PA_TfR: Protease-associated domain containing proteins like transferrin receptor (TfR). This group contains various PA domain-containing proteins similar to human TfR1 and TfR2. TfR1 and TfR2 are type II membrane proteins, belonging to the peptidase M28 family. TfR1 is homodimeric, widely expressed, and a key player in the uptake of iron-loaded transferrin (Tf) into cells. The TfR1 homodimer binds two molecules of Tf and this complex is internalized. In addition to its role in iron uptake, TfR1 may participate in cell growth and proliferation. TfR2 also binds Tf but with a significantly lower affinity than does TfR1. TfR2 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. HFE and TfR2 interact in cells. By one model for serum iron sensing, at low or basal iron concentrations, HFE and TFR1 form a complex at the plasma membrane; at increased Tf, Tf competes with HFE for binding of TfR1, resulting in HFE disassociating from TfR1 and associating with TfR2 . The TfR1-TfR2 association might initiate a signal cascade leading to the induction of hepcidin (a small peptide hormone that controls systemic iron levels). Human mutations in TfR2 are associated with a form of hemochromatosis (HFE3). The significance of the PA domain to TfRs has not been ascertained. It may be a protein-protein interaction domain. At peptidase active sites, the PA domain may participate in substrate binding and/or promoting conformational changes, which influence the stability and accessibility of the site to substrate.