An unexpected loss of domains in the conservative evolution ninth complement component in a higher teleost, Miichthys miiuy

The complement systems of fish are well developed and play an important role in the innate immune response. C9 is the ninth member of complement components, involved in creating the membrane attack complex (MAC). In the present study, a truncated C9 cDNA sequence encoding 461 amino acids was cloned and characterized in the miiuy croaker (Miichthys miiuy). Typical fish C9 molecules have five characteristic modular domains, i.e. TSP1, LDLRA, MACPF, EGF, and a second TSP domain which is absent in mammalian counterparts. While in miiuy croaker, this truncated C9 presents only TSP1, LDLRA and MACPF domains. It is the first time of finding a truncated C9 in teleost components. RT-PCR analysis detected these C9 transcripts among all tissues examined, demonstrating its constitutive expression pattern in healthy fish. The highest levels of transcripts were detected in liver of both healthy and pathogen-infected miiuy croaker. Its constitutive and inducible expression pattern of this truncated C9 in liver is similar to most complement components which belong to the type of acute-phase proteins and are in general of hepatic origin. We cannot exclude the possibility that miiuy croaker presents the typical C9 although it has not yet been found. The molecular evolutionary analysis showed that this truncated C9 of miiuy croaker had a significantly higher omega value comparing with other fish and the positive selection pressure had happened on it after its divergence with other fish.

► A truncated C9 component cDNA (MimiC9) was cloned from Miichthys miiuy.
► MimiC9 only presents the three domains on the N terminal, lack of the EGF and TSP2 domains.
► MimiC9 was highly expressed in liver.
► C9 of fish and mammals, including MimiC9, is under purifying selection pressure.