Coronin

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Significant progress about the function and regulation of this protein family has been made since 2005, which are not covered in the following part. A recent review should provide better understand to this field.

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Coronin is an actin binding protein with unique structural domain. Eugenio L. de Hostos et al. (1991) isolated a 55 kDa protein from actinomyosin complex of Dictyostelium discoidium, which was later shown to bind actin in vitro. This actin binding protein was named coronin after its strong immunolocalisation in the actin rich crown like extension of the cell cortex in D. discoidium. Initially this protein was admitted into club of actin binding proteins with least enthusiasm, as the primary structure did not match any other ABPs. But later on, the protein was identified in many eukaryotic cells and Dictyostelium was found to be impaired in cytokinesis, and many actin mediated processes like endocytosis, cell motility etc. The primary structure of coronin belongs to WD-repeat containing proteins. Yeast coronin and Drosophila Dpod1 were found crosslink actin and microtubule cytoskeleton. C.elegans POD-1 and drosophila coronin homologue regulate actin cytoskeleton and are involved in vesicular trafficking. Although seven different isoforms of coronin have been reported in mammals, their exact functions are not very clear. Mammalian coronin-7 does not interact with actin nor does it execute any actin mediated processes, but rather participate in Golgi trafficking. Although coronin is present almost all eukaryotic organisms and have different functions, but everywhere these proteins have been shown to bind F-actin and localize in the dynamic F-actin rich area of cells.
WD-40-domain –repeat proteins are defined by the presence of at least four WD repeats located centrally in the protein. These domains were discovered in 1986 and are characterized by a partial conserved domain of 40-60 amino acids, starting with GH dipeptide 11-24 residue away from the N-terminus and ending with a tryptophane-aspartic acid (WD) dipeptide at the C-terminus. The WD domain has no ntrinsic catalytic activity and is thought to serve as a stable platform for simultaneous interaction. WD repeat proteins have diverse cellular functions. They play central role in physiological processes like signal transduction, transcriptional regulation, cytoskeleton remodeling, regulation of vesicle trafficking etc.
Coronin homologues both in vertebrates and invertebrates forms a subfamily among WD repeat proteins. Coronin contains 3-5 WD clustered repeats forming the central core domain. Apart from core domain, almost all coronins have a short conserved N-terminal motif and coiled coil motif of 50aa at C-terminus. The N-terminal region contains 12 basic aa which can be taken as signature as it is present in only coronin proteins. A recent study shows that these basic residues are involved in actin binding. Furthermore, each coronin contains a unique divergent region between the WD domain and C-terminal coiled coil region. The number of amino acids in this region varies greatly. The unique region has of dictyostelium has 22aa whereas mammalian coronins contains about 50 aa. The coronin like proteins from budding yeast Crn1 and one of the coronins in C. elegans has a much longer unique region i.e. 194 vs 144aa. The unique region of yeast coronin shows homologies with microtubule binding domains of the MAPs and yeast coronin binds both actin and microtubule and serve as bridge between them.
A second region of variability exists in the fourth β-strand of the third WD repeats.

Coronin Classification

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1. Short conventional coronins 450-650aa with C-terminus coiled coil region of 30-40 aa that mediates homophilic dimerization and or olimerization of coronins. E.g. CRN1, CRN2
2. Long coronins two core domains with no C-terminal coiled coil region N-terminus signature region is reduced to 5aa and appears in front of each WD-repeat core domain. e.g. CRN7, POD-1,

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