Coronin

Originally isolated from Dictyostelium discoideum (de Hostos et al, 1991), and subsequently found in other protists (Tardieux et al, 1998), vertebrates including Xenopus (Mishima & Nishida, 1999), mice and humans (Okumura et al, 1998), (see (de Hostos, 1999) for a recent review), coronin is a homodimeric c 55kDa actin-binding protein. Coronin is localized to cell surface projections (sometime appearing as "crowns", thus the name), and generally appearing in actin -rich regions (de Hostos et al, 1991).  It is now clear that coronin is present in many organisms as a family of proteins, the main feature of which is the five WD domains.  These WD domains, when present in other protein are known to function as protein:protein interacting motifs.  The five WD domains in coronins form a so called b-propeller structure within which the actin binding region lies.  Exactly where, or even if the whole structure is required, is not currently known, however the b-propeller structure itself when expressed alone in bacteria does not bind to actin (Mishima & Nishida, 1999).  This does not of course mean that this is not the actin binding region as it is possible that the protein was not correctly folded in the bacteria.  A region of coronin (435-462) has been suggested to be homologous to YpkA, an actin binding kinase from the bacteria Yersinia suggesting that this may be an actin binding region (Juris, et al, 2000).  At the C-terminal, a coiled-coil region self associated to form the coronin dimer.  This dimer can therefore bind two filaments leading to their gelating and bundling the actin filaments.

    It has been pointed out that emergence of coronin  as an actin binding protein was unlike the many other Dictyostelium ABPs.  Typically, ABPs have been isolated biochemically on the basis of their effects on the viscosity of exogenously added actin and this activity used as a guide for purification to homogeneity. After purification either antibodies or limited peptide sequence information permitted the isolation of cDNAs.  Coronin instead was isolated from a contracted actomyosin pellet and subsequently found to bind actin. However, when it became possible to knock out these genes from the haploid organism (see Dictyostelium ABPs), there was some degree of disappointment as many of the ABPs with strong actin-binding activity produced very little phenotype.  The phenotype of coronin-null cells however, was very obvious, as the resulting cells were reduced in locomotion and their ability to divide was also markedly inhibited.

    Coronin-null cells are not only defective in locomotion, but grow more slowly, possibly as a consequence of inhibition of pinocytosis, phagocytosis (Hacker et al, 1997) and more certainly by inhibition of cytokinesis (de Hostos et al, 1993).  In vertebrate cells too coronin is seen to localize to the lamellopodia and when disrupted by the overexpression with truncated forms inhibits cell spreading and locomotion (Mishima & Nishida, 1999), and the protein appears to be involved in the actin-rich "tails" of Listeria infected cells (David et al, 1998).  The function of coronin in cells has been vividly displayed by GFP -coronin fusion proteins that permit visualization of the protein in living cells (Gerisch et al, 1995; also see the links after the references).

    In neutrophils the actin cytoskeleton seems to have a role in the regulation of the NADPH oxidase enzyme assembly around the phagolysosome membrane and there are indications that coronin is involved in this process (Grogan et al, 1997).

Other Related Web-sites:-

Coronin-GFP dynamics in living cells
de Hostos Lab. pages

References

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.

de Hostas, E. L., B. Bradtke, et al. (1991). "Coronin, an actin binding protein of Dictyostelium discoideum localized to cell surface projections, has sequence similarities to G protein b subunits." EMBO J. 10, 4097-4104.