Vipoxin is a neurotoxic postsynaptic heterodimeric complex from the venom of Vipera ammodytes meridionalis, the most toxic snake in Europe. It consists of a basic and highly toxic phospholipase A(2) and an acidic non-toxic protein inhibitor. The two polypeptide chains have the same chain length and share 62% amino-acid identity. Vipoxin is a unique example of evolution of the catalytic and toxic phospholipase A(2) functions into inhibitory and non-toxic functions. The crystal structure of the complex has been determined by the molecular-replacement method and refined to 1.4 A resolution to an R factor of 18.2%. The complex formation decreases the accessible surface area of the two subunits by approximately 1480 A(2), which results in a reduction of toxicity and catalytic activity. The catalytic and substrate-binding sites of the vipoxin phospholipase A(2) are identical or similar to those of other group I/II enzymes. Two 2-methyl-2,4-pentanediol molecules are present in the hydrophobic channel close to the active site. The two subunits lack calcium ions. The negatively charged Asp49 of the phospholipase A(2), which participates in the Ca(2+)-binding sites of other snake-venom phospholipase A(2)s, is neutralized by the side chain of Lys69 from the inhibitor. Attempts have been made to identify the toxicity region and to explain the reduced catalytic activity and toxicity of the phospholipase A(2) subunit.

Recently, we described an 80-kDa lipopolysaccharide (LPS)-binding membrane protein to be identical to CD55 [decay accelerating factor (DAF)]. Here, we demonstrate that CD55 is able to contribute to lipopolysaccharide (LPS) signaling. Transfection of Chinese hamster ovary (CHO) cells with human CD55 resulted in a translocation of NF-‹ B after stimulation with LPS as well as with free lipid A. In addition, interaction of lipid A and CD55 was shown by co-immuno-precipitation of these molecules from CHO-CD55 cells after incubation with lipid A and anti-lipid A monoclonal antibody, as well as by fluorescence resonance energy transfer (FRET) analysis in human monocytes. The comparison of LPS-induced signaling pathways in CHO-CD55 and CHO-CD14 cells revealed that p38, JNK and ERK MAP kinases are activated upon LPS stimulation in both cell lines, and that the activation by LPS can be blocked at the level of Toll-like receptor 4. Finally, through FRET analysis we could demonstrate LPS-induced clustering of CD55 and CD11/CD18 in human monocytes. Our results imply a new functional role of CD55 as a member of a multimeric LPS receptor complex.

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