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* Laboratory of Marine Biochemistry, Department of Bioscience and Biotechnology, Kyushu University, Hakozaki, Fukuoka, Japan;
Institute of Glycotechnology and Department of Applied Biochemistry, Tokai University, Hiratsuka, Kanagawa, Japan; and
Department of Biochemistry, Fukushima Medical University School of Medicine, and Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Fukushima, Japan
The lectin pathway of complement is considered to be the most ancient complement pathway as inferred from identification of ancient homologs of mannose-binding lectin (MBL) and MBL-associated serine proteases (MASPs) in some invertebrates. MBL homologs with galactose selectivity and an MASP3-like sequence also occur in bony fish, linking the evolution of the lectin complement pathway from invertebrates to higher vertebrates. However, these cannot be considered authentic complement components until confirmatory functional evidence is obtained. Here, we report the isolation and characterization of two MBL homologs from a cyprinid teleost, the common carp, Cyprinus carpio. One, designated GalBL, corresponds to the MBL-like molecule with the galactose specificity. The other is an authentic MBL with mannose specificity. Both were found to associate with a serine protease that cleaves native human C4 into C4b but not C4i with a hydrolyzed thioester. Molecular cloning and phylogenetic analysis revealed this C4-activating protease to be carp MASP2, indicating that MASP2 arose before the emergence of bony fish. Database mining of MBL-like genes reveals that MBL and GalBL genes are arranged in tandem in the zebrafish genome and that both lectins are conserved in the distantly related puffer fish. These results imply that bony fish have developed a diverged set of MBL homologs that function in the lectin complement pathway.
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1 This work was supported in part by grants from the Ministry of Education, Science, Sports, and Technology of Japan.
2 The nucleotide sequence data reported in this paper are available in the DNA Database of Japan, European Molecular Biological Laboratory, and GenBank databases with the following accession numbers: AB110825 for MBL1, AB110826 for MBL2 (clone 3), AB110827 for MBL2 (clone 10), and AB234294 for carp MASP2.
3 Address correspondence and reprint requests to Dr. Miki Nakao, Department of Bioscience and Biotechnology, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan. E-mail address: miki_n{at}agr.kyushu-u.ac.jp
4 Abbreviations used in this paper: MBL, mannose-binding lectin; MASP, MBL-associated serine protease; CRD, carbohydrate recognition domain; GlcNAc, N-acetyl-D-glucosamine; GalBL, galactose-binding lectin with MBL-like structure; MetMan, methyl-
-D-mannopyranoside; ORF, open reading frame; UT, untranslated region; MRP, MASP-related protein; UPM, universal primer mix.
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