Department of Chemistry and Biochemistry, University of California, Santa Cruz, CA 95064, USA.
Department of Cellular and Molecular Pharmacology and Howard Hughes Medical Institute, University of California, San Francisco, CA 94158, USA.
Science. 2019 Dec 13;366(6471). doi: 10.1126/science.aaw2106.
The p27 protein is a canonical negative regulator of cell proliferation and acts primarily by inhibiting cyclin-dependent kinases (CDKs). Under some circumstances, p27 is associated with active CDK4, but no mechanism for activation has been described. We found that p27, when phosphorylated by tyrosine kinases, allosterically activated CDK4 in complex with cyclin D1 (CDK4-CycD1). Structural and biochemical data revealed that binding of phosphorylated p27 (phosp27) to CDK4 altered the kinase adenosine triphosphate site to promote phosphorylation of the retinoblastoma tumor suppressor protein (Rb) and other substrates. Surprisingly, purified and endogenous phosp27-CDK4-CycD1 complexes were insensitive to the CDK4-targeting drug palbociclib. Palbociclib instead primarily targeted monomeric CDK4 and CDK6 (CDK4/6) in breast tumor cells. Our data characterize phosp27-CDK4-CycD1 as an active Rb kinase that is refractory to clinically relevant CDK4/6 inhibitors.
p27 蛋白是细胞增殖的典型负调控因子，主要通过抑制细胞周期蛋白依赖性激酶（CDK）发挥作用。在某些情况下，p27 与活性 CDK4 相关联，但尚未描述其激活机制。我们发现，p27 被酪氨酸激酶磷酸化后，以变构的方式激活与 cyclin D1 （CDK4-CycD1）结合的 CDK4。结构和生化数据表明，磷酸化 p27（phosp27）与 CDK4 的结合改变了激酶三磷酸腺苷（ATP）结合位点，从而促进视网膜母细胞瘤肿瘤抑制蛋白（Rb）和其他底物的磷酸化。令人惊讶的是，纯化的和内源性的 phosp27-CDK4-CycD1 复合物对 CDK4 靶向药物 palbociclib 不敏感。相反，palbociclib 主要靶向乳腺癌肿瘤细胞中的单体 CDK4 和 CDK6（CDK4/6）。我们的数据将 phosp27-CDK4-CycD1 描绘为一种对临床相关 CDK4/6 抑制剂具有抗性的活性 Rb 激酶。
