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


RESULTS
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).


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 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.
Abteilung NMR basierte Strukturbiologie, Max-Planck-Institut für Biophysikalische Chemie, Am Fassberg 11, D-37077, Göttingen, Germany.
Christian
C
Eckert
Agnieszka
A
Goretzki
Maria
M
Faberova
Martin
M
Kollmar
D051344:Actin Capping Proteins / Q000737:chemistry*; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D051347:CapZ Actin Capping Protein / Q000737:chemistry; D002645:Chickens; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D003593:Cytoplasm / Q000378:metabolism*; D004023:Dictyostelium / Q000737:chemistry*; D008055:Lipids; D008840:Microfilament Proteins / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009132:Muscles / Q000378:metabolism*; D009928:Organ Specificity; D011485:Protein Binding; D017433:Protein Structure, Secondary; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D015800:Protozoan Proteins / Q000737:chemistry*; D016415:Sequence Alignment
D051344:Actin Capping Proteins; D051347:CapZ Actin Capping Protein; D008055:Lipids; D008840:Microfilament Proteins; D021122:Protein Subunits; D015800:Protozoan Proteins; C069272:cap32-34 protein, Dictyostelium