Complement factor I (FI) is a plasma serine proteinase that plays an essential role in the modulation of the complement cascade. In the presence of substrate modulating cofactors (Factor H, C4bp, CR1, etc), FI cleaves the activation products of C3 (i.e. C3b) and C4 (i.e. C4b) to limit complement activity. In this study, the full length cDNA of factor I (CpFI) is isolated from the liver of the whitespotted bamboo shark (Chiloscyllium plagiosum). The CpFI cDNA is 2326 bp in length, encoding a protein of 671 amino acids, which shares 72-80% identity with FI molecules of other sharks, higher than the teleosts (37-40%) and mammals (44-47%). The sequence alignment and comparative analysis indicates the FI proteins are well conserved, with the typical modular architecture and identical active sites throughout vertebrate evolution, suggesting the conserved function. However, the additional sequence present between the leader peptide (LP) and the factor I membrane attack complex (FIMAC) domain in other fishes is also found in CpFI, which consists of two kind of tandem repeats. Phylogenetic analysis suggests that CpFI belongs to the elasmobranch clade, in parallel with the higher vertebrates, to form a sister taxa to teleosts. Expression analysis revealed that CpFI is ubiquitously distributed in a variety of tissues, with the constitutive expression in liver, which might reflect the species-specific distribution patterns of FI. Together with earlier reports, the presence of FI in various sharks might suggest the existence of a well-developed complement regulation mechanism in cartilaginous fish.
Keywords: Chiloscyllium plagiosum; Complement; Factor I; Regulatory protein; Shark.
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