Pentraxins (PTX), also known as pentaxins, are an evolutionary conserved family of

proteins 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, re ...

characterised by containing a pentraxin

protein domain In molecular biology, a protein domain is a region of a protein's Peptide, polypeptide chain that is self-stabilizing and that Protein folding, folds independently from the rest. Each domain forms a compact folded Protein tertiary structure, thre ...

. Proteins of the pentraxin family are involved in acute immunological responses. They are a class of pattern recognition receptors (PRRs). They are a superfamily of multifunctional conserved proteins, some of which are components of the humoral arm of

innate immunity The innate immune system or nonspecific immune system is one of the two main immunity strategies in vertebrates (the other being the adaptive immune system). The innate immune system is an alternate defense strategy and is the dominant immune s ...

and behave as functional ancestors of

antibodies An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that caus ...

(Abs). They are known as classical acute phase proteins (APP), known for over a century.

Structure

Pentraxins are characterised by

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

dependent

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

binding and a distinctive flattened β-jellyroll structure similar to that of the

legume Legumes are plants in the pea family Fabaceae (or Leguminosae), or the fruit or seeds of such plants. When used as a dry grain for human consumption, the seeds are also called pulses. Legumes are grown agriculturally, primarily for human consum ...

lectins Lectins are carbohydrate-binding proteins that are highly specific for sugar groups that are part of other molecules, so cause agglutination of particular cells or precipitation of glycoconjugates and polysaccharides. Lectins have a role in r ...

. The name "pentraxin" is derived from the Greek word for five (, pente) and axle (axis) relating to the radial symmetry of five

monomer A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization. Classification Chemis ...

s forming a ring approximately 95Å across and 35Å deep observed in the first members of this family to be identified. The "short" pentraxins include Serum Amyloid P component (SAP) and C reactive protein (CRP). The "long" pentraxins include PTX3 (a cytokine modulated molecule) and several neuronal pentraxins.

Family members

Three of the principal members of the pentraxin family are serum proteins: namely, CRP, SAP, and hamster female protein (FP). PTX3 (or TSG-14) protein is a

cytokine Cytokines () are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are produced by a broad range of cells, including immune cells like macrophages, B cell, B lymphocytes, T cell, T lymphocytes ...

-induced protein that is homologous to CRPs and SAPs.

C-reactive protein

C-reactive protein C-reactive protein (CRP) is an annular (ring-shaped) pentameric protein found in blood plasma, whose circulating concentrations rise in response to inflammation. It is an acute-phase protein of hepatic origin that increases following interleukin ...

is expressed during the

acute phase Acute may refer to: Language * Acute accent, a diacritic used in many modern written languages * Acute (phonetic), a perceptual classification Science and mathematics * Acute angle ** Acute triangle ** Acute, a leaf shape in the glossary of leaf m ...

response to tissue injury or

inflammation Inflammation (from ) is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function (Latin ''calor'', '' ...

in mammals. The protein resembles antibody and performs several functions associated with host defence: it promotes

agglutination In linguistics, agglutination is a morphology (linguistics), morphological process in which words are formed by stringing together morphemes (word parts), each of which corresponds to a single Syntax, syntactic feature. Languages that use agglu ...

, bacterial capsular swelling and

phagocytosis Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...

, and activates the

classical complement pathway The classical complement pathway is one of three pathways which activate the complement system, which is part of the immune system. The classical complement pathway is initiated by antigen-antibody complexes with the antibody isotypes IgG and I ...

through its calcium-dependent binding to

phosphocholine Phosphocholine is an intermediate in the synthesis of phosphatidylcholine in tissues. Phosphocholine is made in a reaction, catalyzed by choline kinase, that converts ATP and choline into phosphocholine and ADP. Phosphocholine is a molecule ...

. CRPs have also been sequenced in an invertebrate, '' Limulus polyphemus'' (Atlantic horseshoe crab), where they are a normal constituent of the hemolymph.

Pentraxin 3

Pentraxin 3 (PTX3) is an acute phase protein whose levels rise during severe infections in humans. In case of central nervous system infections PTX3 helps distinguishes between

bacterial Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the ...

and aseptic meningoencephalitis. It is significantly higher in bacterial meningoencephalitis.

Serum amyloid P component

Serum amyloid P component is a vertebrate protein that is identical to tissue forms of amyloid P component. It is found in all types of amyloid deposits, in

glomerular basement membrane The glomerular basement membrane of the kidney is the basal lamina layer of the glomerulus. The glomerular endothelial cells, the glomerular basement membrane, and the filtration slits between the podocytes perform the filtration function of th ...

and in

elastic fibre Elastic fibers (or yellow fibers) are an essential component of the extracellular matrix composed of bundles of proteins (elastin) which are produced by a number of different cell types including fibroblasts, endothelial, smooth muscle, and air ...

s in

blood vessels Blood vessels are the tubular structures of a circulatory system that transport blood throughout many animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the tissues of a body. They also take waste an ...

. SAP binds to various

lipoprotein A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, sur ...

ligands in a calcium-dependent manner, and it has been suggested that, in mammals, this may have important implications in

atherosclerosis Atherosclerosis is a pattern of the disease arteriosclerosis, characterized by development of abnormalities called lesions in walls of arteries. This is a chronic inflammatory disease involving many different cell types and is driven by eleva ...

and

amyloidosis Amyloidosis is a group of diseases in which abnormal proteins, known as amyloid fibrils, build up in tissue. There are several non-specific and vague signs and symptoms associated with amyloidosis. These include fatigue, peripheral edema, weigh ...

Hamster female protein

Hamster female protein is a SAP homologue found in ''

Mesocricetus auratus The golden hamster or Syrian hamster (''Mesocricetus auratus'') is a rodent belonging to the hamster subfamily, Cricetinae. Their natural geographical range is in an arid region of northern Syria and southern Turkey. Their numbers have been d ...

'' (the Golden hamster). The concentration of this plasma protein is altered by

sex steroid Sex hormones, also known as sex steroids, gonadocorticoids and gonadal steroids, are steroid hormones that interact with vertebrate steroid hormone receptors. The sex hormones include the androgens, estrogens, and progestogens. Their effects a ...

s and stimuli that elicit an acute phase response.

Nervous system

Pentraxin proteins expressed in the nervous system are neural pentraxin I ( NPTXI) and II ( NPTXII). NPTXI and NPTXII are homologous and can exist within one species. It is suggested that both proteins mediate the uptake of synaptic macromolecules and play a role in

synaptic plasticity In neuroscience, synaptic plasticity is the ability of synapses to Chemical synapse#Synaptic strength, strengthen or weaken over time, in response to increases or decreases in their activity. Since memory, memories are postulated to be represent ...

. Apexin, a sperm acrosomal protein, is a homologue of NPTXII found in ''

Cavia porcellus The guinea pig or domestic guinea pig (''Cavia porcellus''), also known as the cavy or domestic cavy ( ), is a species of rodent belonging to the genus ''Cavia'', family Caviidae. Animal fancy, Breeders tend to use the name "cavy" for the ani ...

'' (Guinea pig).

Human

Human genes encoding proteins that contain this domain include: * APCS * CRP * GPR144 * NPTX1; NPTX2; NPTXR * PTX3 * SVEP1

References

{{InterPro content, IPR001759 Protein families Protein domains Single-pass transmembrane proteins