The protein phosphatase 2A (PP2A) heterotrimer PP2A-B56α is a human tumour suppressor. However, the molecular mechanisms inhibiting PP2A-B56α in cancer are poorly understood. Here, we report molecular level details and structural mechanisms of PP2A-B56α inhibition by an oncoprotein CIP2A. Upon direct binding to PP2A-B56α trimer, CIP2A displaces the PP2A-A subunit and thereby hijacks both the B56α, and the catalytic PP2Ac subunit to form a CIP2A-B56α-PP2Ac pseudotrimer. Further, CIP2A competes with B56α substrate binding by blocking the LxxIxE-motif substrate binding pocket on B56α. Relevant to oncogenic activity of CIP2A across human cancers, the N-terminal head domain-mediated interaction with B56α stabilizes CIP2A protein. Functionally, CRISPR/Cas9-mediated single amino acid mutagenesis of the head domain blunted MYC expression and MEK phosphorylation, and abrogated triple-negative breast cancer in vivo tumour growth. Collectively, we discover a unique multi-step hijack and mute protein complex regulation mechanism resulting in tumour suppressor PP2A-B56α inhibition. Further, the results unfold a structural determinant for the oncogenic activity of CIP2A, potentially facilitating therapeutic modulation of CIP2A in cancer and other diseases.

蛋白磷酸酶 2A (PP2A) 异源三聚体 PP2A-B56α 是一种人类肿瘤抑制剂。然而，人们对癌症中抑制 PP2A-B56α 的分子机制知之甚少。在这里，我们报告了癌蛋白 CIP2A 抑制 PP2A-B56α 的分子水平细节和结构机制。直接与 PP2A-B56α 三聚体结合后，CIP2A 取代 PP2A-A 亚基，从而劫持 B56α 和催化 PP2Ac 亚基，形成 CIP2A-B56α-PP2Ac 假三聚体。此外，CIP2A 通过阻断 B56α 上的 LxxIxE 基序底物结合口袋来与 B56α 底物结合竞争。与 CIP2A 在人类癌症中的致癌活性相关，N 端头结构域介导的与 B56α 的相互作用可稳定 CIP2A 蛋白。从功能上讲，CRISPR/Cas9介导的头域单氨基酸突变削弱了MYC表达和MEK磷酸化，并消除了三阴性乳腺癌体内肿瘤生长。总的来说，我们发现了一种独特的多步劫持和静音蛋白复合物调节机制，导致肿瘤抑制因子 PP2A-B56α 受到抑制。此外，这些结果揭示了 CIP2A 致癌活性的结构决定因素，可能促进 CIP2A 在癌症和其他疾病中的治疗调节。