5.B.6 The Transmembrane Epithelial Antigen Protein-3  Ferric Reductase (STEAP) Family 

The daily production of 200 billion erythrocytes requires 20 mg of iron, accounting for nearly 80% of the iron demand in humans. Thus, erythroid precursor cells possess an efficient mechanism for iron uptake in which iron loaded transferrin (Tf) binds to the transferrin receptor (TfR) at the cell surface. The Tf:TfR complex then enters the endosome via receptor-mediated endocytosis. Upon endosomal acidification, iron is released from Tf, reduced to Fe2+ by Steap3 (also called TSAP6), and transported across the endosomal membrane by divalent metal iron transporter 1. Steap3, the major ferrireductase in erythrocyte endosomes, is a member of a unique family of reductases. Steap3 is comprised of an N-terminal cytosolic oxidoreductase domain and a C-terminal heme-containing transmembrane domain (YedZ). Cytosolic NADPH and a flavin are predicted cofactors, but the NADPH/flavin binding domain differs from those in other eukaryotic reductases. Instead, Steap3 shows limited homology to FNO, an archaeal oxidoreductase. Sendamarai et al. (2008) have determined the crystal structure of the human Steap3 oxidoreductase domain in the absence and presence of NADPH. The structure reveals an FNO-like domain with an unexpected dimer interface and substrate binding sites that are positioned to direct electron transfer from the cytosol to a heme moiety predicted to be fixed within the transmembrane domain. 

Six-transmembrane epithelial antigen of the prostate 3 (Steap3) is the major ferric reductase in developing erythrocytes. Steap family proteins are defined by a shared transmembrane domain that in Steap3 has been shown to function as a transmembrane electron shuttle, moving cytoplasmic electrons derived from NADPH across the lipid bilayer to the extracellular face where they are used to reduce Fe3+ to Fe2+ and potentially Cu2+ to Cu1+ (Kleven et al. 2015). The cytoplasmic N-terminal oxidoreductase domains of Steap3 and Steap4 are relatively well characterized. High affinity FAD, iron and b-type heme binding sites are in the Steap3 transmembrane domain and Steap3 is functional as a homodimer. It utilizes an intrasubunit electron transfer pathway through the single heme moiety rather than an intersubunit electron pathway through a potential domain-swapped dimer. The sequence motifs in the transmembrane domain that are associated with the FAD and metal binding sites are not only present in Steap2 and Steap4 but also in Steap1 which lacks the N-terminal oxidoreductase domain, suggesting that Steap1 harbors latent oxidoreductase activity (Kleven et al. 2015). STEAP is not present in Dictyostelium discoideum, but three genes encode ferric chelate reductases belonging to the Cytochrome b561 family and containing an N-terminal DOMON domain (DOpamine beta-MONooxygenase N-terminal domain). We have cloned the three genes, naming them fr1A, fr1B and fr1C. fr1A and fr1B are mainly expressed in the vegetative stage while fr1C is highly expressed in the post aggregative stage. All three reductases are localized in the endoplasmic reticulum, but Fr1A is also found in endolysosomal vesicles, in the Golgi and, to a much lower degree, in the plasma membrane. Fr1C is homogeneously distributed in the plasma membrane and in macropinosomal andphagosomal membranes (Peracino et al. 2022).




Arner, P., B.M. Stenson, E. Dungner, E. Näslund, J. Hoffstedt, M. Ryden, and I. Dahlman. (2008). Expression of six transmembrane protein of prostate 2 in human adipose tissue associates with adiposity and insulin resistance. J Clin Endocrinol Metab 93: 2249-2254.

Barroca-Ferreira, J., A.M. Gonçalves, M. Santos, T. Santos-Silva, C.J. Maia, and L.A. Passarinha. (2022). A chromatographic network for the purification of detergent-solubilized six-transmembrane epithelial antigen of the prostate 1 from Komagataella pastoris mini-bioreactor lysates. J Chromatogr A 1685: 463576.

Batista-Silva, J., D. Gomes, J. Barroca-Ferreira, E. Gallardo, &.#.1.9.4.;. Sousa, and L.A. Passarinha. (2023). Specific Six-Transmembrane Epithelial Antigen of the Prostate 1 Capture with Gellan Gum Microspheres: Design, Optimization and Integration. Int J Mol Sci 24:.

