Molecular characterization of complement factor I reveals constitutive expression in channel catfish

The complement system in vertebrates plays a crucial role in immune defense via recognition and removal of pathogens. Complement is tightly regulated by a group of both soluble and cell-associated proteins. Complement factor I is a soluble serine protease that regulates multiple pathways in complement activation. In this work, a complement factor I transcript was isolated and sequenced from channel catfish (Ictalurus punctatus) liver after screening expressed sequence tags. The full-length cDNA is comprised of 2284 bp in length, encoding a polypeptide of 668 amino acids. The complement factor I protein was found to be well conserved, with similar domain structures and architecture from fish to mammals. The catfish complement factor I exists as a single-copied gene in the catfish genome. Expression analysis revealed that the catfish complement factor I is constitutively expressed in all tissues and leukocyte cell lines tested, indicating its importance as a regulatory enzyme throughout channel catfish. While expression of complement factor I is often found to be in the liver in mammals, it is constitutively expressed in channel catfish and carp throughout in various tissues and organs.