Pentraxins (earlier also termed pentaxins), named for their homopentameric
quaternary structure, are a superfamily of multifunctional conserved proteins
that are characterized by a cyclic multimeric structure and by the presence in
their carboxyl-terminal of an ~200 aa-long conserved domain, called pentraxin
(PTX) domain. In addition, all the members of this family share an 8 aa-long
conserved sequence (HxCxS/TWxS, in which x is any amino acid) in the pentraxin
domain, called pentraxin signature. Some pentraxins, together with collectins
and ficolins, constitute the humoral arm of innate immunity and behave as
functional ancestors of Abs by mediating agglutination, complement activation,
and opsonisation [1,2,3]. They are divided into short:
- C-reactive protein (CRP), a protein which, in mammals, is expressed during
acute phase response to tissue injury or inflammation. CRP displays several
functions associated with host defense: it promotes agglutination,
bacterial capsular swelling, phagocytosis and complement fixation through
its calcium-dependent binding to phosphorylcholine. CRPs have also been
sequenced in an invertebrate, the Atlantic horseshoe crab, where they are a
normal constituent of the hemolymph.
- Serum Amyloid P-component (SAP), a precursor of amyloid component P which
is found in basement membrane and is associated with amyloid deposits.
- Hamster female protein (FP), a plasma protein whose concentration is
altered by sex steroids and stimuli that elicit an acute phase response.
and long pentraxins:
- Human PTX3 (or TSG-14). PTX3 is a cytokine-induced protein.
- Vertebrate PTX4 .
- Guinea pig apexin , a sperm acrosomal protein. Apexin seems to be the
ortholog of human neuronal pentraxin II (gene NPTX2) .
- Rat neuronal pentraxin I .
The pentraxin domain has also been found in multidomain proteins, such as:
- Mammalian polydom, an extracellular protein which includes an N-terminal
von Willebrand factor A domain (see <PDOC50234>), hyalin repeats (HYRs)
(see <PDOC50825>), epidermal growth factor repeats (EGFs) (see
<PDOC00021>), sushi domains (see <PDOC50923>), and a single pentraxin
- Vertebrate adhesion G-protein–coupled receptors (GPRs), in particular
GPR144, GPR112, and GPR126.
The function of these proteins has not been defined yet, nor has the role of
the pentraxin domain in multidomain proteins.
The pentraxin domain consists of two anti-parallel β-sheets in the form of
a flattened β-barrel with a jellyroll topology (see <PDB:3KQR>) [7,8].
The pentraxin domain is related to the laminin G (LamG) (see <PDOC50025>) and
thrombospondin N-terminal (TspN) domains .
The sequences of the different members of this family are quite conserved. As
a signature, we selected a six residue pattern which includes a cysteine
known to be involved in a disulfide bridge in CRPs and SAP. We have also
developed a profile that covers the entire pentraxin domain.
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