Peptidase M28 family; Transferrin Receptor (TfR) and prostate-specific membrane antigen (PSMA, also called glutamate carboxypeptidase or GCP-II) subfamily. TfR and PSMA are homodimeric type II transmembrane proteins containing three distinct domains: protease-like, apical or protease-associated (PA) and helical domains. The protease-like domain is a large extracellular portion (ectodomain). In TfR, it contains a binding site for the transferrin molecule and has 28% identity to membrane glutamate carboxypeptidase II (mGCP-II or PSMA). The PA domain is inserted between the first and second strands of the central beta sheet in the protease-like domain. TfR1 is widely expressed, and is a key player in the uptake of iron-loaded transferrin (Tf) into cells. The TfR1 homodimer binds two molecules of Tf and the complex is then internalized. TfR1 may also participate in cell growth and proliferation. TfR2 binds Tf but with a significantly lower affinity than TfR1. It is expressed chiefly in hepatocytes, hematopoietic cells, and duodenal crypt cells; its expression overlaps with that of hereditary hemochromatosis protein (HFE). TfR2 is involved in iron homeostasis; in humans, mutations in TfR2 are associated with a form of hemochromatosis (HFE3). PSMA is over-expressed predominantly in prostate cancer (PCa) as well as in the neovasculature of most solid tumors, but not in the vasculature of normal tissues. PSMA is considered a biomarker for PCa and possibly for use as an imaging and therapeutic target. The extracellular domain of PSMA possesses two unique enzymatic functions: N-acetylated, alpha-linked acidic dipeptidase (NAALADase) which cleaves terminal glutamate from the neurodipeptide N-acetyl-aspartyl-glutamate (NAAG), and folate hydrolase (FOLH) which cleaves the terminal glutamates from gamma-linked polyglutamates (carboxypeptidase). A mutation in this gene may be associated with impaired intestinal absorption of dietary folates, resulting in low blood folate levels and consequent hyperhomocysteinemia. Expression of this protein in the brain may be involved in a number of pathological conditions associated with glutamate excitotoxicity. This gene likely arose from a duplication event of a nearby chromosomal region. Alternative splicing gives rise to multiple transcript variants. While related in sequence to peptidase M28 GCP-II, TfR lacks the metal ion coordination centers and protease activity.
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