, which we believe may be critical in inducing such a change (see below) (69,70).…”

Section: Triad-lysmentioning

confidence: 96%

The Crystal Structure of Iron-free Human Serum Transferrin Provides Insight into Inter-lobe Communication and Receptor Binding

Wally

Halbrooks

Vonrhein

et al. 2006

Journal of Biological Chemistry

229

259

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Serum transferrin reversibly binds iron in each of two lobes and delivers it to cells by a receptor-mediated, pH-dependent process. The binding and release of iron result in a large conformational change in which two subdomains in each lobe close or open with a rigid twisting motion around a hinge. We report the structure of human serum transferrin (hTF) lacking iron (apo-hTF), which was independently determined by two methods: 1) the crystal structure of recombinant non-glycosylated apo-hTF was solved at 2.7-Å resolution using a multiple wavelength anomalous dispersion phasing strategy, by substituting the nine methionines in hTF with selenomethionine and 2) the structure of glycosylated apo-hTF (isolated from serum) was determined to a resolution of 2.7 Å by molecular replacement using the human apo-N-lobe and the rabbit holo-C1-subdomain as search models. These two crystal structures are essentially identical. They represent the first published model for full-length human transferrin and reveal that, in contrast to family members (human lactoferrin and hen ovotransferrin), both lobes are almost equally open: 59.4°and 49.5°rotations are required to open the N-and C-lobes, respectively (compared with closed pig TF). Availability of this structure is critical to a complete understanding of the metal binding properties of each lobe of hTF; the apo-hTF structure suggests that differences in the hinge regions of the N-and C-lobes may influence the rates of iron binding and release. In addition, we evaluate potential interactions between apo-hTF and the human transferrin receptor.The transferrins are a family of bilobal iron-binding proteins that play the crucial role of binding ferric iron and keeping it in solution, thereby controlling the levels of this important metal in the body (1, 2). Human serum transferrin (hTF) 4 is synthesized in the liver and secreted into the plasma; it acquires Fe(III) from the gut and delivers it to iron requiring cells by binding to specific transferrin receptors (TFR) on their surface. The entire hTF⅐TFR complex is taken up by receptor-mediated endocytosis culminating in iron release within the endosome (3). Essential to the re-utilization of hTF, iron-free hTF (apo-hTF) remains bound to the TFR at low pH. When the apo-hTF⅐TFR complex is returned to the cell surface, apo-hTF is released to acquire more iron.Strong homologies exist, both between TF family members, and between the two lobes of any given TF (4, 5). Each N-and C-lobe is divided into two subdomains (designated N1 and N2, and C1 and C2) connected by a hinge that gives rise to a deep cleft containing the iron-binding ligands. Iron is coordinated by four highly conserved amino acid residues: an aspartic acid (the sole ligand from the N1-or C1-subdomain), a tyrosine in the hinge at the edge of the N2-or C2-subdomain, a second tyrosine within the N2-or C2-subdomain, and a histidine at the hinge bordering the N1-or C1-subdomain. In addition, the iron atom is bound by two oxygen atoms from the synergistic anion (carbonate), w...

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“…Iron within each lobe of hTF is transported into cells by receptor-mediated endocytosis in which lower pH approximately 5.6, the participation of the TFR, and an unidentified chelator within the endosome orchestrate the efficient and balanced release of iron from each lobe of hTF (16)(17)(18)(19). Prior to exiting the endosome via the divalent metal transporter 1, Fe 3þ must be reduced to Fe 2þ .…”

mentioning

confidence: 99%

How the binding of human transferrin primes the transferrin receptor potentiating iron release at endosomal pH

Eckenroth

Steere

Chasteen

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

137

200

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Delivery of iron to cells requires binding of two iron-containing human transferrin (hTF) molecules to the specific homodimeric transferrin receptor (TFR) on the cell surface. Through receptor-mediated endocytosis involving lower pH, salt, and an unidentified chelator, iron is rapidly released from hTF within the endosome. The crystal structure of a monoferric N-lobe hTF/TFR complex (3.22-Å resolution) features two binding motifs in the N lobe and one in the C lobe of hTF. Binding of Fe N hTF induces global and site-specific conformational changes within the TFR ectodomain. Specifically, movements at the TFR dimer interface appear to prime the TFR to undergo pH-induced movements that alter the hTF/TFR interaction. Iron release from each lobe then occurs by distinctly different mechanisms: Binding of His349 to the TFR (strengthened by protonation at low pH) controls iron release from the C lobe, whereas displacement of one N-lobe binding motif, in concert with the action of the dilysine trigger, elicits iron release from the N lobe. One binding motif in each lobe remains attached to the same α-helix in the TFR throughout the endocytic cycle. Collectively, the structure elucidates how the TFR accelerates iron release from the C lobe, slows it from the N lobe, and stabilizes binding of apohTF for return to the cell surface. Importantly, this structure provides new targets for mutagenesis studies to further understand and define this system.

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Receptor-induced switch in site-site cooperativity during iron release by transferrin

Cited by 56 publications

References 13 publications

Mutagenesis of the aspartic acid ligands in human serum transferrin: lobe–lobe interaction and conformation as revealed by antibody, receptor-binding and iron-release studies

Mutagenesis of the aspartic acid ligands in human serum transferrin: lobe–lobe interaction and conformation as revealed by antibody, receptor-binding and iron-release studies

The Crystal Structure of Iron-free Human Serum Transferrin Provides Insight into Inter-lobe Communication and Receptor Binding

How the binding of human transferrin primes the transferrin receptor potentiating iron release at endosomal pH

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