Cordon-bleu protein

Protein cordon-bleu is a protein that in humans is encoded by the COBL gene.

The Cordon-bleu protein was first described in 2007. Cobl was demonstrated to be a brain-enriched, Wiskott-Aldrich Homology 2 WH2 domain–based actin nucleator playing a pivotal role in morphogenetic processes in the vertebrate central nervous system (CNS) that give rise to the complex dendritic arbor of neuronal cells.

The Cobl gene was originally identified in lacZ-gene-trap experiments in mice. Cobl mRNA was detected as early as at day 7.5 post-coitum (E7.5) in the gastrula organizer and extended towards the axial midline at E8. The organizer is a small group of embryonal cells that organizes the entire body plan because it gives rise to the axial midline – an important source of patterning and morphogenesis cues.

The human COBL gene encodes a 1261-amino acid protein with a mass of about 136 kDa. The mouse protein is 1337 amino acids long. The cobl gene in mice and human gives rise to a variety of putative splice variants, which, however, have not yet been analyzed at the protein level. Cobl genes seem to be restricted to vertebrates.

Cobl was discovered as an actin nucleator in F-actin formation assays with brain cytosol depleted for the Arp2/3 complex (the only other actin nucleator known at this time) and with beads coated with the SH3 domain of the actin binding protein 1 (Abp1) mAbp1|SH3P7|HIP-55|drebin-like and with the SH3 domain of syndapin I, respectively, and subsequent yeast-two hybrid screen for putatively responsible components using a brain cDNA library. Cobl was then cloned fully and its neuronal functions were characterized in primary neuronal cultures in functional slices of the cerebellum, at the animal level in zebra fish, and in mice.

The WH2 domain-based actin nucleator Cobl turned out to be critical for the induction of dendritic branches in both hippocampal neurons and Purkinje cells of the cerebellum, as revealed by RNAi experiments. Cobl hereby specifically accumulates at nascent dendritic branch sites prior to a burst of local F-actin formation at these sites.

Cobl gives rise to actin filaments by bringing together three actin monomers by the use of its three C-terminal WH2 domains, as shown by in vitro reconstitutions with the purified C-terminus of Cobl. In addition to the three C-terminal WH2 domains, the presence of the linker region L2 between the second and third WH2 domain was also of importance for Cobl-mediated actin nucleation. Cobl assembles non-bundled, unbranched actin filaments.

In contrast to other actin nucleators, which are directly or indirectly via their activators controlled by Rho type GTPases, Cobl hereby is controlled by calcium/calmodulin signalling and by arginine methylation brought about by the arginine methyltransferase 2 PRMT2. Consistently, both CaM inhibitors and inhibitors of arginine methylation fully suppressed Cobl-induced dendritic branch induction. Furthermore, Cobl mutants lacking the PRMT2 binding site or lacking CaM-binding regions failed to power dendritic branching. The molecular mechanisms, by which Ca2+ signals control the actin nucleator Cobl, are quite complex and may reflect a high level of fine-tuning Cobl's physiological functions to Ca2+ transients in neuronal cells. Cobl is regulated by Ca2+ signals in at least three distinct ways (Izadi and Hou et al., 2018). These modes involve all three WH2 domains of Cobl, which closely need to work together to nucleate actin effectively. Interestingly, Ca2+/CaM controls not only the actin cytoskeletal aspects of Cobl functions but also its targeting to the plasma membrane - a prerequisite for F-actin-driven force generation shaping the morphology of cells. Mutational analyses showed that both individual CaM-dependent regulatory mechanisms are crucial for Cobl-mediated dendritogenesis. The required recruitment of Cobl to the dendritic plasma membrane is brought about by Cobl's own ability to associate with membrane lipids and by the membrane-binding protein syndapin I, which is critical for Cobl's membrane targeting and function.