Transferrin receptor is an essential protein involved in iron uptake and the regulation of cell growth. TfR knockout embryos die before embryonic day 12 due to defect in erythropoiesis and neurological development. Iron delivery and uptake from Tf into cells occur through the internalization of iron-loaded Tf and are mediated by the TfR.
CHEMICAL STRUCTURE AND IMAGES
When relevant for the function
- Primary structure
- Secondary structure
- Tertiary structure
- Quaternary structure
The primary structure of the human transferrin receptor 1 (TfR1) has been deduced from the nucleotide sequence of its cDNA. TfR1 is a type II transmembrane glycoprotein found primarily as a homodimer of 180 kDa and consisting of identical monomers linked by disulfide bridges at cysteines 89 and 98. Each monomer is characterized by (Figure 1):
- intracellular N-terminal domain of 61 amino acids
- single pass transmembrane domain of 28 amino acids
- large extracellular C-terminal domain of 671 amino acids which is soluble and contains the Tf binding site and a trypsin-sensitive site
The extracellular part, known as ectodomain, is separated from the membrane by a stalk; it contains 3 N-linked glycosylation sites and one O-linked glycosylation site; for an adeguate function of the receptor, glycosylation at these sites is required. Phophorylation on serine 24 in the intracellular domain is also demonstrated but is not necessary for the internalization or recycling of the receptor. The amino acid sequence of the globular ectodomain of TfR1 is 28% identical to that of membrane glutamate carboxypeptidase II, which hydrolyzes the most prevalent mammalian neuropeptide, N-acetyl-α-L-aspartyl-L-glutamate. Therefore, it has been suggested that TfR1 is evolved from a peptidase related to membrane glutamate carboxypeptidase II although it lacks peptidase activity.
Crystallographic studies of the ectodomain of human TfR1 revealed that homodimer is organized as a butterfly-like shape (Figure2). Each monomer consists of three distinct globular domains. Three domains, identified as the protease-like, apical and helical domains, form a lateral cleft which is likely to be in contact with the docked transferrin molecules.
Protein Aminoacids Percentage
The Protein Aminoacids Percentage gives useful information on the local environment and the metabolic status of the cell (starvation, lack of essential AA, hypoxia)
SYNTHESIS AND TURNOVER
TFR1 expression is regulated by iron primarily by a post-transcriptional mechanism. The transcript contains 5 iron-responsive elements (IRE) .
, iron regulatory proteins (IRP) bind to the IREs, placing an inhibition on the instability elements, increasing the half-life of the mRNA and, hence, increasing translation.
, IRPs do not bind IREs, resulting in a decrease of the stability of the transferrin receptor message.
Two isoforms of IRP have been identified: IRP1 and IRP2 .
TFR1 is also regulated by other mechanisms. The gene contains an hypoxia response element in its promoter region which mediates up-regulation of transcription in the presence of hypoxia-inducible factor 1. Transcription is also up-regulated by cytokines, such as interleukin-2 , mitogens and growth factors.
The TfR is ubiquitously expressed at low levels on normal cells and is expressed at greater levels on cells with a high proliferation rate such as cells of the basal epidermis and intestinal epithelium. Activated peripheral blood mononuclear cells express high levels of the TfR. The TfR is also expressed on cells that require large amounts of iron such as placental trophoblasts, the cells responsible for the delivery of iron to the fetus, and maturing erythroid cells that require iron for heme synthesis. However, mature erythroid cells do not express the TfR. TfR expression has also been observed on non-proliferating cells including those of the vascular endothelium of brain capillaries, endocrine pancreas, seminiferous tubules of the testes, cells of the pituitary, luminal membranes of the breast, hepatocytes, Kupffer cells of the liver, and tubules of the kidney. Little or no TfR expression has been detected on pluripotent hematopoietic stem cells (from the mouse, rat, or human), while late cycling murine and human erythroid progenitor cells do express the receptor.
