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CAP (Srv2p)

Updated 27/6/03

Originally described as a cyclase associated protein (hence CAP) from Saccharomyces cerevisiae (Field et al, 1988; Field et al, 1990; Fedor-Chaiken et al, 1990; see Hubberstey & Mottillo, 2002 for a recent review). The actin binding region  (Freeman et al, 1995)  seems to function purely as a G-actin monomer sequestering protein as is inhibits polymerization of actin from both  barbed and pointed ends.  The stoichiometry of binding appears to be 1:1.  The actin monomer sequestering activity may be important in vivo as profilin can suppress the CAP-null phenotype in yeast (Gerst et al, 1991; Vojtek et al, 1991).  Like many actin-binding proteins (see PIs and ABPs), the interaction of CAP with actin is inhibited by phosphatidylinositol 4,5 bisphosphate (Gottwald et al, 1996).  Although many have found that the actin binding site resides in the N-terminal region, after the proline rich region some disagreement exist as to where this site is.  Is has been suggested that the actin binding region is in the last 27 amino-acids of yeast CAP (Zelicof et al, 1996), but others (Baum & Perrimon, 2000) have pointed out that a verprolin homology (VH) domain exist next to the proline rich region.  As VH domains are known to bind G-actin in other proteins (WASP, Verprolin), it is considered by these workers that this is likely to be the actin binding site. CAP has been crystallized (Hofmann et al, 2002) to 2.2 angstroms so we can expect a structure any time now!

Figure 1. CAP structure. N terminal half binds adenylyl cyclase, the proline rich region binds Abp1p and Abl tyrosine kinase.  VP is homologous to verprolin and WASP. The molecule dimerises through the C-terminal region (Dimer).
The Proline rich region binds the SH3 containing protein Abp1p another yeast actin-binding protein (Freeman et al, 1996). It is through this region that CAP is localized to the actin-rich regions in the yeast (Yu et al, 1999), as Abp1p null cells have reduced concentration of CAP in cortical actin patches (Lila & Drubin, 1997). CAP homologs have been described from Dictyostelium (Gottwald et al, 1996), pig (Gieselman & Mann, 1992), Drosophila (Baum et al, 2000) plants (), and two related genes in humans (Matviw et al, 1992; Yu et al, 1994).
Adenylate Cyclase activation by CAP (In Yeast)
In the budding yeast Saccharomyces cerevisiae, it is known that adenylate cyclase (CYR1) is the major down stream activator of the RAS G-proteins RAS1 and RAS2 and that CAP is essential for CYR1 activation (
Feder-Chaiken et al, 1990). There appears to be two RAS-binding sites in the CYR1/CAP complex (Shima et al, 2000).  This activation only requires the N-terminal half of CAP and not the actin-binding C-terminal half (Gerst et al, 1991), but it is possible the actin binding site and the Abp1p binding concentrate the activated complex at the bud site so locally increasing cAMP levels?  The N-terminal of CAP bind to CYR1 via a coiled-coil interaction (Nishida et al, 1998).  CAP is reported to regulate the bud to hypha in Candida albicans. In conditions that normally lead to hypha formation, the CAP-null yeast was found to bud (Bahn & Sundstrom, 2001).  The addition of dibutryl-cAMP (cell permeant) reversed this effect showing that it was likely that the adenylyl cyclase function that was missed. So far only the yeast CAP proteins have been shown to bind adenylate cyclase.
Figure 2.  The active Adenylate Cyclase/CAP Yeast complex.  Adenylate cyclase is in cyan (marked AC), it is shown bound to the CAP via the N-terminal cyclase-binding region.  Two active RAS molecules bind the complex.  The proline rich region is shown complexed to Abp1p and two CAP molecules are dimerised via the C-terminal domain (blue). The PIP2 binding site on the CAP may  localise complex at the membrane in addition to the RAS protein's farnesylation. The details of the actin-binding side are not shown as its not really clear at this stage (to me at least!).  Presumably, actin binding localises the complex to regions that require a localized burst of cAMP.
Control of Actin dynamics by CAP in Drosophila
Work in the Drosophila system has begun to tease out the details of how CAP functions to control actin polymerization (
Baum & Perrimon 2000 & 2001; Stevenson & Theurkauf, 2000).  How applicable this is to yeast or any other organisms is probably similar but this is far from certain.
A role for CAP in Cytokinesis and Endocytosis?
Experiments with Dictyostelium CAP-null cells revealed a requirement for the protein in endocytosis and cytokinesis as these cells were found to be deficient in both (
Noegel et al, 1999).  CAP-null Dictyostelium cells grew more slowly and were half as fast at taking up FITC dextran indicating inefficient endocytosis, in agreement with earlier yeast studies (Wesp et al, 1997).  Dictyostelium, CAP-null cells were almost twice the diameter of normal cells (Noegel et al, 1999), and overexpression of CAP in Arabidopsis (a plant) caused a decrease in cell expansion (Barrero et al, 2003), and reduced organ size (such as leaves).
Cooperation of CAP1 and cofilin
Cofilin is a member of the ADF/cofilin family of actin-binding proteins that are pivotally involved in the accelerated turn over actin in cells.   It has been reported (
Moriyama & Yahara, 2002) that CAP1 interacts with cofilin/actin through the N-terminus and that the monomer binding domain in the C-terminus increases the exchange of nucleotide in actin.  This later property may be pivotal in the action of CAP in cells as it may function to strip cofilin from actin thus permitting cofilin to sever once more.