Chen, W.J., H.T. Wu, C.L. Li, Y.K. Lin, Z.X. Fang, W.T. Lin, and J. Liu. (2021). Regulatory Roles of Six-Transmembrane Epithelial Antigen of the Prostate Family Members in the Occurrence and Development of Malignant Tumors. Front Cell Dev Biol 9: 752426.

Gauss, G.H., M.D. Kleven, A.K. Sendamarai, M.D. Fleming, and C.M. Lawrence. (2013). The crystal structure of six-transmembrane epithelial antigen of the prostate 4 (Steap4), a ferri/cuprireductase, suggests a novel interdomain flavin-binding site. J. Biol. Chem. 288: 20668-20682.

Hasegawa, H., C. Li, B.M. Alba, D.M. Penny, Z. Xia, M.R. Dayao, P. Li, J. Zhang, J. Zhou, D. Lim, C.M. Murawsky, and A.C. Lim. (2018). Membrane cholesterol modulates STEAP2 conformation during dynamic intracellular trafficking processes leading to broad subcellular distribution. Exp Cell Res 370: 208-226.

Inoue, A., I. Matsumoto, Y. Tanaka, K. Iwanami, A. Kanamori, N. Ochiai, D. Goto, S. Ito, and T. Sumida. (2009). Tumor necrosis factor α-induced adipose-related protein expression in experimental arthritis and in rheumatoid arthritis. Arthritis Res Ther 11: R118.

Kim, H.Y., W.Y. Kwon, J.B. Park, M.H. Lee, Y.J. Oh, S. Suh, Y.H. Baek, J.S. Jeong, and Y.H. Yoo. (2020). Hepatic STAMP2 mediates recombinant FGF21-induced improvement of hepatic iron overload in nonalcoholic fatty liver disease. FASEB J. 34: 12354-12366.

Kleven, M.D., M. Dlakić, and C.M. Lawrence. (2015). Characterization of a Single b-type Heme, FAD, and Metal Binding Sites in the Transmembrane Domain of Six-transmembrane Epithelial Antigen of the Prostate (STEAP) Family Proteins. J. Biol. Chem. 290: 22558-22569.

Oosterheert, W. and P. Gros. (2020). Cryo-EM structure and potential enzymatic function of human six-transmembrane epithelial antigen of the prostate 1. J. Biol. Chem. [Epub: Ahead of Print]

Oosterheert, W., L.S. van Bezouwen, R.N.P. Rodenburg, J. Granneman, F. Förster, A. Mattevi, and P. Gros. (2018). Cryo-EM structures of human STEAP4 reveal mechanism of iron(III) reduction. Nat Commun 9: 4337.

Peracino, B., V. Monica, L. Primo, E. Bracco, and S. Bozzaro. (2022). Iron metabolism in the social amoeba Dictyostelium discoideum: A role for ferric chelate reductases. Eur J. Cell Biol. 101: 151230. [Epub: Ahead of Print]

Rocha, S.M., F.M. Santos, S. Socorro, L.A. Passarinha, and C.J. Maia. (2023). Proteomic analysis of STEAP1 knockdown in human LNCaP prostate cancer cells. Biochim. Biophys. Acta. Mol. Cell Res 1870: 119522. [Epub: Ahead of Print]

Sendamarai, A.K., R.S. Ohgami, M.D. Fleming, and C.M. Lawrence. (2008). Structure of the membrane proximal oxidoreductase domain of human Steap3, the dominant ferrireductase of the erythroid transferrin cycle. Proc. Natl. Acad. Sci. USA 105: 7410-7415.

Tamura, T. and J. Chiba. (2009). STEAP4 regulates focal adhesion kinase activation and CpG motifs within STEAP4 promoter region are frequently methylated in DU145, human androgen-independent prostate cancer cells. Int J Mol Med 24: 599-604.

Yang, D., G.E. Holt, M.P. Velders, E.D. Kwon, and W.M. Kast. (2001). Murine six-transmembrane epithelial antigen of the prostate, prostate stem cell antigen, and prostate-specific membrane antigen: prostate-specific cell-surface antigens highly expressed in prostate cancer of transgenic adenocarcinoma mouse prostate mice. Cancer Res 61: 5857-5860.

