Structural proteomics defines a sequential priming mechanism for the progesterone receptor (XL-MS)

The progesterone receptor (PR) is a steroid-responsive nuclear receptor with two isoforms: PR-A and PR-B. Disruption of PR-A:PR-B signaling is associated with breast cancer through interactions with oncogenic co-regulatory proteins (CoRs). However, molecular details of isoform-specific PR-CoR interactions remain poorly understood. Using structural mass spectrometry, we investigate the sequential binding mechanism of purified full-length PR and intact CoRs, steroid receptor coactivator 3 (SRC3) and p300, as complexes on target DNA. Our findings reveal selective CoR NR-box binding by PR and novel interaction surfaces between PR and CoRs during complex assembly, providing a structural basis for CoR sequential binding on PR. Antagonist-bound PR showed persistent CoR interactions, challenging the classical model of nuclear receptor activation and repression. Collectively, we offer a peptide-level perspective on the organization of the PR transcriptional complex and infer the mechanisms behind the interactions of these proteins, both in active and inactive conformations.

孕酮受体（progesterone receptor, PR）属于类固醇应答型核受体，存在PR-A与PR-B两种亚型。PR-A与PR-B的信号通路失衡可通过与致癌性共调控蛋白（co-regulatory proteins, CoRs）的相互作用，与乳腺癌发生相关联。然而，目前对亚型特异性PR与CoRs相互作用的分子细节仍知之甚少。本研究采用结构质谱技术，针对结合于靶DNA的复合物，探究纯化全长PR与完整CoRs——类固醇受体共激活因子3（steroid receptor coactivator 3, SRC3）及p300的顺序结合机制。研究结果揭示了PR对CoRs的NR框（NR-box）的选择性结合能力，以及复合物组装过程中PR与CoRs之间的新型相互作用界面，为CoRs在PR上的顺序结合机制提供了结构基础。拮抗剂结合型PR仍可与CoRs维持相互作用，这一发现对核受体激活与抑制的经典模型提出了挑战。综上，本研究从肽段层面解析了PR转录复合物的组织形式，并推导了这些蛋白在活性与非活性构象下的相互作用机制。