Department of Biochemistry and Winship Cancer Institute, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
BMC Biochem. 2011 Oct 10;12:54. doi: 10.1186/1471-2091-12-54.
Striatin, a putative protein phosphatase 2A (PP2A) B-type regulatory subunit, is a multi-domain scaffolding protein that has recently been linked to several diseases including cerebral cavernous malformation (CCM), which causes symptoms ranging from headaches to stroke. Striatin association with the PP2A A/C (structural subunit/catalytic subunit) heterodimer alters PP2A substrate specificity, but targets and roles of striatin-associated PP2A are not known. In addition to binding the PP2A A/C heterodimer to form a PP2A holoenzyme, striatin associates with cerebral cavernous malformation 3 (CCM3) protein, the mammalian Mps one binder (MOB) homolog, Mob3/phocein, the mammalian sterile 20-like (Mst) kinases, Mst3, Mst4 and STK25, and several other proteins to form a large signaling complex. Little is known about the molecular architecture of the striatin complex and the regulation of these sterile 20-like kinases.
To help define the molecular organization of striatin complexes and to determine whether Mst3 might be negatively regulated by striatin-associated PP2A, a structure-function analysis of striatin was performed. Two distinct regions of striatin are capable of stably binding directly or indirectly to Mob3--one N-terminal, including the coiled-coil domain, and another more C-terminal, including the WD-repeat domain. In addition, striatin residues 191-344 contain determinants necessary for efficient association of Mst3, Mst4, and CCM3. PP2A associates with the coiled-coil domain of striatin, but unlike Mob3 and Mst3, its binding appears to require striatin oligomerization. Deletion of the caveolin-binding domain on striatin abolishes striatin family oligomerization and PP2A binding. Point mutations in striatin that disrupt PP2A association cause hyperphosphorylation and activation of striatin-associated Mst3.
Striatin orchestrates the regulation of Mst3 by PP2A. It binds Mst3 likely as a dimer with CCM3 via residues lying between striatin's calmodulin-binding and WD-domains and recruits the PP2A A/C heterodimer to its coiled-coil/oligomerization domain. Residues outside the previously reported coiled-coil domain of striatin are necessary for its oligomerization. Striatin-associated PP2A is critical for Mst3 dephosphorylation and inactivation. Upon inhibition of PP2A, Mst3 activation appears to involve autophosphorylation of multiple activation loop phosphorylation sites. Mob3 can associate with striatin sequences C-terminal to the Mst3 binding site but also with sequences proximal to striatin-associated PP2A, consistent with a possible role for Mob 3 in the regulation of Mst3 by PP2A.
Striatin 是一种假定的蛋白磷酸酶 2A（PP2A）B 型调节亚基，是一种具有多个结构域的支架蛋白，最近与包括脑静脉畸形（CCM）在内的几种疾病有关，CCM 可引起从头痛到中风等各种症状。Striatin 与 PP2A A/C（结构亚基/催化亚基）异源二聚体的结合改变了 PP2A 的底物特异性，但 striatin 相关的 PP2A 的靶标和作用尚不清楚。除了与 PP2A A/C 异源二聚体结合形成 PP2A 全酶外，Striatin 还与脑静脉畸形 3（CCM3）蛋白、哺乳动物 Mps one binder（MOB）同源物 Mob3/phocein、哺乳动物 sterile 20-like（Mst）激酶 Mst3、Mst4 和 STK25 以及其他几种蛋白质结合，形成一个大型信号复合物。关于 striatin 复合物的分子结构及其对这些 sterile 20-like 激酶的调控知之甚少。
为了帮助定义 striatin 复合物的分子结构，并确定 Mst3 是否可能受到 striatin 相关的 PP2A 的负调控，对 striatin 进行了结构功能分析。Striatin 有两个不同的区域能够稳定地直接或间接结合 Mob3--一个是 N 端，包括卷曲螺旋结构域，另一个是更 C 端，包括 WD 重复结构域。此外，striatin 残基 191-344 包含与 Mst3、Mst4 和 CCM3 有效结合所必需的决定因素。PP2A 与 striatin 的卷曲螺旋结构域结合，但与 Mob3 和 Mst3 不同，其结合似乎需要 striatin 寡聚化。Striatin 上的 caveolin 结合结构域缺失会破坏 striatin 家族的寡聚化和 PP2A 的结合。破坏 PP2A 结合的 striatin 点突变会导致 striatin 相关 Mst3 的过度磷酸化和激活。
Striatin 调节 Mst3 被 PP2A 调控。它可能作为二聚体与 CCM3 结合，通过位于 striatin 的钙调蛋白结合和 WD 结构域之间的残基，招募 PP2A A/C 异源二聚体到其卷曲螺旋/寡聚化结构域。先前报道的 striatin 卷曲螺旋结构域之外的残基对于其寡聚化是必需的。与 striatin 相关的 PP2A 对 Mst3 的去磷酸化和失活至关重要。在抑制 PP2A 后，Mst3 的激活似乎涉及多个激活环磷酸化位点的自磷酸化。Mob3 可以与 striatin 序列结合，该序列位于与 Mst3 结合位点的 C 端，但也可以与与 striatin 相关的 PP2A 近端的序列结合，这与 Mob3 在 Mst3 被 PP2A 调控中的作用一致。
