THE OV20 PROTEIN OF THE PARASITIC NEMATODE ONCHOCERCA-VOLVULUS - A STRUCTURALLY NOVEL CLASS OF SMALL HELIX-RICH RETINOL-BINDING PROTEINS

Ov20 is a major antigen of the parasitic nematode Onchocerca volvulus, the causative agent of river blindness in humans, and the protein is secreted into the tissue occupied by the parasite, DNA encoding Ov20 w as isolated, and the protein was expressed in Escherichia coli, Fluore scence-based ligand binding assays show that the protein contains a hi gh affinity binding site for retinol, fluorescent fatty acids ethylami no-naphthalene-1-sulfonyl)amino)undecanoic acid, dansyl-DL-alpha-amino caprylic acid, and parinaric acid) and, by competition, oleic and arac hidonic acids, but not cholesterol, The fluorescence emission of dansy lated fatty acids is significantly blue-shifted upon binding in compar ison to similarly sized beta-sheet-rich mammalian retinol-and fatty ac id-binding proteins, Secondary structure prediction algorithms indicat e that a alpha-helix predominates in Ov20, possibly in a coiled coil m otif, with no evidence of beta structures, and this was confirmed by c ircular dichroism, The protein is highly stable in solution, requiring temperatures in excess of 90 degrees C or high denaturant concentrati ons for unfolding, Ov20 there fore represents a novel class of small r etinol-binding protein, which appears to be confined to nematodes, The retinol binding activity of Ov20 could possibly contribute to the eye defects associated with onchocerciasis and, because there is no count erpart in mammals? rep resents a strategic target for chemotherapy.