Mechanisms of the Scaffold Subunit in Facilitating Protein Phosphatase 2A Methylation

The function of the biologically essential protein phosphatase 2A (PP2A) relies on formation of diverse heterotrimeric holoenzymes, which involves stable association between PP2A scaffold (A) and catalytic (C or PP2Ac) subunits and binding of variable regulatory subunits. Holoenzyme assembly is highly regulated by carboxyl methylation of PP2Ac-tail; methylation of PP2Ac and association of the A and C subunits are coupled to activation of PP2Ac. Here we showed that PP2A-specific methyltransferase, LCMT-1, exhibits a higher activity toward the core enzyme (A–C heterodimer) than free PP2Ac, and the A-subunit facilitates PP2A methylation via three distinct mechanisms: 1) stabilization of a proper protein fold and an active conformation of PP2Ac; 2) limiting the space of PP2Ac-tail movement for enhanced entry into the LCMT-1 active site; and 3) weak electrostatic interactions between LCMT-1 and the N-terminal HEAT repeats of the A-subunit. Our results revealed a new function and novel mechanisms of the A-subunit in PP2A methylation, and coherent control of PP2A activity, methylation, and holoenzyme assembly.生物学必需的蛋白磷酸酶2A（PP2A）的功能依赖于多样化异源三聚体全酶的形成，该过程涉及PP2A支架（A）亚基与催化（C或PP2Ac，protein phosphatase 2A catalytic subunit）亚基的稳定结合，以及可变调控亚基的结合。全酶的组装受到PP2Ac尾区的羧基甲基化的高度调控；PP2Ac的甲基化以及A、C亚基的结合，均与PP2Ac的激活相互偶联。本研究发现，PP2A特异性甲基转移酶LCMT-1对核心酶（A–C异二聚体）的活性高于游离状态的PP2Ac；A亚基通过三种不同机制促进PP2A的甲基化：1）稳定PP2Ac的正确蛋白质折叠与活性构象；2）限制PP2Ac尾区的运动空间，以增强其进入LCMT-1活性位点的效率；3）介导LCMT-1与A亚基N端HEAT重复序列（HEAT repeats）之间的弱静电相互作用。本研究结果揭示了A亚基在PP2A甲基化中的全新功能与调控机制，以及对PP2A活性、甲基化与全酶组装的协同调控。