Group:MUZIC:CapZ

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Contents

Introduction

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 and 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]

Crystal structure of chicken CapZ (PDB entry: 1IZN)

Drag the structure with the mouse to rotate

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 P47755 for subunit α and P47756 for the β subunit.

Structure

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 . 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 [2].

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 [3]. 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 [4]. 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 [5]. 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 [6],[7]. 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 [8]. 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 [9].

Actin binding model

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 [10].

Pathology

The absence of capping protein prevented the reconstruction of motility in Shigella and Listeria, in vitro [11]. 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 [12].

References

  1. ↑ Yamashita A, Maeda K, Maeda Y. Crystal structure of CapZ: structural basis for actin filament barbed end capping. EMBO J. 2003 Apr 1;22(7):1529-38. PMID:12660160 doi:10.1093/emboj/cdg167
  2. ↑ Au Y. The muscle ultrastructure: a structural perspective of the sarcomere. Cell Mol Life Sci. 2004 Dec;61(24):3016-33. PMID:15583864 doi:10.1007/s00018-004-4282-x
  3. ↑ Au Y. The muscle ultrastructure: a structural perspective of the sarcomere. Cell Mol Life Sci. 2004 Dec;61(24):3016-33. PMID:15583864 doi:10.1007/s00018-004-4282-x
  4. ↑ Yamashita A, Maeda K, Maeda Y. Crystal structure of CapZ: structural basis for actin filament barbed end capping. EMBO J. 2003 Apr 1;22(7):1529-38. PMID:12660160 doi:10.1093/emboj/cdg167
  5. ↑ Frank D, Kuhn C, Katus HA, Frey N. The sarcomeric Z-disc: a nodal point in signalling and disease. J Mol Med. 2006 Jun;84(6):446-68. Epub 2006 Jan 17. PMID:16416311 doi:10.1007/s00109-005-0033-1
  6. ↑ Frank D, Frey N. Cardiac Z-disc signaling network. J Biol Chem. 2011 Mar 25;286(12):9897-904. Epub 2011 Jan 21. PMID:21257757 doi:10.1074/jbc.R110.174268
  7. ↑ Pyle WG, Hart MC, Cooper JA, Sumandea MP, de Tombe PP, Solaro RJ. Actin capping protein: an essential element in protein kinase signaling to the myofilaments. Circ Res. 2002 Jun 28;90(12):1299-306. PMID:12089068
  8. ↑ Yamashita A, Maeda K, Maeda Y. Crystal structure of CapZ: structural basis for actin filament barbed end capping. EMBO J. 2003 Apr 1;22(7):1529-38. PMID:12660160 doi:10.1093/emboj/cdg167
  9. ↑ dos Remedios CG, Chhabra D, Kekic M, Dedova IV, Tsubakihara M, Berry DA, Nosworthy NJ. Actin binding proteins: regulation of cytoskeletal microfilaments. Physiol Rev. 2003 Apr;83(2):433-73. PMID:12663865 doi:10.1152/physrev.00026.2002
  10. ↑ Narita A, Takeda S, Yamashita A, Maeda Y. Structural basis of actin filament capping at the barbed-end: a cryo-electron microscopy study. EMBO J. 2006 Nov 29;25(23):5626-33. Epub 2006 Nov 16. PMID:17110933 doi:10.1038/sj.emboj.7601395
  11. ↑ Frank D, Kuhn C, Katus HA, Frey N. The sarcomeric Z-disc: a nodal point in signalling and disease. J Mol Med. 2006 Jun;84(6):446-68. Epub 2006 Jan 17. PMID:16416311 doi:10.1007/s00109-005-0033-1
  12. ↑ Pappas CT, Bhattacharya N, Cooper JA, Gregorio CC. Nebulin interacts with CapZ and regulates thin filament architecture within the Z-disc. Mol Biol Cell. 2008 May;19(5):1837-47. Epub 2008 Feb 13. PMID:18272787 doi:10.1091/mbc.E07-07-0690

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