Pentraxin / C-reactive protein / pentaxin family
SMART accession number:SM00159
Description: This family form a doscoid pentameric structure. Human serum amyloid P demonstrates calcium-mediated ligand-binding.
Interpro abstract (IPR001759):

This entry represents Pentaxins and its related proteins such as CRP (C-reactive protein) and SAP (serum amyloid P component protein) [ (PUBMED:9480764) ]. This entry also includes adhesion G-protein coupled receptors D2 and G6 from humans.

Pentraxins (or pentaxins) [ (PUBMED:6356809) (PUBMED:7772283) ] are a family of proteins which show, under electron microscopy, a discoid arrangement of five noncovalently bound subunits. Proteins of the pentraxin family are involved in acute immunological responses [ (PUBMED:7772283) ]. Three of the principal members of the pentraxin family are serum proteins and Ca 2 dependent: namely, C-reactive protein (CRP) [ (PUBMED:9614930) ], serum amyloid P component protein (SAP) [ (PUBMED:9514915) ], and female protein (FP) [ (PUBMED:9583999) ]. CRP binds to ligands containing phosphocholine, SAP binds to amyloid fibrils, DNA, chromatin, fibronectin, C4-binding proteins and glycosaminoglycans.

CRP is expressed during acute phase response to tissue injury or inflammation in mammals. The protein resembles antibody and performs several functions associated with host defence: it promotes agglutination, bacterial capsular swelling and phagocytosis, and activates the classical complement pathway through its calcium-dependent binding to phosphocholine. CRPs have also been sequenced in an invertebrate, Limulus polyphemus (Atlantic horseshoe crab), where they are a normal constituent of the hemolymph [ (PUBMED:7881902) ].

SAP is a vertebrate protein that is a precursor of amyloid component P. It is found in all types of amyloid deposits, in glomerular basement menbrane and in elastic fibres in blood vessels. SAP binds to various lipoprotein ligands in a calcium-dependent manner, and it has been suggested that, in mammals, this may have important implications in atherosclerosis and amyloidosis [ (PUBMED:8114934) ].

FP is a SAP homologue found in Mesocricetus auratus (golden hamster). The concentration of this plasma protein is altered by sex steroids and stimuli that elicit an acute phase response.

"Long" pentraxins have N-terminal extensions to the common pentraxin domain [ (PUBMED:8899296) ]; one group, the neuronal pentraxins, may be involved in synapse formation and remodeling, and they may also be able to form heteromultimers [ (PUBMED:10748068) ]. Pentraxin proteins expressed in the nervous system are neural pentraxin I (NPI) and II (NPII) [ (PUBMED:8884281) ]. NPI and NPII 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. Apexin, a sperm acrosomal protein, is a homologue of NPII found in Cavia porcellus (Guinea pig) [ (PUBMED:7798266) ].

PTX3 is a long pentraxin that provides defence against infectious agents and plays several functions in tissue repair and regulation of cancer-related inflammation [ (PUBMED:31019517) ].

Family alignment:
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There are 1293 PTX domains in 1204 proteins in SMART's nrdb database.

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