Eckert Christian, Goretzki Agnieszka, Faberova Maria, Kollmar Martin
Abteilung NMR basierte Strukturbiologie, Max-Planck-Institut für Biophysikalische Chemie, Am Fassberg 11, D-37077, Göttingen, Germany.
BMC Struct Biol. 2012 Jun 1;12:12. doi: 10.1186/1472-6807-12-12.
Capping protein (CP), also known as CapZ in muscle cells and Cap32/34 in Dictyostelium discoideum, plays a major role in regulating actin filament dynamics. CP is a ubiquitously expressed heterodimer comprising an α- and β-subunit. It tightly binds to the fast growing end of actin filaments, thereby functioning as a "cap" by blocking the addition and loss of actin subunits. Vertebrates contain two somatic variants of CP, one being primarily found at the cell periphery of non-muscle tissues while the other is mainly localized at the Z-discs of skeletal muscles.
To elucidate structural and functional differences between cytoplasmic and sarcomercic CP variants, we have solved the atomic structure of Cap32/34 (32=β- and 34=α-subunit) from the cellular slime mold Dictyostelium at 2.2 Å resolution and compared it to that of chicken muscle CapZ. The two homologs display a similar overall arrangement including the attached α-subunit C-terminus (α-tentacle) and the flexible β-tentacle. Nevertheless, the structures exhibit marked differences suggesting considerable structural flexibility within the α-subunit. In the α-subunit we observed a bending motion of the β-sheet region located opposite to the position of the C-terminal β-tentacle towards the antiparallel helices that interconnect the heterodimer. Recently, a two domain twisting attributed mainly to the β-subunit has been reported. At the hinge of these two domains Cap32/34 contains an elongated and highly flexible loop, which has been reported to be important for the interaction of cytoplasmic CP with actin and might contribute to the more dynamic actin-binding of cytoplasmic compared to sarcomeric CP (CapZ).
The structure of Cap32/34 from Dictyostelium discoideum allowed a detailed analysis and comparison between the cytoplasmic and sarcomeric variants of CP. Significant structural flexibility could particularly be found within the α-subunit, a loop region in the β-subunit, and the surface of the α-globule where the amino acid differences between the cytoplasmic and sarcomeric mammalian CP are located. Hence, the crystal structure of Cap32/34 raises the possibility of different binding behaviours of the CP variants toward the barbed end of actin filaments, a feature, which might have arisen from adaptation to different environments.
封端蛋白（CP），在肌肉细胞中也被称为CapZ，在盘基网柄菌中被称为Cap32/34，在调节肌动蛋白丝动力学中起主要作用。CP是一种普遍表达的异二聚体，由α亚基和β亚基组成。它紧密结合在肌动蛋白丝快速生长的末端，通过阻止肌动蛋白亚基的添加和丢失而起到“封端”作用。脊椎动物含有两种CP的体细胞变体，一种主要存在于非肌肉组织的细胞周边，另一种主要定位于骨骼肌的Z盘。
为了阐明细胞质和肌节CP变体之间的结构和功能差异，我们以2.2埃的分辨率解析了细胞黏菌盘基网柄菌的Cap32/34（32 = β亚基，34 = α亚基）的原子结构，并将其与鸡肌肉CapZ的结构进行了比较。这两种同源物显示出相似的总体排列，包括附着的α亚基C末端（α触角）和灵活的β触角。然而，这些结构存在明显差异，表明α亚基内具有相当大的结构灵活性。在α亚基中，我们观察到与C末端β触角相对位置的β折叠区域向连接异二聚体的反平行螺旋弯曲。最近，有人报道了主要归因于β亚基的两结构域扭曲。在这两个结构域的铰链处，Cap32/34包含一个细长且高度灵活的环，据报道该环对于细胞质CP与肌动蛋白的相互作用很重要，并且可能导致细胞质CP与肌节CP（CapZ）相比具有更动态的肌动蛋白结合。
盘基网柄菌Cap32/34的结构使得能够对CP的细胞质和肌节变体进行详细分析和比较。在α亚基、β亚基中的一个环区域以及细胞质和肌节哺乳动物CP之间存在氨基酸差异的α球蛋白表面，尤其可以发现显著的结构灵活性。因此，Cap32/34的晶体结构增加了CP变体对肌动蛋白丝倒刺端具有不同结合行为的可能性，这一特征可能是由于适应不同环境而产生的。