Zhang, C.M., X. Chi, B. Wang, M. Zhang, Y.H. Ni, R.H. Chen, X.N. Li, and X.R. Guo. (2008). Downregulation of STEAP4, a highly-expressed TNF-α-inducible gene in adipose tissue, is associated with obesity in humans. Acta Pharmacol Sin 29: 587-592.


TC#NameOrganismal TypeExample

TSAP6 (dudulin 2 isoform B) The dominant ferrireductase (metaloreductase) of the human erythroid transferrin cycle (Sendamarai et al. 2008). Also called six transmembrane epithelial antigen of protein-3 (Steap3) and tumor suppressor-activated pathway protein-6 (TSAP6). Consists primarily of a YedZ domain (9.B.43)


TSAP6 of Homo sapiens (NM_018234)


Metalloreductase STEAP2 or STAMP1 (EC 1.16.1.-) (Prostate cancer-associated protein 1) (Protein up-regulated in metastatic prostate cancer; PUMPCn) Six-transmembrane epithelial antigen of prostate 2) (6TMS protein of prostate 1). Membrane cholesterol modulates the STEAP2 conformation during intracellular trafficking, leading to a broad subcellular distribution (Hasegawa et al. 2018). The molecular mechanism and function of STEAPs in the occurrence and development of different cancers have been reviewed (Chen et al. 2021).


STEAP2 of Homo sapiens


STEAP4 metaloreductase of 459 aas.  It is also called Stamp2, TNFAIP9. It is an integral membrane protein that functions as an NADPH-dependent ferric-chelate reductase, using NADPH from one side of the membrane to reduce a Fe3+ chelate that is bound on the other side of the membrane. STAMP2 mediates sequential transmembrane electron transfer from NADPH to FAD and onto heme, and finally to the Fe3+ chelate (Oosterheert et al. 2018) and can also reduce Cu2+ to Cu1+. It plays a role in systemic metabolic homeostasis, integrating inflammatory and metabolic responses and is associated with obesity and insulin-resistance (Zhang et al. 2008, Arner et al. 2008). It is also involved in inflammatory arthritis, through the regulation of inflammatory cytokines (Inoue et al. 2009). It inhibits anchorage-independent cell proliferation (Tamura and Chiba 2009). The crystal structure is known (Gauss et al. 2013). Hepatic STAMP2 mediates recombinant FGF21-induced improvement of hepatic iron overload in nonalcoholic fatty liver disease (Kim et al. 2020).


STEAP4 of Homo sapiens


The epithelial plasma membrane antigen of the prostate (STEAP; STEAP1; DIS3) (Yang et al. 2001). It is a metalloreductase that can reduce both Fe3+ to Fe2+ and Cu2+ to Cu1+ using NAD+ as the electron acceptor (Oosterheert and Gros 2020). However, STEAP1 (in contrast to STREAP2-4) lacks an intracellular NADPH-binding domain. Oosterheert and Gros 2020 presented a ~3.0 Å cryo-EM structure of trimeric human STEAP1. The structure shows that it adopts a reductase-like conformation and interacts with the Fabs through its extracellular helices. STEAP1 promotes iron(III) reduction when fused to the intracellular NADPH-binding domain of its family member STEAP4, suggesting that STEAP1 functions as a ferric reductase in STEAP hetero-trimers. It has been purified using gellan gum microspheres (Batista-Silva et al. 2023). Proteomic analyses of STEAP1 knockdown mutants in human LNCaP prostate cancer cells have been published (Rocha et al. 2023). Proteins involved in endocytosis, RNA transport and apooptosis were up-regulated (including calhepsin B, intersectin-1 (see TC family 1.F.3) and syntaxin 4 (see TC# 8.A.91.1.12) while HPas, PIK3c2a and DIS3 were down regulated (Rocha et al. 2023). The Six-Transmembrane Epithelial Antigen of the Prostate 1 (STEAP1) is involved in cellular communication, in the stimulation of cell proliferation by increasing Reactive Oxygen Species levels, and in the transmembrane-electron transport and reduction of extracellular metal-ion complexes. STEAP1, which has been purified, is over-expressed in prostate cancer, in contrast with non-tumoral tissues and vital organs, contributing to tumor progression and aggressiveness (Barroca-Ferreira et al. 2022).



STEAP of Homo sapiens (AAH11802)


Uncharacterized protein of 230 aas and 6 TMSs.

UP of Candidatus Wolfebacteria bacterium