The Par complex polarizes diverse animal cells through the concerted action of multiple regulators. Binding to the multi-PDZ domain containing protein Par-3 couples the complex to cortical flows that construct the Par membrane domain. Once localized properly, the complex is thought to transition from Par-3 to the Rho GTPase Cdc42 to activate the complex. While this transition is a critical step in Par-mediated polarity, little is known about how it occurs. Here, we used a biochemical reconstitution approach with purified, intact Par complex and qualitative binding assays and found that Par-3 and Cdc42 exhibit strong negative cooperativity for the Par complex. The energetic coupling arises from interactions between the second and third PDZ protein interaction domains of Par-3 and the aPKC Kinase-PBM (PDZ binding motif) that mediate the displacement of Cdc42 from the Par complex. Our results indicate that Par-3, Cdc42, Par-6, and aPKC are the minimal components that are sufficient for this transition to occur and that no external factors are required. Our findings provide the mechanistic framework for understanding a critical step in the regulation of Par complex polarization and activity.

Par 复合体通过多个调节因子的协同作用使不同的动物细胞极化。与含有 Par-3 蛋白的多 PDZ 结构域结合，将复合物与构建 Par 膜结构域的皮质流耦合。一旦正确定位，该复合物被认为会从 Par-3 过渡到 Rho GTPase Cdc42 以激活该复合物。虽然这种转变是 Par 介导的极性的关键步骤，但人们对其如何发生知之甚少。在这里，我们使用生化重建方法进行纯化、完整的 Par 复合物和定性结合测定，发现 Par-3 和 Cdc42 对 Par 复合物表现出强烈的负协同作用。这种能量耦合源于 Par-3 的第二个和第三个 PDZ 蛋白相互作用结构域与 aPKC 激酶-PBM（PDZ 结合基序）之间的相互作用，介导 Cdc42 从 Par 复合物中的置换。我们的结果表明 Par-3、Cdc42、Par-6 和 aPKC 是足以发生这种转变的最小组件，并且不需要外部因素。我们的研究结果为理解 Par 复合物极化和活性调节的关键步骤提供了机制框架。