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

ABSTRACT: BACKGROUND: 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 ?- and ?-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. RESULTS: To elucidate structural and functional differences between cytoplasmic and sarcomercic CP variants, we have solved the atomic structure of Cap32/34 (32=?- and 34=?-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 ?-subunit C-terminus (?-tentacle) and the flexible ?-tentacle. Nevertheless, the structures exhibit marked differences suggesting considerable structural flexibility within the ?-subunit. In the ?-subunit we observed a bending motion of the ?-sheet region located opposite to the position of the C-terminal ?-tentacle towards the antiparallel helices that interconnect the heterodimer. Recently, a two domain twisting attributed mainly to the ?-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). CONCLUSIONS: 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 ?-subunit, a loop region in the ?-subunit, and the surface of the ?-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.

SUBMITTER: Eckert C 

PROVIDER: S-EPMC3472329 | biostudies-literature | 2012

REPOSITORIES: biostudies-literature