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CapZ (capping protein, CP)

Page updated 15/5/03

An actin filament barbed end capping protein (CP) from the Z-disc of muscle (thus the name CapZ).  Members of this group have been isolated from a variety of phyla, and those that have been sequenced demonstrate extensive homology in both subunits.  The protein is composed of two subunits alpha and beta. Similar proteins have been isolated from species across several phyla  including Drosophila, Saccharomyces, Dictyostelium (Schleicher et al, 1984), Acanthamoeba (Isenberg et al, 1980), Caenorhabditis (Waddle et al, 1993) as well as vertebrates (Kilimann & Isenberg, 1982; Casella et al, 1986).  The a subunits range between 32,000 and 36,000 while the b subunits are generally smaller ranging between 28,000 and 32,00. Although both subunits are required for capping activity, thus far actin binding has only been ascribed to the b-subunit (Hug et al, 1992). Puzzlingly, although the CP demonstrate extensive homology in both subunits, it is the very end domain which seems to possess actin binding properties (Hug et al, 1992) despite the fact that at this particular part there is no detectable sequence homology between this and other species! (Hartmann et al, 1989). 

Structure
Although there is no obvious homology between the
a and b subunits, recent crystallographic solution (Yamashita et al, 2003) of CP from chicken muscle reveals that there is a striking resemblance of the fold of the two subunits (Figure 1). 

 

Regulation
CPs are calcium insensitive in their binding to actin (
Isenberg et al, 1980; Kilimann & Isenberg, 1982; Hartmann et al, 1989; Nachmias et al, 1996), nor or they regulated by Ca2+/calmodulin (Kilimann & Isenberg, 1982).  Like many other actin-binding proteins, CP is regulated by phosphatidylinositol 4,5 bisphosphate (PIP2) (Heiss & Cooper, 1991; Schafer et al, 1996; DiNubile & Huang 1997). The activity of CP are also modulates by interaction with a number of CP binding proteins (Table 1).

Protein Function Regulation  Reference
Actin nucleating PIP2 Heiss & Cooper, 1991; Schafer et al, 1996; DiNubile & Huang 1997
Carmil A scaffold protein   Jung et al, 2001
Hsc70 An agonist activating capping activity in Dictyostelium and Plasmodium.   Eddy et al, 1993; Tardieux et al,1998
S-100b Localises capZ to vesicles ?   Ivanenkov et al, 1993
Twinfilin Does not interfere with actin-binding but may localise it   Palmgren et al, 2001
V-1 (myotrophin) Inhibits actin nucleation

?

Taoka et al, 2003
Table 1.  CP  binding proteins

 

Localization
Localization studies in non-muscle cells have not always produced a consistent picture; one member displays a nuclear distribution, while chicken CP is concentrated in cell-cell junction complexes of epithelial cells. Yeast capping proteins are found at the membrane in regions generally rich in actin (
Amatruda & Cooper, 1992).  In muscle cells CP is present at the Z-line independently of actin so may bind another protein partner at the Z-line (Schafer et al, 1994).

References:-

Amatruda, J. F., J. F. Cannon, et al. (1990). "Disruption of the Actin Cytoskeleton in Yeast Capping Protein Mutants." Nature 344: 352--354.

Amatruda, J. F. and J. A. Cooper (1992). "Purification, Characterization and Immunofluorescence Localization of Saccharomyces cerevisiae Capping Protein." J Cell Biol 117: 1067-1076.

Amatruda, J. F., D. J. Gattermeir, et al. (1992). "Effects of Null Mutations and Overexpression of Capping Protein on Morphogenesis, Actin Distribution and Polarized Secretion in Yeast." The Journal of Cell Biology 119(5), 1151-1162.

Bear, J. E., Svitkina, T. M., Krause, M., Schafer, D. A., Loureiro, J. J., Strasser, G. A., Maly, I. V., Chaga, O. Y., Cooper, J. A., Borisy, G. G. & Gertler, F. B. (2002) Antagonism between Ena/VASP proteins and actin filament capping regulates fibroblast motility. Cell. 109, 509-521.

