CAP

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.
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(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. 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