Various studies have also shown elevated levels of expression of the TfR on cancer cells when compared to their normal counterparts. This could be attributed to the increased need for iron as a cofactor of the ribonucleotide reductase enzyme involved in DNA
synthesis of rapidly dividing cells. When benign and malignant breast epithelium were compared in the same section, TfR expression could be up to 4- to 5- fold higher in malignant breast cells when compared to non-neoplastic breast cells. Bladder transitional cell carcinomas, breast cancer, gliomas, lung adenocarcinoma, chronic lymphocytic leukemia and non-Hodgkin’s lymphoma also showed increased TfR expression that correlated with tumor grade and stage or prognosis.
Primary bladder malignancies with high expression of TfR demonstrated a higher rate of recurrence than those with low TfR expression. Increased TfR expression was also detected on peripheral blood mononuclear cells from lymphoma, myeloma, or leukemic tumor patients compared to those taken from normal patients. These data suggest that TfR expression is increased on circulating tumor cells, tumor precursor cells, or cells that have been activated during tumorigenesis.
Regulation of TfR expression
The molecular mechanism underlying the regulation of TfR gene expression by iron has been generally accepted. The regulation is largely post transcriptional and is mediated by specific mRNA protein interactions in the cytoplasm (Figure3). The 3’-untranslated region of receptor mRNA contains a series of five hairpin stem-loop structures required for iron-dependent regulation. The stem-loop structures called iron-responsive elements (IREs) are recognized by trans-acting proteins, known as iron-regulatory proteins (IRPs), that control the rate of mRNA translation or stability.
Two closely related IRP
s (IRP-1 and IRP
-2) have been identified to date. Both display IRE
-binding properties under conditions of iron deprivation. IRP
-1 has been regarded as a bi-functional ‘‘sensor’’ of iron, switching between RNA
binding and enzymatic activities as aconitase depending on cellular iron status. In iron-depleted cells, IRP
-1 inhibits translation of ferritin by binding to IRE
s of ferritin mRNA located in the 5’-untranslated region. Binding of IRP
-1 to IRE
s in the3’-untranslated region of transferrin receptor mRNA stabilized this transcript, thus increase cellular iron uptake and availability. When iron is high, IRP
-1 is enzymatically active and no longer binds well to the IRE
-hairpins, which leads to the degradation of TfR mRNA, thus the inverse effect ensues. IRP
-2 binds specifically to all known mRNA IRE
s with an affinity equally as high as that of IRP
-1. The two proteins are encoded by separate genes. However, IRP
-2 is enzymatically inactive. The IRP
s both respond to iron, but via different pathway. IRP
-1 is post-translationally converted between active and inactive RNA
-binding forms, while IRP
-2 is induced following iron starvation through renewed synthesis of stable IRP
-2 protein and its inactivation by iron reflects degradation of IRP
-2 by a translation-dependent mechanism.
TFR1-mediated uptake of diferric transferrin.
Transferrin binds to TFR1 and is endocytosed. At the pH of the extracellular fluid, diferric transferrin is bound preferentially, the affinity of the receptor being higher for diferric transferrin than for either monoferric- or apo- transferrin. Following formation of the endosome, it is acidified which results in a decrease in the affinity of transferrin for iron and subsequent detachment of the metal. The affinity of the receptor for the (now) apotransferrin is increased by the acidic environment and apotransferrin remains bound to the receptor as the endosome returns to and fuses with the plasma membrane. At the higher extracellular pH, the affinity of the receptor for apotransferrin decreases and apotransferrin is released back into the circulation.
TFR2 - Transferrin Receptor 2 (Transferrin cycle)
A second transferrin-mediated route of iron uptake is probably responsible for the bulk of iron uptake by hepatocytes since, at the concentrations of transferrin present in the plasma, TFR1 would be saturated. The mechanism of uptake is similar to that of the TFR1-mediated pathway. After binding to the low affinity binding site, transferrin is endocytosed and iron is removed following acidification of the vesicle. Iron is sequestered away from the vesicle and apotransferrin is exocytosed.
TFR2 binds diferric transferrin specifically in a pH-dependent manner with an affinity 25-30 times lower than TFR1.
* Cell signaling and Ligand transport
* Structural proteins