CNN1 | |||||||||||||||||||||||||||||||||||||||||||||||||||
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Aliases | CNN1 , HEL-S-14, SMCC, Sm-Calp, calponin 1 | ||||||||||||||||||||||||||||||||||||||||||||||||||
External IDs | OMIM: 600806; MGI: 104979; HomoloGene: 995; GeneCards: CNN1; OMA:CNN1 - orthologs | ||||||||||||||||||||||||||||||||||||||||||||||||||
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Wikidata | |||||||||||||||||||||||||||||||||||||||||||||||||||
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Calponin 1 is a basic smooth muscle protein that in humans is encoded by the CNN1 gene. [5]
The CNN1 gene is located at 19p13.2-p13.1 in the human chromosomal genome and contains 7 exons, encoding the protein calponin 1, an actin filament-associated regulatory protein. [6] Human calponin 1 is a 33.2-KDa protein consists of 297 amino acids with an isoelectric point of 9.1, [7] thus calponin 1 is also known as basic calponin.
Three homologous genes, Cnn1, Cnn2 and Cnn3, have evolved in vertebrates, encoding three isoforms of calponin: calponin 1, [7] [8] calponin 2, [9] calponin 3, [10] respectively. Protein sequence alignment shows that calponin 1 is highly conserved in mammals but more diverged among lower vertebrates.
The expression of CNN1 is specific to differentiated mature smooth muscle cells, suggesting a role in contractile functions. Calponin 1 is up-regulated in smooth muscle tissues during postnatal development [11] with a higher content in phasic smooth muscle of the digestive tract. [12]
The majority of structure-function relationship studies of calponin were with experiments using chicken calponin 1. Primary structure of calponin consists of a conserved N-terminal calponin homology (CH) domain, a conserved middle region containing two actin-binding sites, and a C-terminal variable region that contributes to the differences among there isoforms.
The CH domain was found in a number of actin-binding proteins (such as α-actinin, spectrin, and filamin) to form the actin-binding region or serve as a regulatory structure. [13] However, the CH domain in calponin is not the binding site for actin nor does it regulate the modes of calponin-F-actin binding. [14] Nonetheless, CH domain in calponin was found to bind to extra-cellular regulated kinase (ERK) for calponin to play a possible role as an adaptor protein in the ERK signaling cascades. [15]
Calponin binds actin to promote and sustain polymerization. The binding of calponin to F-actin inhibits the MgATPase activity of smooth muscle myosin. [16] [17] [18] Calponin binds F-actin through two sites at residues 144-162 and 171–188 in chicken calponin 1. The two actin-binding sites are conserved in the three calponin isoforms.
There are three repeating sequence motifs in calponin next to the C-terminal region. This repeating structure is conserved in all three isoforms and across species. Outlined in Fig. 2, the first repeating motif overlaps with the second actin-binding site and contains protein kinase C (PKC) phosphorylation sites Ser175 and Thr184 that are not present in the first actin-binding site. This feature is consistent with the hypothesis that the second actin-binding site plays a regulatory role in the binding of calponin to the actin filament. Similar sequences as well as potential phosphorylation sites are present in repeats 2 and 3 whereas their function is unknown.
The C-terminal segment of calponin has diverged significantly among the three isoforms. The variable lengths and amino acid sequences of the C-terminal segment produce the size and charge differences among the calponin isoforms. The corresponding charge features rendered calponin 1, 2 and 3 the names of basic, neutral and acidic calponins. [19] [20] [21]
The C-terminal segment of calponin has an effect on weakening the binding of calponin to F-actin. Deletion of the C-terminal tail strongly enhanced the actin-binding and bundling activities of all three isoforms of calponin. [22] [23] The C-terminal tail regulates the interaction with F-actin by altering the function of the second actin-bing site of calponin. [24]
Numerous in vitro experimental data indicate that calponin 1 functions as an inhibitory regulator of smooth muscle contractility through inhibiting actomyosin interactions. [6] [25] [26] In this regulation, binding of Ca2+-calmodulin and PKC phosphorylation dissociate calponin 1 from the actin filament and facilitate smooth muscle contraction. [27]
In vivo data also support the role of calponin 1 as regulator of smooth muscle contractility. While aortic smooth muscle of adult Wistar Kyoto rats, which naturally lacks calponin 1, is fully contractile, it has a decreased sensitivity to norepinephrine activation. [28] [29] Matrix metalloproteinase-2 proteolysis of calponin 1 resulted in vascular hypocontractility to phenylephrine. [30] Vas deferens smooth muscle from calponin 1 knockout mice showed faster maximum shortening velocity. [31] Calponin 1 knockout mice exhibited blunted MAP response to phenylephrine administration. [32]
There is a large collection of in vitro evidences demonstrating the phosphorylation regulation of calponin. The primary phosphorylation sites are Ser175 and Thr184 in the second actin-binding site (Fig. 2). Experimental data showed that Ser175 and Thr184 in calponin 1 are phosphorylated by PKC in vitro. [27] Direct association was found between calponin 1 and PKCα [33] and PKCε. [15] Calmodulin-dependent kinase II and Rho-kinase are also found to phosphorylate calponin at Ser175 and Thr184 in vitro. [34] [35] Of these two residues, the main site of regulatory phosphorylation by calmodulin-dependent kinase II and Rho-kinase is Ser175. Dephosphorylation of calponin is catalyzed by type 2B protein phosphatase [36] [37]
Unphosphorylated calponin binds to actin and inhibits actomyosin MgATPase. Ser175 phosphorylation alters the molecular conformation of calponin and dissociates calponin from F-actin. [38] The consequence is to release the inhibition of actomyosin MgATPase and increase the production of force. [18] [39] [40]
Despite the overwhelming evidence for the phosphorylation regulation of calponin obtained from in vitro studies, phosphorylated calponin is not readily detectable in vivo or in living cells under physiological conditions. [41] [42] Based on the observation that PKC phosphorylation of calponin 1 weakens the binding affinity for the actin filaments, [38] the phosphorylated calponin may not be stable in the actin cytoskeleton thus be degraded in the cell.
The 2016 version of this article was updated by an external expert under a dual publication model. The corresponding academic peer reviewed article was published in Gene and can be cited as: Rong Liu; J-P Jin (9 March 2016). "Calponin isoforms CNN1, CNN2 and CNN3: Regulators for actin cytoskeleton functions in smooth muscle and non-muscle cells". Gene . Gene Wiki Review Series. 585 (1): 143–153. doi:10.1016/J.GENE.2016.02.040. ISSN 0378-1119. PMC 5325697 . PMID 26970176. Wikidata Q37666020. |
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