Amberg, D. C., Basart, E.,  & Botstein D. (1995). Defining protein interactions with yeast actin in vivo. Nature Struct. Biol. 2(1): 28-35.

Bahn, Y.-S. & Sundstrom, P. (2001) CAP1, an adenylate cyclase-associated protein gene, regulates bud-hypha transitions, filamentous growth, and cyclic AMP levels and is required for virulence of Candida albicans.  J. Bacteriology. 183, 3211-3223.

Barrero, R. A., Umeda, M., Yamamura, S. & Uchimiya, H. (2002) Arabidopsis CAP regulates the actin cytoskeleton necessary for plant cell elongation and division. The Plant Cell. 14, 149-163.

Baum, B., Li, W. & Perrimon, N. (2000) A cyclase-associated protein regulates actin and cell polarity during Drosophila oogenesis and in yeast., Current Biology. 10, 964-973.

Baum, B. & Perrimon, N. (2001) Spatial control of the actin cytoskeleton in Drosophila epithelial cells., Nature Cell Biol. 3, 883-890.

Feder-Chaiken, M., Deschenes, R.J. & Broach, J.R. (1990). SRV2, a gene required for RAS activation of adenylate cyclase in yeast. Cell 61, 329-340.

Field, J., Nikawa, J.-I., Broek, D., Macdonald, B., Rodgers, L., Wilson, I. A., Lerner, R. A. & Wigler, M. (1988) Purification of a RAS-responsive adenyl cyclase complex from Saccharomyces cerevisiae by the use of an epitope addition method.  Mol.Biol. Cell. 8, 2159-2165.

Field, J. et al (1990). Cloning and characterization of CAP, the S. cerevisiae gene encoding the 70kd adenyl cyclase-associated protein. Cell 61, 319-327.

Freeman, N. L., Chen, Z., Horenstein, J., Weber, A. & Field, J. (1995) An actin monomer binding activity localizes to the carboxyl-terminal half of the Saccharomyces cerevisiae cyclase-associated protein. J. Biol. Chem. 270, 5680-5685.

Freeman, N. L., Lila, T., Mintzer, K. A., Chen, Z., Pahk, A. J., Ren, R., Drubin, D. G. & Field, J. (1996) A conserved proline-rich region of the Saccharomyces cerevisiae cyclase-associated protien binds SH3 domains and modulates cytoskeletal localization.  Mol.Biol. Cell. 16, 548-556.

Freeman, N. L. & Field, J. (2000) Mammalian homolog of the yeast cyclase associated protein, CAP/Srv2p, regulates actin filament assembly. Cell Motilty and Cytoskeleton. 45, 106-120.

Gerst, J., Ferguson, K., Vojtek, A., Wigler, M. & Field, C. M. (1991) CAP: a bifunctional component of the Saccharomyces cerevisiae adenylyl cyclase complex.  Mol.Cell Biol. 11, 1248-1257.

Gieselmann, R. and K. Mann (1992). ASP-56, a new actin sequestering protein from pig platelets with homology to CAP, an adenylate cyclase-associated protein from yeastFEBS. Lett. 298(2,3), 149-153.

Goldschmidt-Clermont, P. J. and P. A. Janmey (1991). Profilin, a weak Cap for actin and Ras.  Cell 66, 419-421.

