Group:MUZIC:CapZ: Difference between revisions

CapZ 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 <scene name='User:Mara_Camelia_Rusu/Workbench/CapZ/Central_b_sheet/1'>a central 10-stranded β-sheet</scene>. The N- and C-terminus of each monomer are on opposite faces of the central β -sheet. The C-termini of the subdomains are at opposite ends of the elongated molecule.

One CapZ heterodimer binds two actin molecules; this may explain why it is selective for the F-actin barbed end as opposed to monomeric G-actin. CapZ is thought to bind between actin subdomains 1 and 3 (the barbed-end). CapZ also binds a spectrin domain of α-actinin and  the C-terminus of nebulin <ref>PMID:15583864</ref>.

Capping protein binds to the barbed end with high affinity (Kd > 1 nM) with 1:1 stoichiometry and prevents the loss and addition of actin monomers. CapZ is important in the dynamics of actin filaments and 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 <ref>PMID:12660160</ref>. 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 <ref>PMID:16416311</ref>. 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 CapZ and nebulin plays an important role in Z-disk architecture <ref>PMID:18272787</ref>. CapZ regulates the activity of cardiac protein kinase C (PKC): down regulation of CapZ leads to a decrease and alteration of the PKC signaling pathways. Cardiac CapZ regulates binding of PKC II to the myofilaments with effects on cardiac contractility <ref>PMID:21257757</ref>,<ref>PMID:12089068</ref>. Other binding partners of CapZ 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 <ref>PMID:12660160</ref>. 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 CapZ from actin filaments <ref>PMID:12663865</ref>.  

== Actin binding model ==

Proposed by Narita ''et al'' <ref>PMID:17110933</ref>

First, CapZ 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.