Caldwell, J. E., S. G. Heiss, et al. (1989). "Effects of CapZ, an Actin Capping Protein of Muscle, on the Polymerization of Actin." Biochemistry 28, 8506--8514.

Casella, J. F., Maack, D. J. & Lin, S. (1986) Purification and initial characterization of a protein from skeletal muscle that caps the barbed ends of actin filaments., J.Biol.Chem. 261, 10915-10921.

Casella, J. F., E. A. Barron-Casella, et al. (1995). "Quantitation of cap Z in conventional actin preparations and methods for further purification of actin." Cell Mot. and Cytoskeleton 30, 164-170.

Dabiri, G. A., C. L. Youngs, et al. (1992). "Molecular cloning of human macrophage capping protein cDNA." J. Biol. Chem. 267(23), 16545-16552.

David, V., E. Gouin, et al. (1998). "Identification of cofilin, coronin, Rac and capZ in actin tails using a Listeria affinity approach." J.Cell Sci. 111, 2877-2884.

DiNubile, M. J. (1999). "Erythrocyte membrane fractions contain free barbed filament ends despite sufficient concentrations of retained capper(s) to prevent barbed end growth." Cell Mot. Cytoskeleton 43, 10-22.

DiNubile, M. J. and S. Huang (1997). "High concentrations of phosphatidylinositol-4,5-bisphosphate may promote actin filament growth by three potetial mechanisms: Inhibiting capping by neutrophil lysates, severing actin filaments and removing capping protein-b2 from barbed ends." Biochim.Biophys.Acta. 1358, 261-278.

Eddy, R. J., J. Han, et al. (1997). "Capping protein terminates but does not initiate chemoattractant-induced actin assembly in Dictyostelium." J.Cell Biol. 130, 1243-1253.

Eddy, R. J., R. A. Sauterer, et al. (1993). "Aginactin, an agonist-regulated F-actin capping activity is associated with an Hsc70 in Dictyostelium." J. Biol. Chem. 268(31), 23267-23274.

Hart, M. C., Y. O. Korshunova, et al. (1997). "Vertebrates have conserved capping protein a isoforms with specific expression patterns." Cell Mot.Cytoskel. 38, 120-132.

Hartmann, H., A. A. Noegel, et al. (1989). "Calcium-dependent F-actin capping proteins. Cap32/334, a Capping Protein from Dictyostelium discoideum, does not share sequence homologies with known Actin-Binding Proteins." J Biol Chem 264, 12639-12647.

Haung, M., C. Yang, et al. (1999). "Cdc42-induced actin filaments are protected from capping protein." Curr.Biol. 9, 979-082.

Heiss, S. G. and J. A. Cooper (1991). "Regulation of CapZ, an Actin Capping Protein of chicken Muscle, by Anionic Phospholipids." Biochemistry 30, 8753--8758.

Hopmann, R., J. A. Cooper, et al. (1995). "Capping protein mutations are lethal in Drosophila and affect multiple actin-dependent processes." Molecular Biology Of The Cell 6, 794-794.

Hug, C., P. Y. Jay, et al. (1995). "Capping protein levels influence actin assembly and cell motility in Dictyostelium." Cell 81, 591-600.

Hug, C., T. Miller, et al. (1992). "Identification and Characterization of an Actin-Binding Site of CapZ." J Cell Biol 116, 923-931.

Isenberg, G., Aebi, U. & Pollard, T. D. (1980) An actin-binding protein from Acanthamoeba regulates actin filament polymerization and interactions. Nature. 288, 455-459.

Ivanenkov, V. V., G. A. Jamieson, et al. (1995). "Characterization of S-100b binding epitopes. Identification of a novel target, the actin capping protein, CapZ." J.Biol.Chem. 270(24), 14651-14658.

Jung, G., Remmert, K., Wu, X., Volosky, J. M. & Hammer III, J. A. (2001) The Dictyostelium CARMIL protein links capping protein and the Arp2/3 complex to type I myosins through their SH3 domains. J. Cell Biol. 153, 1475-1497.

Karpova, T. S. and J. A. Cooper (1992). "Yeast mutations that are lethal in combination with null mutations of capping protein." Molecular Biology Of The Cell 3, A 153-A 153.

