Calponin 1 is a basic smooth muscle

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respon ...

that in

human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

s is encoded by the ''CNN1''

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

. The ''CNN1'' gene is located at 19p13.2-p13.1 in the human chromosomal

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...

and contains 7

exons An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequenc ...

, encoding the protein calponin 1, an

actin Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in myofibril, muscle fibrils. It is found in essentially all Eukaryote, eukaryotic cel ...

filament-associated regulatory protein. Human calponin 1 is a 33.2-KDa protein consists of 297

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

s with an

isoelectric point The isoelectric point (pI, pH(I), IEP), is the pH at which a molecule carries no net electrical charge or is electrically neutral in the statistical mean. The standard nomenclature to represent the isoelectric point is pH(I). However, pI is also ...

of 9.1, thus calponin 1 is also known as basic calponin.

Evolution

Three homologous genes, ''Cnn1'', ''Cnn2'' and ''Cnn3'', have evolved in

vertebrate Vertebrates () comprise all animal taxon, taxa within the subphylum Vertebrata () (chordates with vertebral column, backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the ...

s, encoding three

isoforms A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some iso ...

calponin Calponin is a calcium binding protein. Calponin tonically inhibits the ATPase activity of myosin in smooth muscle. Phosphorylation of calponin by a protein kinase, which is dependent upon calcium binding to calmodulin, releases the calponin's ...

: calponin 1,

calponin 2 Calponin 2 is a protein that in humans is encoded by the ''CNN2'' gene. The ''CNN2'' gene is located at 19p13.3 in the human chromosomal genome, encoding the protein calponin 2. Calponin 2 is one of the three isoforms of calponin and an actin fi ...

, calponin 3, respectively. Protein

sequence alignment In bioinformatics, a sequence alignment is a way of arranging the sequences of DNA, RNA, or protein to identify regions of similarity that may be a consequence of functional, structural, or evolutionary relationships between the sequences. Ali ...

shows that calponin 1 is highly conserved in mammals but more diverged among lower vertebrates.

Smooth muscle-specific expression

The

expression Expression may refer to: Linguistics * Expression (linguistics), a word, phrase, or sentence * Fixed expression, a form of words with a specific meaning * Idiom, a type of fixed expression * Metaphorical expression, a particular word, phrase, ...

of CNN1 is specific to differentiated mature smooth muscle cells, suggesting a role in contractile functions. Calponin 1 is up-regulated in

smooth muscle tissue Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit m ...

s during postnatal development with a higher content in phasic smooth muscle of the

digestive tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans an ...

Structure-function relationship

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 The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...

calponin homology (CH) domain Calponin homology domain (or CH domain) is a family of actin binding domains found in both cytoskeletal proteins and signal transduction proteins. The domain is about 100 amino acids in length and is composed of four alpha helices. It compris ...

, a conserved middle region containing two actin-binding sites, and a

C-terminal The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...

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 Actinin is a microfilament protein. Alpha-actinin-1 is necessary for the attachment of actin myofilaments to the Z-lines in skeletal muscle cells, and to the dense bodies in smooth muscle cells. The functional protein is an anti-parallel dimer ...

spectrin Spectrin is a cytoskeletal protein that lines the intracellular side of the plasma membrane in eukaryotic cells. Spectrin forms pentagonal or hexagonal arrangements, forming a scaffold and playing an important role in maintenance of plasma memb ...

, and

filamin Filamins are a class of proteins that hold two actin filaments at large angles. Filamin protein in mammals is made up of an actin-binding domain at its N-terminus that is followed by 24 immunoglobulin-like repeat modules of roughly 95 amino acids. ...

) to form the actin-binding region or serve as a regulatory structure. However, the CH domain in calponin is not the binding site for actin nor does it regulate the modes of calponin-F-actin binding. Nonetheless, CH domain in calponin was found to bind to extra-cellular regulated

kinase In biochemistry, a kinase () is an enzyme that catalysis, catalyzes the transfer of phosphate groups from High-energy phosphate, high-energy, phosphate-donating molecules to specific Substrate (biochemistry), substrates. This process is known as ...

(ERK) for calponin to play a possible role as an adaptor protein in the ERK signaling cascades.

Actin-binding sites

Calponin binds actin to promote and sustain

polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...

. The binding of calponin to F-actin inhibits the MgATPase activity of smooth muscle

myosin Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility. The first myosin (M ...

