The Ov20 Protein of the Parasitic Nematode Onchocerca volvulus

Abstract

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 was isolated, and the protein was expressed in Escherichia coli. Fluorescence-based ligand binding assays show that the protein contains a high affinity binding site for retinol, fluorescent fatty acids (11-((5-dimethylaminonaphthalene-1-sulfonyl)amino)undecanoic acid, dansyl-DL-␣-aminocaprylic acid, and parinaric acid) and, by competition, oleic and arachidonic acids, but not cholesterol. The fluorescence emission of dansylated fatty acids is significantly blue-shifted upon binding in comparison to similarly sized ␤-sheet-rich mammalian retinoland fatty acid-binding proteins. Secondary structure prediction algorithms indicate that a ␣-helix predominates in Ov20, possibly in a coiled coil motif, with no evidence of ␤ structures, and this was confirmed by circular dichroism. The protein is highly stable in solution, requiring temperatures in excess of 90°C or high denaturant concentrations for unfolding. Ov20 therefore represents a novel class of small retinol-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 counterpart in mammals, represents a strategic target for chemotherapy.

Hydrophobic Ligand Binding by Zn-α2-glycoprotein, a Soluble Fat-depleting Factor Related to Major Histocompatibility Complex Proteins

“…A blue shift to 512 nm with DAUDA is substantial, but is less than that observed upon DAUDA binding to liver or intestinal fatty acid binding proteins (496 and 492 nm, respectively) (29,30), tear lipocalin (490 nm) (31); the most extreme blue shifts in DAUDA emission have been recorded upon interaction with unusual lipid-binding proteins from nematode worms (475 and 485 nm) (32)(33)(34). The relatively small blue shift when DAUDA interacts with ZAG is perhaps indicative of a binding site that is more solvent-exposed than in lipid transporter proteins, such as would be expected for an open-sided binding groove.…”

Fatty Acid- and Retinoid-binding Proteins Have Distinct Binding Pockets for the Two Types of Cargo

“…For Ce-FAR-7, retinol cannot use cavity P1, because of the more bulky and rigid isoprenoid chain. Nevertheless other FARs do bind retinol with an affinity similar to that for fatty acids (15,16,18,19), which means that P2 in other FARs might be better matched to these ligands. That the binding pockets for fatty acids and retinol are distinct (Fig.…”

“…Both groups are allergens and are generally secreted from the parasite into the host tissues (13)(14)(15). There are no available three-dimensional structural data, but circular dichroism (CD) measurements and secondary structure predictions suggest these proteins are predominantly ␣-helical.…”

“…Their importance for lipid metabolism, their antigenic properties, and the structural difference from their host FABP proteins makes them an interesting target for structural work. The first described FAR family member was Ov-FAR-1 from the filarial agent Onchocerca volvulus, which causes human river blindness (15).…”

The Highly Abundant Protein Ag-lbp55 from Ascaridia galli Represents a Novel Type of Lipid-binding Proteins

“…The first representatives were identified as major nematode antigens (As-NPA-1 from Ascaris suum and Ov-FAR-1 from Onchocerca volvulus). They show a high affinity for amphipathic molecules, such as fatty acids and retinoids, and these features appear to be true for all members of both families (12)(13)(14). The completion of the genome from the model nematode Caenorhabditis elegans revealed eight FAR protein sequences, Ce-FAR-1 to 8 (11).…”