Gottwald, U., Brokamp, R., Karakesisoglou, I., Schleicher, M. & Noegel, A. A. (1996) Identification of a cylcase-associated protein (CAP) homologue in Dictyostelium discoideum and characterization of its interaction with actin.  Mol.Biol. Cell. 7, 261-272.

Hofmann, A., Hess, S., Noegel, A. A., Schleicher, M. & Wlodawer, A. (2002) Crystallization of cyclase-associated protein from Dictyostelium discoideum. Acta Crystallographica Sect.D. 58, 1858-1861.

Ishimaru, Y., Yasuoka, A., Asano-Miyoshi1, M., Abe1, K. & Emori, Y. (2001) An actin-binding protein, CAP, is expressed in a subset of rat taste bud cells, Neuroreport. 12, 233-235.

Kawamukai, M., Gerst, J., Field, J., Riggs, M., Rodgers, L., Wigler, M. & Young, D. (1992) Generic and biochemical analysis of the adenylyl cyclase-associated protein, CAP, in Schizosaccharomyces pombe. Mol.Biol.Cell. 3, 167-180.

Lila, T. & Drubin, D. (1997). Mol.Biol.Cell 8, 367-385.

Matviw, H., Yu, G. & Young, D. (1992) Identification of a human cDNA encoding a protein that is structurally and functionally related to the yeast adenyl cyclase-associated CAP proteins. Mol.Cell Biol. 12, 5033-5040.

Moriyama, K. & Yahara, I. (2002) Human CAP1 is a key factor in the recycling of cofilin and actin for rapid actin turnover. J.Cell Sci. 115, 1591-1601.

Nishida, Y., Shima, F., Sen, H., Tanaka, Y., Yanagihara, C., Yamawaki-Kataoka, Y., Kariya, K.-I. & Kataoka, T. (1998) Coiled-coil interaction of N-terminal 36 residues of cyclase-associated protein with adenylyl cyclase is sufficient for its function in Saccharomyces cerevisiae Ras pathway. J.Biol.Chem. 273, 28019-28024.

Noegel, A. A., Rivero, F., Albrecht, R., Janssen, K.-P., Kohler, J., Parent, C. A. & Schleicher, M. (1999) Assessing the role of the ASP56/CAP homologue of Dictyostelium discoideum and the requirements for subcellular localization.  J.Cell Sci. 112, 3195-3203.

Shima, F., Okado, T., Kido, M., Sen, H., Tanaka, Y., Tamada, M., Hu, C.-D., Yamawaki-Kataoka, Y., Kariya, K.-I. & Kataoka, T. (2000) Association of yeast adenylyl cyclase with cyclase-associated protein CAP forms a second Ras-binding site which mediates its Ras-dependent activation. Mol.Cell Biol. 20, 26-33.

Stevenson, V. A. & Theurkauf, W. E. (2000) Actin cytoskeleton: Putting a CAP on actin polymerization. Curr. Biol. 10, R695-R697.

Swiston, J., Hubberstey, A. V., Yu, G. & Young, D. (1995) Differential expression of CAP and CAP2 in adult rat tissues. Gene. 165, 273-277.

Vojtek, A., Haarar, B., Field, J., Gerst, J., Pollard, T. D., Brown, S. & Wigler, M. (1991) Evidence for a functional link between profilin and CAP in the yeast S. cerevisiae. Cell. 66, 497-505.

Wesp, A., Hicke, L., Palecek, J., Lombardi, R., Aust, T., Munn, A. L. & Riezman, H. (1997) End4p/Sla2p interacts with actin-associated proteins for endocytosis in Saccharomyces cerevisiae. Mol. Biol. Cell. 8, 2291-2306.

Yu, G., Swiston, J. & Young, D. (1994) Comparison of human CAP and CAP2, homologues of yeast adenylyl cyclase associated proteins. J. Cell Sci. 107, 1671-1678.

Yu, J., Wang, C., Palmieri, S. J., Haarer, B. K. & Field, J. (1999) A cytoskeletal localizing domain in the cyclase-associated protein, CAP/Srv2p, regulates access to a distant SH3-binding site. J.Biol.Chem. 274, 19985-19991.

Zelicof, A., Protopopov, V., David, D., Lin, X.-Y., Lustgarten, V. & Gerst, J. E. (1996) Two separate functions are encoded by the carboxyl-terminal domains of the yeast cyclase-associated protein and its mammalian homologs. J.Biol.Chem. 271, 18243-18252.

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