Crystal and Structural Chemistry, Bijvoet Centre for Biomolecular Research, Department of Chemistry, Faculty of Science, Utrecht University, Utrecht, The Netherlands
J Biol Chem. 2020 Jul 10;295(28):9502-9512. doi: 10.1074/jbc.RA120.013690. Epub 2020 May 14.
Six-transmembrane epithelial antigen of the prostate 1 (STEAP1) is an integral membrane protein that is highly up-regulated on the cell surface of several human cancers, making it a promising therapeutic target to manage these diseases. It shares sequence homology with three enzymes (STEAP2-STEAP4) that catalyze the NADPH-dependent reduction of iron(III). However, STEAP1 lacks an intracellular NADPH-binding domain and does not exhibit cellular ferric reductase activity. Thus, both the molecular function of STEAP1 and its role in cancer progression remain elusive. Here, we present a ∼3.0-Å cryo-EM structure of trimeric human STEAP1 bound to three antigen-binding fragments (Fabs) of the clinically used antibody mAb120.545. The structure revealed that STEAP1 adopts a reductase-like conformation and interacts with the Fabs through its extracellular helices. Enzymatic assays in human cells revealed that 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 heterotrimers. Our work provides a foundation for deciphering the molecular mechanisms of STEAP1 and may be useful in the design of new therapeutic strategies to target STEAP1 in cancer.
前列腺六跨膜上皮抗原 1（STEAP1）是一种高度上调于几种人类癌症细胞表面的完整膜蛋白，使其成为管理这些疾病的有前途的治疗靶标。它与三种催化 NADPH 依赖性铁（III）还原的酶（STEAP2-STEAP4）具有序列同源性。然而，STEAP1 缺乏细胞内 NADPH 结合域，并且不表现出细胞内铁还原酶活性。因此，STEAP1 的分子功能及其在癌症进展中的作用仍然难以捉摸。在这里，我们展示了与人 STEAP1 三聚体结合的三种临床使用的抗体 mAb120.545 的抗原结合片段（Fabs）的约 3.0-Å 冷冻电镜结构。该结构表明，STEAP1 采用还原酶样构象，并通过其细胞外螺旋与 Fabs 相互作用。在人细胞中的酶促测定表明，当与家族成员 STEAP4 的细胞内 NADPH 结合域融合时，STEAP1 促进铁（III）还原，表明 STEAP1 在 STEAP 异三聚体中充当铁还原酶。我们的工作为解析 STEAP1 的分子机制提供了基础，并可能有助于设计针对癌症中 STEAP1 的新治疗策略。
