SECRETION OF A NOVEL, DEVELOPMENTALLY-REGULATED FATTY-ACID-BINDING PROTEIN INTO THE PERIVITELLINE FLUID OF THE PARASITIC NEMATODE, ASCARIS-SUUM

Early development of the parasitic nematode, Ascaris suum, occurs insi de a highly resistant eggshell, and the developing larva is bathed in perivitelline fluid. Two-dimensional gel analysis of perivitelline flu id from infective larvae reveals seven major proteins; a cDNA encoding one of these, As-p18, has been cloned, sequenced, and protein express ed in Escherichia coli. The predicted amino acid sequence of As-p18 ex hibits similarities to the intracellular lipid-binding protein (iLBP) family including retinoid- and fatty acid-binding proteins (FABP), As- p18 is unusual in that it possesses a hydrophobic leader that is not p resent in the mature protein, the developmental regulation of its expr ession, and in terms of its predicted structure. Recombinant As-p18 is a functional FABP with a high affinity for both a fluorescent fatty a cid analog ((5-(dimethylamino)-1-naphthalenyl)sulfonyl)amino) undecano ic acid) and oleic acid, but not retinol. Circular dichroism of rAs-p1 8 reveals a high beta-sheet content (62%), which is consistent with se condary structure for the protein predicted from sequence algorithms, and the structure of iLBPs, Unusual features are apparent in a structu ral model of As-p18 generated from existing crystal structures of iLBP s. As-p18 is not found in unembryonated eggs, begins to be synthesized at about day 3 of development, reaches a maximal concentration with t he formation of the first stage larva and remains abundant in the peri vitelline fluid of the second-stage larva. Since As-p18 is not present in the post-infective third-stage larva or adult worm tissues, it app ears to be exclusive to the egg. Surprisingly, however, Northern blot analysis yields mRNA for As-p18 not only in the early larval stages, b ut also the unembryonated egg, third-stage larvae, and ovaries of adul t worms, even though the protein is not detectable from any of those s ources. As-p18 may play a role in sequestering potentially toxic fatty acids and their peroxidation products, or it may be involved in the m aintenance of the impermeable lipid layer of the eggshell.