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== CapZ (Actin Capping Protein) ==
== Introduction ==


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.  
Capping protein, also known as CapZ in muscle, is expressed in all eukaryotic cells. BLAST analysis shows high sequence conservation across mammals.  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. CapZ is a heterodimer composed of two subunits <scene name='User:Mara_Camelia_Rusu/Workbench/CapZ/Alpha_subunit/1'>α</scene> and <scene name='User:Mara_Camelia_Rusu/Workbench/CapZ/Beta_subunit/1'>β</scene> 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.  
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). <ref>PMID:12660160</ref>
The crystal structure of the sarcomeric form has been resolved to a resolution of 2.1 Å by X-ray crystallography (1IZN).
<Structure load='1IZN' size='350' frame='true' align='right' caption='Crystal structure of chicken CapZ (PDB entry: [http://www.pdb.org/pdb/explore/explore.do?structureId=1izn 1IZN])' scene='Insert optional scene name here' />
<references/>[http://www.pdb.org/pdb/explore/explore.do?structureId=1IZN]
<Structure load='1IZN' size='500' frame='true' align='right' caption='Insert caption here' scene='Insert optional scene name here' />


{{ABSTRACT_PUBMED_12660160}}
== Sequence annotation ==
 
CapZ was shown to be a heterodimer with α and β subunits of 286 and 277 residues, respectively. The aminoacid sequences are available from UniProtKB [http://www.uniprot.org/uniprot/P47755 P47755] for subunit α and [http://www.uniprot.org/uniprot/P47756 P47756] for the β subunit.


== Structure ==
== Structure ==


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 <scene name='User:Mara_Camelia_Rusu/Workbench/CapZ/Central_b_sheet/1'>a central 10-stranded β-sheet</scene>. 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.
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 <ref>PMID:15583864</ref>.
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 <references/>[http://www.ncbi.nlm.nih.gov/pubmed/15583864].


== Function ==
== Function and Interactions==


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>. 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>. 
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>.


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 <references/>[http://www.ncbi.nlm.nih.gov/pmc/articles/PMC152911/]. The absence of capping protein prevented the reconstruction of motility in ''Shigella'' and ''Listeria'', ''in vitro''.
'''Actin binding model'''
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 <references/>[http://ukpmc.ac.uk/abstract/MED/16416311/reload=0;jsessionid=ABE500AEEA23925FCAF83BDC4F984B6A.jvm1].
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 <references/>[http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2366866/?tool=pubmed].
Firstly, CapZ is attracted to the barbed-end of the actin filament through the electrostatic interactions between basic residues on the α subunit and acidic residues on the extreme surface at the barbed-end of the actin filament. Secondly, the β-tentacle finds the hydrophobic binding site on the front surface of actin. The binding of the β -tentacle acts as a lock, and thus reduces the off-rate. 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 <ref>PMID:17110933</ref>.
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 <references/>[http://www.jbc.org/content/286/12/9897.abstract].
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 <references/>[http://www.ncbi.nlm.nih.gov/pubmed/12660160].
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 <references/>[http://www.ncbi.nlm.nih.gov/pubmed/12663865].


== Actin binding model == proposed by Narita ''et al'' <references/>[http://www.ncbi.nlm.nih.gov/pubmed/17110933]
== Pathology ==
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.


The absence of capping protein prevented the reconstruction of motility in Shigella and Listeria, in vitro <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>.


==References==
<references/>


[[Category: actin binding proteins, actin regulation, CapZ, actin capping protein]]
[[Category: actin binding proteins]]
[[Category: actin regulation]]
[[Category: CapZ]]
[[Category: actin capping protein]] [[Category: Z-disk]]
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