Calponin 1

Calponin 1 is a basic smooth muscle protein that in humans is encoded by the CNN1 gene.[5]

CNN1
Available structures
PDBOrtholog search: PDBe RCSB
Identifiers
AliasesCNN1, HEL-S-14, SMCC, Sm-Calp, calponin 1
External IDsOMIM: 600806 MGI: 104979 HomoloGene: 995 GeneCards: CNN1
Orthologs
SpeciesHumanMouse
Entrez

1264

12797

Ensembl

ENSG00000130176

ENSMUSG00000001349

UniProt

P51911

Q08091

RefSeq (mRNA)

NM_001299
NM_001308341
NM_001308342

NM_009922

RefSeq (protein)

NP_001290
NP_001295270
NP_001295271

NP_034052

Location (UCSC)Chr 19: 11.54 – 11.55 MbChr 9: 22.01 – 22.02 Mb
PubMed search[3][4]
Wikidata
View/Edit HumanView/Edit Mouse

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.

Evolution
Figure 1: Evolutionary lineage of vertebrate CNN1.

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.

Smooth muscle-specific expression

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]

Structure-function relationship
Figure 2. Structural and functional domains of calponin. The linear structural map summarized primarily from studies of chicken calponin 1 illustrates the structural and functional domains of calponin. The CH domain, two actin-binding sites, three repeating sequence motifs, and the C-terminal variable region are outlined. The CH domain overlaps with the ERK signaling binding region. Amino acid sequences of the two actin-binding sites in the three isoforms and the three repeating motifs of calponin 1 are shown in the insets. The regulatory phosphorylation sites Ser175 and Thr184 are located in the second actin-binding site that overlaps with the first repeating motif. Potentially phosphorylatable serine residues corresponding to Ser175 are conserved in repeats 2 and 3, while a Thr184 equivalent is conserved in repeat 2. Different from calponin 1 and calponin 3, calponin 2 has a potentially phosphorylatable additional serine at position 177.

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

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]

Actin-binding sites

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.

C-terminal variable region

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]

Regulation of smooth muscle contractility

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]

Phosphorylation regulation

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.

Notes

References
  1. GRCh38: Ensembl release 89: ENSG00000130176 - Ensembl, May 2017
  2. GRCm38: Ensembl release 89: ENSMUSG00000001349 - Ensembl, May 2017
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  4. "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  5. "Entrez Gene: calponin 1, basic, smooth muscle".
  6. Takahashi K, Abe M, Hiwada K, Kokubu T (December 1988). "A novel troponin T-like protein (calponin) in vascular smooth muscle: interaction with tropomyosin paracrystals". Journal of Hypertension Supplement. 6 (4): S40–3. doi:10.1097/00004872-198812040-00008. PMID 3241227. S2CID 38679688.
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