. 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 motif In biology, a sequence motif is a nucleotide or amino-acid sequence pattern that is widespread and usually assumed to be related to biological function of the macromolecule. For example, an ''N''-glycosylation site motif can be defined as '' ...

s in calponin next to the C-terminal region. This repeating structure is conserved in all three isoforms and across

species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...

. Outlined in Fig. 2, the first repeating motif overlaps with the second actin-binding site and contains

protein kinase C In cell biology, Protein kinase C, commonly abbreviated to PKC (EC 2.7.11.13), is a family of protein kinase enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and ...

(PKC)

phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, ...

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 Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other pro ...

. 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. 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. The C-terminal tail regulates the interaction with F-actin by altering the function of the second actin-bing site of calponin.

Regulation of smooth muscle contractility

Numerous

in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology and ...

experimental data indicate that calponin 1 functions as an inhibitory regulator of smooth muscle contractility through inhibiting

actomyosin Myofilaments are the three protein filaments of myofibrils in muscle cells. The main proteins involved are myosin, actin, and titin. Myosin and actin are the ''contractile proteins'' and titin is an elastic protein. The myofilaments act toge ...

interactions. In this regulation, binding of Ca²⁺-calmodulin and PKC phosphorylation dissociate calponin 1 from the actin filament and facilitate smooth muscle contraction.

In vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and ...

data also support the role of calponin 1 as regulator of smooth muscle contractility. While

aortic The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes o ...

smooth muscle of adult Wistar Kyoto

rat Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include '' Neotoma'' ( pack rats), '' Bandicota'' (bandico ...

s, which naturally lacks calponin 1, is fully contractile, it has a decreased sensitivity to

norepinephrine Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain and body as both a hormone and neurotransmitter. The name "noradrenaline" (from Latin '' ad ...

activation. Matrix metalloproteinase-2

proteolysis Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Uncatalysed, the hydrolysis of peptide bonds is extremely slow, taking hundreds of years. Proteolysis is typically catalysed by cellular enzymes called protease ...

of calponin 1 resulted in

vascular Blood vessels are the structures of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away from ...

hypocontractility to

phenylephrine Phenylephrine is a medication primarily used as a decongestant, to dilate the pupil, to increase blood pressure, and to relieve hemorrhoids. In the United States, it was previously used orally as an over-the-counter decongestant to relieve na ...

Vas deferens The vas deferens or ductus deferens is part of the male reproductive system of many vertebrates. The ducts transport sperm from the epididymis to the ejaculatory ducts in anticipation of ejaculation. The vas deferens is a partially coiled t ...

smooth muscle from calponin 1

knockout mice A knockout mouse, or knock-out mouse, is a genetically modified mouse (''Mus musculus'') in which researchers have inactivated, or " knocked out", an existing gene by replacing it or disrupting it with an artificial piece of DNA. They are importan ...

showed faster maximum shortening velocity. Calponin 1 knockout mice exhibited blunted MAP response to phenylephrine administration.

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. Direct association was found between calponin 1 and PKCα and

PKCε Protein kinase C epsilon type (PKCε) is an enzyme that in humans is encoded by the ''PRKCE'' gene. PKCε is an isoform of the large PKC family of protein kinases that play many roles in different tissues. In cardiac muscle cells, PKCε regulate ...

. Calmodulin-dependent kinase II and

Rho-kinase Rho-associated protein kinase (ROCK) is a kinase belonging to the AGC (PKA/ PKG/PKC) family of serine-threonine specific protein kinases. It is involved mainly in regulating the shape and movement of cells by acting on the cytoskeleton. ROCKs (R ...

are also found to phosphorylate calponin at Ser175 and Thr184 in vitro. 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 Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycl ...

by type 2B protein

phosphatase In biochemistry, a phosphatase is an enzyme that uses water to cleave a phosphoric acid monoester into a phosphate ion and an alcohol. Because a phosphatase enzyme catalyzes the hydrolysis of its substrate, it is a subcategory of hydrolases. Ph ...

Unphosphorylated calponin binds to actin and inhibits actomyosin MgATPase. Ser175 phosphorylation alters the molecular conformation of calponin and dissociates calponin from F-actin. The consequence is to release the inhibition of actomyosin MgATPase and increase the production of force. 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. Based on the observation that PKC phosphorylation of calponin 1 weakens the

binding affinity In biochemistry and pharmacology, a ligand is a substance that forms a complex with a biomolecule to serve a biological purpose. The etymology stems from ''ligare'', which means 'to bind'. In protein-ligand binding, the ligand is usually a mo ...

for the actin filaments, the phosphorylated calponin may not be stable in the actin cytoskeleton thus be degraded in the cell.

Notes

References

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