Conservation and divergence between cytoplasmic and muscle-specific actin capping proteins: insights from the crystal structure of cytoplasmic Cap32/34 from Dictyostelium discoideum

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？？ 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.Actin is a key component in all eukaryotic cells and plays an essential role in a wide range of cellular processes, such as migration, endocytosis, cytokinesis and generation of contraction [1-4]. Actin monomers (G-actin) are able to polymeriz