PTX

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

Pentaxins (or pentraxins) [(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 pentaxin family are involved in acute immunological responses [(PUBMED:7772283)]. Three of the principal members of the pentaxin family are serum proteins: namely, C-reactive protein (CRP) [(PUBMED:9614930)], serum amyloid P component protein (SAP) [(PUBMED:9514915)], and female protein (FP) [(PUBMED:9583999)].

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.

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.

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.

Pentaxin proteins expressed in the nervous system are neural pentaxin 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 (or TSG-14) protein is a cytokine-induced protein that is homologous to CRPs and SAPs, but its function is not yet known.

Family alignment:
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There are 198 PTX domains in 145 proteins in SMART's nrdb database.

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