title: Complete cDNA sequence of SAP-like pentraxin from Limulus polyphemus: implications for pentraxin evolution.
abstract: The serum amyloid P component (SAP)-like pentraxin Limulus polyphemus SAP is a recently discovered, distinct pentraxin species, of known structure, which does not bind phosphocholine and whose N-terminal sequence has been shown to differ markedly from the highly conserved N terminus of all other known horseshoe crab pentraxins. The complete cDNA sequence of Limulus SAP, and the derived amino acid sequence, the first invertebrate SAP-like pentraxin sequence, have been determined. Two sequences were identified that differed only in the length of the 3' untranslated region. Limulus SAP is synthesised as a precursor protein of 234 amino acid residues, the first 17 residues encoding a signal peptide that is absent from the mature protein. Phylogenetic analysis clusters Limulus SAP pentraxin with the horseshoe crab C-reactive proteins (CRPs) rather than the mammalian SAPs, which are clustered with mammalian CRPs. The deduced amino acid sequence shares 22% identity with both human SAP and CRP, which are 51% identical, and 31-35% with horseshoe crab CRPs. These analyses indicate that gene duplication of CRP (or SAP), followed by sequence divergence and the evolution of CRP and/or SAP function, occurred independently along the chordate and arthropod evolutionary lines rather than in a common ancestor. They further indicate that the CRP/SAP gene duplication event in Limulus occurred before both the emergence of the Limulus CRP variants and the mammalian CRP/SAP gene duplication. Limulus SAP, which does not exhibit the CRP characteristic of calcium-dependent binding to phosphocholine, is established as a pentraxin species distinct from all other known horseshoe crab pentraxins that exist in many variant forms sharing a high level of sequence homology.
authors.affiliation: School of Life Sciences, Keele University, Keele, Staffordshire, ST5 5BG, UK.
authors.forename: Hazel A
authors.initials: HA
authors.lastname: Tharia
authors.forename: Annette K
authors.initials: AK
authors.lastname: Shrive
authors.forename: John D
authors.initials: JD
authors.lastname: Mills
authors.forename: Chris
authors.initials: C
authors.lastname: Arme
authors.forename: Gwyn T
authors.initials: GT
authors.lastname: Williams
authors.forename: Trevor J
authors.initials: TJ
authors.lastname: Greenhough
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000235:genetics; D019143:Evolution, Molecular; D020131:Genes, Duplicate / Q000235:genetics; D006737:Horseshoe Crabs / Q000737:chemistry / Q000235:genetics*; D006801:Humans; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D010455:Peptides / Q000235:genetics; D010802:Phylogeny; D016415:Sequence Alignment; D000683:Serum Amyloid P-Component / Q000737:chemistry* / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism
chemical_list: D018076:DNA, Complementary; D010446:Peptide Fragments; D010455:Peptides; D000683:Serum Amyloid P-Component; C019529:polyalanine