Iron-dependent binding of 8-anilinonaphthalene-1-sulphonate by both lactoferrin and transferrin.
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ABSTRACT:
SUBMITTER: Bilic N
PROVIDER: S-EPMC1163837 | biostudies-other | 1976 Jul
REPOSITORIES: biostudies-other
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Project description:Transferrin, the major plasma iron-binding molecule, interacts with cell-surface receptors to deliver iron, modulates hepcidin expression, and regulates erythropoiesis. Transferrin binds and releases iron via either or both of 2 homologous lobes (N and C). To test the hypothesis that the specificity of iron occupancy in the N vs C lobe influences transferrin function, we generated mice with mutations to abrogate iron binding in either lobe (TfN-bl or TfC-bl). Mice homozygous for either mutation had hepatocellular iron loading and decreased liver hepcidin expression (relative to iron concentration), although to different magnitudes. Both mouse models demonstrated some aspects of iron-restricted erythropoiesis, including increased zinc protoporphyrin levels, decreased hemoglobin levels, and microcytosis. Moreover, the TfN-bl/N-bl mice demonstrated the anticipated effect of iron restriction on red cell production (ie, no increase in red blood cell [RBC] count despite elevated erythropoietin levels), along with a poor response to exogenous erythropoietin. In contrast, the TfC-bl/C-bl mice had elevated RBC counts and an exaggerated response to exogenous erythropoietin sufficient to ameliorate the anemia. Observations in heterozygous mice further support a role for relative N vs C lobe iron occupancy in transferrin-mediated regulation of iron homeostasis and erythropoiesis.
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Project description: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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Project description:Moraxella catarrhalis is a Gram-negative bacterium that is responsible for a substantial proportion of upper respiratory infections in children and lower respiratory infections in the elderly. Moraxella catarrhalis resides exclusively on the mucosal surfaces of the upper respiratory tract of humans and is capable of directly acquiring iron for growth from the host glycoproteins human transferrin (hTf) and human lactoferrin (hLf). The iron-bound form of these glycoproteins is initially captured by the surface lipoproteins Tf or Lf binding protein B (TbpB or LbpB) and delivered to the integral outer membrane TonB-dependent transport (TBDT) proteins, Tf binding protein A (TbpA) or Lf binding protein A (LbpA). The extraction of iron involves conformational changes in Lf and Tf to facilitate iron removal followed by its transport across the outer membrane by a well characterized process for TBDTs. Surprisingly the disruption of the gene encoding another TBDT, CopB, results in a reduction in the ability to grow on human Tf or Lf. The possibility that this could have been due to an artifact of mutant construction that resulted in the inhibition of TonB-mediated process was eliminated by a complete deletion of the CopB gene. A systematic evaluation of the impact on growth under various conditions by deletions of the genes encoding TbpA, LbpA, and CopB as well as mutations of the iron liganding residues and TonB box region of CopB was implemented. The results indicate that although CopB is capable of effectively acquiring iron from the growth medium, it does not directly acquire iron from Tf or Lf. We propose that the indirect effect on iron transport from Tf and Lf by CopB could possibly be explained by the association of TBDTs at gaps in the peptidoglycan layer that may enhance the efficiency of the process. This concept is supported by previous studies demonstrating an indirect effect on growth of Tf and Lf by deletion of the peptidoglycan binding outer membrane lipoprotein RmpM in Neisseria that also reduced the formation of larger complexes of TBDTs.
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Project description:Plutonium is a toxic synthetic element with no natural biological function, but it is strongly retained by humans when ingested. Using small-angle X-ray scattering, receptor binding assays and synchrotron X-ray fluorescence microscopy, we find that rat adrenal gland (PC12) cells can acquire plutonium in vitro through the major iron acquisition pathway--receptor-mediated endocytosis of the iron transport protein serum transferrin; however, only one form of the plutonium-transferrin complex is active. Low-resolution solution models of plutonium-loaded transferrins derived from small-angle scattering show that only transferrin with plutonium bound in the protein's C-terminal lobe (C-lobe) and iron bound in the N-terminal lobe (N-lobe) (Pu(C)Fe(N)Tf) adopts the proper conformation for recognition by the transferrin receptor protein. Although the metal-binding site in each lobe contains the same donors in the same configuration and both lobes are similar, the differences between transferrin's two lobes act to restrict, but not eliminate, cellular Pu uptake.
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