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Free keywords:
Capping protein, Actin-binding, Dictyostelium discoideum, Structural flexibility, Cap32/34, CapZ
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
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.