Karpova, T. S., M. M. Lepetit, et al. (1993). "Mutations that enhance the cap2 null mutant phenotype in Saccharomyces cerevisiae affect the actin cytoskeleton, morphogenesis and pattern of growth." Genetics 135, 693-709.

Karpova, T. S., K. Tatchell, et al. (1995). "Actin filaments in yeast are unstable in the absence of capping protein or fimbrin." J. Cell Biol. 131(6 part 1), 1483-1493.

Korshunova, Y. O., M. C. Hart, et al. (1995). "Mouse capping protein - cDNA sequence, tissue expression, gene- mapping and disruption." Molecular Biology Of The Cell 6, 795-795.

Kuhlman, P. A. and V. M. Fowler (1997). "Purification and characterization of an a1b2 isoform of CapZ from human erythrocytes: Cytosolic location and inability to bind to Mg2+ ghosts suggest that erythrocytes actin filaments are capped by adducin." Biochemistry 36, 13461-13472.

Maruyama, K., H. Kurakawa, et al. (1990). "b-Actinin Is Equivalent to Cap Z Protein." J. Biol. Chem. 265(15), 8712-8715.

Maun, N. A., D. W. Speicher, et al. (1996). "Purification and properties of a Ca2+-independent barbed-end actin filament capping protein, capz, from human polymorphonuclear leukocytes." Biochemistry 35, 3518-3524.

Nachmias, V. T., Golla, R., Casella, J. F. & Barron-Casella, E. A. (1996) Cap Z, a calcium insensitive capping protein in resting and activated platelets. FEBS Lett. 378, 258-262.

Palmgren, S., Ojala, P. J., Wear, M. A., Cooper, J. A. & Lappalainen, P. (2001) Interactions with PIP2, ADP-actin monomers, and capping protein regulate the activity and localization of yeast twinfilin. J. Cell Biol. 155, 251-260.

Pantaloni, D., R. Boujemaa, et al. (2000). "The Arp2/3 complex branches filament barbed ends: functional antagonism with capping proteins." Nature Cell Biol. 2, 385-391.

Sauterer, R. A., R. J. Eddy, et al. (1991). "Purification and Characterization of Aginactin, a Newly Identified Agonist-regulated Actin-capping Protein from Dictyostelium Amoebae." J Biol Chem 266, 24533-24539.

Schafer, D. A., C. hug, et al. (1995). "Inhibition of CapZ during myofibrillogenesis alters assembly of actin filaments." J. Cell Biol. 128, 61-70.

Schafer, D. A., P. B. Jennings, et al. (1995). "Dynamics of capping protein-interaction with actin-filaments." Molecular Biology Of The Cell 6, 820-820.

Schafer, D. A., P. B. Jennings, et al. (1996). "Dynamics of capping protein and actin assembly in vitro: Uncapping barbed ends by polyphosphoinositides." J.Cell Biol. 135, 169-179.

Schafer, D. A., Y. O. Korshunova, et al. (1994). "Differential localization and sequence analysis of capping protein a-subunit isoforms of vertebrates." J. Cell Biol. 127(2), 453-465.

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Taoka, M., Ichimura, T., Wakamiya-Tsuruta, A., Kubota, Y., Araki, T., Obinata, T. & Isobe, T. (2003) V-1, a Protein Expressed Transiently during Murine Cerebellar Development, Regulates Actin Polymerization via Interaction with Capping Protein. J. Biol. Chem. 278, 5864-5870.

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Waddle, J. A., J. A. Cooper, et al. (1993). "The a-subunit and b-subunit of nematode actin capping protein function in yeast." Molecular Biology Of The Cell 4, 907-917.

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Xu, J., J. F. Cassella, et al. (1999). "Effect of capping protein, CapZ, on the length of actin filaments and mechanical properties of actin filament networks." Cell Mot.Cytoskel. 42, 73-81.

Yamashita, A., Maeda, K. & Maeda, Y. (2003) Crystal structure of CapZ: structural basis for actin filament barbed end capping. EMBO J. 22, 1529-1538.

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