Group:MUZIC:CapZ
CapZ (Actin Capping Protein, CP)CapZ (Actin Capping Protein, CP)
CapZ is expressed in all eukaryotic cells. It binds to the fast growing barbed ends of actin filaments and blocks G-actin association and disassociation, thus regulating actin filament dynamics. In skeletal muscle it localizes at the Z-disk. Cap Z is a heterodimer composed of two subunits α and β and there are at least two isoforms of each of the subunits. In cardiomyocites the β1 containing isoform localizes to the Z-disk and β2 containing isoform localizes to the cell periphery and intercalated disc. The crystal structure of the sarcomeric form has been resolved to a resolution of 2.1 Å by X-ray crystallography (1IZN). [1]
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Template:ABSTRACT PUBMED 12660160
StructureStructure
Cap Z was shown to be a stable heterodimer with α and β subunits of 286 and 277 residues, respectively. It is a mixed α-helix and β-sheet protein. CapZ has an elongated structure, with overall dimensions of~90 x 50 x55 Å. The capZ dimer has a pseudo two-fold symmetry, with the monomers joining together to form a central 10-stranded antiparallel . This creates an elongated molecule with the N- and C-terminus of each monomer on opposite faces of the central β -sheet. The C-termini of the subdomains are at opposite ends of the elongated molecule. One Cap Z heterodimer appears to be able to bind two actin molecules; this may explain why it is selective for the F-actin barbed end as opposed to monomeric G-actin. Cap Z is thought to bind between actin subdomains 1 and 3 (the barbed-end). Cap Z also binds a spectrin domain of α-actinin and the C-terminus of nebulin [2].
FunctionFunction
Capping protein binds to the barbed end with high affinity (Kd > 1 nM) and the stoichiometry is 1:1 and it prevents the loss and addition of actin monomers. Cap Z is important in the dynamics of actin filaments as it is crucial for rapid filament elongation as a response to signaling. It does so by blocking the barbed ends, thus ensuring a high steady state concentration of G-actin in the cytoplasm [3]. The absence of capping protein prevented the reconstruction of motility in Shigella and Listeria, in vitro. CapZ plays a role in targeting the actin filaments to other structural components. The sarcomeric isoform interacts with α-actinin and anchors the thin filament system to the Z-disk [4]. Small interference RNA (siRNA) studies showed that knockdown of nebulin in chick skeletal myotubes leads to a reduction of assembled CapZ and a loss of the characteristic uniform alignment of the barbed ends of F-actin and this suggests that the interaction of Cap Z and nebulin plays a very important role in Z-disk architecture [5]. Cap Z regulates the activity of cardiac protein kinase C (PKC): down regulation of Cap Z leads to a decrease and alteration of the PKC signaling pathways. Cardiac Cap Z regulates binding of PKC II to the myofilaments with effects on cardiac contractility [6]. Other binding partners of Cap Z include the CARMIL protein which further interacts with Arp complex2/3 and myosin I, both of which are key players in actin based cell motility [7]. In vivo the capping of actin filaments is regulated by second messengers PIP and PIP 2 (Phosphatidylinositol 4,5-bisphosphate), upon signal transduction these molecules promote removal of Cap Z from actin filaments [8].
Actin binding modelActin binding model
Proposed by Narita et al [9]
First, CP is attracted to the barbed-end of the actin filament through the electrostatic interactions between the basic residues, which are mainly but not exclusively on/around the α-tentacle and the acidic residues on the extreme surface at the barbed-end of the actin filament. The electrostatic interactions through the α-tentacle may be the major determining factors of the on-rate of the binding. This is because the deletion of the β-tentacle altered only the off-rate of the binding, without changing the on-rate. In contrast, the deletion of the a-tentacle reduced both the on- and off rates. Second, the β-tentacle finds the hydrophobic binding site on the front surface of actin. The binding of the b-tentacle acts as a lock, and thus reduces the off-rate as suggested previously. This two-step binding mechanism implies that the binding is possible even without the β-tentacle. This is because the first step alone fulfills two requirements for the barbed-end capping: the recognition of the barbed-end and the inhibition of polymerization and depolymerization.
Resolved structuresResolved structures
3LK2 - CapZ + CPI motif of CARMIL 3LK3 - CPI and CSI motifs from CARMIL 3LK4 - capZ + CD2AP