Molecular cloning and characterization of a complement-depleting factor from king cobra, Ophiophagus hannah

Cobra venom factor (CVF) is an anti-complement factor existing in cobra venom. CVF proteins have been purified from the venoms of Naja haje, Naja siamensis, Naja atra, Naja kaouthia, Naja naja, Naja melanoleuca and Austrelaps superbus, but only three full-length cDNA sequences of CVF are available. In the present work, a cobra venom factor termed OVF was purified from the crude venom of Ophiophagus hannah by successive gel filtration, ion-exchange and heparin affinity chromatography steps. The purified OVF was homogenous on the SDS-PAGE gel with an apparent molecular weight of 140 kDa under non-reducing conditions. Under reducing conditions, OVF was divided into three bands with apparent molecular weight of 72 kDa (α chain), 45 kDa (β chain) and 32 kDa (γ chain), respectively. OVF consumed complement components with anti-complement activity of 154 units per mg. By using Reverse transcription-PCR and 5'-RACE assay, the open reading frame of OVF was obtained. MALDI-TOF and protein sequencing assays confirmed the cloned cDNA coding for OVF protein. The cDNA sequence of OVF is conservative when aligned with that of other CVFs. Phylogenetic analysis revealed OVF is closer to CVF from N. kaouthia than to AVF-1 and AVF-2 from A. superbus. Our results demonstrated that OVF has its unique features as following: 1) The N-terminal amino acid sequence of OVF γ chain is different from that of other known CVFs, suggesting that the OVF γ chain might be further processed; 2) Unlike N. kaouthia CVF and A. superbus AVF-1, which have potential N-linked glycosylation sites located in both α and β chain, OVF only has N-linked glycosylation site in its α chain as revealed by Schiff's reagent staining and protein sequence analysis; 3) In addition to the 27 well conserved cysteine residues in all known CVFs, OVF have an additional cysteine residue in its γ chain. Understanding the importance of above mentioned specific characteristics might provide useful information on structure-function relationship between CVF and complement system.