|Cordon-bleu WH2 repeat protein|
|Locus||Chr. 7 p12.1|
Cordon-bleu protein (Cobl) is an actin nucleator protein, which seems to have a pivotal role in morphogenetic processes of the vertebrate central nervous system (CNS). The human COBL gene encodes a 1261-amino acid protein with a mass of about 136 kDa. It was re-discovered by Ahuja et al. in a yeast-two hybrid screen using a brain cDNA library for interactors with syndapin I.
The "functional" domain of this actin-binding protein is the Wiskott-Aldrich syndrome protein (WASp) homology 2 (WH2) domain, which is involved in very diverse functional interactions between actin monomers and actin-binding proteins. Cobl in humans is composed of three WH2 domains located at the C-terminus (the first WH2 domain, WH2.1, lies between 1109 and 1129, WH2.2 between 1149 and 1169 and WH2.3 between 1237 and 1257), that are responsible for forming an "actin nucleus" composed of 3 actin monomers from which an actin filament can be further elongated. The cobl-actin-nucleus remains at the pointed end of the emerging filament. Near the N-terminus Cobl has two KKRRAP motifs, whose roles are unresolved. The polypeptide also contains proline-rich sequences, which may bind profilin-actin heterodimers, and selected Ser residues are phosphorylated. In humans there are 5 isoforms of Cobl generated though alternative splicing.
The WH2 domain is an ~18-21 amino acids actin-binding motif. This domain was first recognized as an essential element for the regulation of the cytoskeleton by the mammalian Wiskott-Aldrich syndrome protein (WASp) and is present in at least 6 classes of proteins WIP/Verprolin, Cibulot, Svv2/CAP, β-Thymosines, WAVE/Scar. WH2 proteins occur in eukaryotes from yeast to mammals, in insect viruses, and in some bacteria. The WH2 domain is found as a modular part of larger proteins and can occur as a tandem repeat. The WH2 domain binds actin monomers and can facilitate the assembly of actin monomers into newly forming actin filaments.
Cobl is responsible for the actin polymerization of filaments with fast-growing barbed ends. Cobl stabilizes a longitudinal actin-GTP dimer by two consecutive WH2 domains and it can interact laterally with another actin monomer to form an actin trimer. Rearrangements of the actin–actin contacts have to occur within this trimer to make a helical nucleus; the third WH2 domain of Cobl has been proposed to interact with the third actin subunit of this trimer. This Cobl-actin tetramer is forming a nucleus to facilitate further G-actin addition.
Cobl-mediated actin nucleation is very efficient. In fact already low nanomolar concentrations of Cobl can generate unbranched filaments with similar characterists as WASp–Arp2/3-complex-mediated actin nucleation. Like Spire-1, expression of Cobl is mainly restricted to the brain; much weaker expression was detected in other tissues.
Cobl localizes in neurons to dendrites, axons and in the cell body, but is enriched at axonal and dendritic growth cones. Cobl is involved in axis patterning in embryonic stem cells. The role of Cobl in neural morphogenesis has been investigated ex vivo in hippocampal neurons in rat-brain. Overexpression leads to abnormal dendrites and axons production, whereas RNAi-mediated Cobl depletion results in a reduction of dendrites and neurites branching.
Recent reports don’t give a significant inside on a pivotal clinical role of Cobl. Due to its role in vertebrate neural morphogenesis we can speculate that some neurodegenative disorders may be consequence of a mutated or less functional Cobl. Cobl seems to play a role in the skeletal asymmetry of the Silver-Russel syndrome; the disease is caused by the duplication of the p11.2-p13 segment of the chromosome 7. However, the removal of the gene in the mouse doesn't show any phenotype.
Screening studies on patients having neurological disorders didn’t report until now (June 2009) a specific involvement of Cobl.
Comparison with Spire and Arp2/3 complex
Several actin nucleators are present in mammalian cells. Cobl, Spire and Leiomodin form a group of such proteins because of the presence of the WH2 domain, which bind G-actin. Other protein containing WH2 domain have other functions. And other nucleators, such as formins, nucleate actin filaments using a different actin-binding motif, e.g. the FH2 (formin homology 2) domain.
Cordon-bleu protein, uses two WH2 motifs (blue segment) for the recruitment of ATP-actin monomers (dark orange) to form a linear actin dimer (Fig. 3). A short linker (short green segment) permits a close association between the linearly arranged actin subunits; a longer linker (long green segment) permits a third ATP-actin monomer to bind to the most carboxy-terminal WH2 motif to assemble in a cross-filament orientation, creating a trimeric actin nucleus.
In contrast, Spire, which contains four closely spaced WH2s, is proposed to nucleate actin filaments by stabilizing the association of four actin monomers along a single, linear strand. Additional four ATP-actin monomers associate laterally along this tetramer to form a short typical two-strand helical actin filament (cf. Fig. 2C).
In the case of the WASp-Arp2/3-complex, WASp (yellow segment, Fig. 3) binds to and activates the Arp2/3 complex (purple structures, the nucleator complex) by orienting the two ARP subunits in a conformation similar to the barbed end of an actin filament. The WH2 motif in WASp recruits ATP-G-actin that binds to either Arp2 or Arp3 to complete the formation of a trimeric actin nucleus.
- Eija Paunola et al., WH2 domain: a small, versatile adapter for actin monomers, FEBS (2001).
- R. Ahuja et al., Cordon-bleu is an actin nucleation factor and controls neuronal morphology, Cell 131 (2007), pp. 337–350.
- Quinlan et al., Drosophila Spire is an actin nucleation factor, Nature 433 (2005), pp. 382–388.
- Louis Renault et al., Spire and Cordon-bleu: multifunctional regulators of actin dynamics, Trends Cell Biol. 18(10) (2008), 494-504.
- Megan P. Hitchins et al., DDC and COBL, flanking the imprinted GRB10 gene on 7p12, are biallelically expressed, Mamm Genome. 13(12) (2002) pp. 686-91.
- Winckler B, Schafer DA, Cordon-bleu: a new taste in actin nucleation, Cell 131 (2007), 337-50.