CLONING AND CHARACTERIZATION OF A CDNA REPRESENTING A PUTATIVE COMPLEMENT-REGULATORY PLASMA-PROTEIN FROM BARRED SAND BASS (PARABLAX NEBLIFER)

It has been demonstrated previously that plasma from a number of verte brate species including the phylogenetically old barred sand bass poss esses molecules that cleave the alpha'-chain of the activated third (C 3b) and fourth (C4b) components of the human complement system. A spec ific protease and a cofactor protein were identified to be responsible for this cleavage. The cofactor activity in sand bass correlated with a 110 kDa polypeptide chain of a 360 kDa plasma protein. The evolutio nary conservation was probed at the cDNA level and subsequently a cDNA clone of barred sand bass was isolated that represents a protein with structural similarity to mammalian complement regulatory proteins. Th e cDNA (SB1) was identified by immuno-screening of a sand bass liver e xpression library using affinity-purified IgG antibodies raised agains t the isolated 110 kDa material. The cDNA is 3397 bp in size and the o pen reading frame represents a protein of 1053 amino acid residues wit h a hydrophobic signal peptide indicative of a secreted protein. The c alculated mass of the mature protein (SBP1) is 115.2 kDa which is in g ood agreement with the molecular mass of 110 kDa determined for the sa nd bass serum protein. Similarly to mammalian complement-regulatory pr oteins, the protein deduced from the sand bass cDNA is organized into short consensus repeats (SCR). It consists of 17 SCRs, of which SCRs 2 , 12 and 16 exhibit significant homology to SCRs 2, 15 and 19 of human factor H, and SCRs 11, 12 and 13 have homology to SCRs 1, 2 and 3 of human C4b-binding protein. For the first time a complete cDNA represen ting a putative complement-regulatory protein which is structurally re lated to mammalian complement proteins has been isolated from a bony f ish.