Hemolymph proteins in ticks.

In comparison to insects and Crustacea, our knowledge of the predominant hemolymph proteins in ticks is minimal. The hemolymph protein most studied in ticks has been vitellogenin (Vg). Vg is synthesized by the tick fat body after female adults obtain a blood meal, is released into the hemolymph and is absorbed by developing oocytes as vitellin (Vn). Much of what we know about Vg is from studies of Vn. In general, the carbohydrate, lipid and amino acid composition is similar to insects except that in the tick, Vg contains heme, most likely from the digestion of host hemoglobin. In the American dog tick, Dermacentor variabilis, Vg is comprised of two native proteins and seven subunits on SDS-PAGE. Vg has been characterized in five tick species but the amino acid sequence is not yet available. Another predominant hemolymph protein, apparently a carrier protein (CP), has recently been studied in two tick species. This protein is found in the hemolymph of both male and females adults, in adult tissues outside of the hemolymph in some tick species, in coxal fluid of soft ticks and in whole body homogenates from eggs, larvae and nymphs. CP from the hard tick, D. variabilis, contains cholesterol, phospholipids, monoacylglycerides, triacylglycerides, free fatty acids, carbohydrate and heme. Under identical assay conditions, the analogous protein in the soft tick, Ornithodoros parkeri, did not contain heme. CP in the American dog tick consists of two subunits, one of which has 61% identity to the biliprotein, artemocyanin, from the fairy shrimp. CP is identical to a heme-lipoprotein (HeLp) from Boophilus microplus. The exact roles of CP and HeLp have not yet been fully determined, but they apparently are important in heme sequestration and as a storage depot for protein and lipid. Macroglobulin, lectin, antimicrobial, JH binding, JH esterase, and other tick hemolymph proteins are also discussed.