C9, a component of the membrane attack complex, participates in the final stage of the complement
cascade which lyses foreign organisms by disrupting the integrity of their cell membranes. In the present study, a
full-length ayu C9 (aC9) cDNA was cloned which contains 2,125 nucleotides and encodes a protein of 592 amino acids.
A signal peptide was deposited in the N-terminal 22 residues. The deduced amino acid sequence of aC9 showed 56.8%
identity to the C9 of rainbow trout, and 40.9% to 53.8% identity to the C9 of other teleosts. RT-PCR analysis
demonstrated that the mRNA of aC9
was expressed in the liver, spleen, intestine, gill and muscle of healthy ayu fish with
the highest level in the liver. Quantitative RT-PCR analysis showed that aC9
transcripts were significantly up-regulated
in the liver at 4 h post Listonella anguillarum
infection, peaked at 16 h post injection. Western blotting analysis revealed
that serum aC9 significantly increased in Listonella anguillarum
infected ayu fish. Our results suggested that aC9 may
play an important role in fish immune response of anti-bacteria.