Characterization of the tick gut receptor that binds OspA. (a) Binding of OspA to different-sized TGE fractions. Tick gut extracts were subjected to size fractionation using Centricon concentrator tubes with molecular mass cutoffs of either 10, 30, 50, or 100 kDa. In each separate fractionation, black and gray bars represent the separated TGE containing the smaller and larger material, respectively. The means ± SD from two experiments are shown. (b) Effect of trypsin treatment on OspA binding to TGE. TGE was treated with soybean trypsin inhibitor (I), soybean trypsin inhibitor and trypsin (I + T), or trypsin (T). The trypsin-treated and control TGEs were then used to coat microtiter wells and probed with labeled OspA. The OD of the binding of labeled OspA to BSA is also shown as a control (B). In an additional series of experiments, marked “W,” TGE-coated microtiter wells were treated with soybean trypsin inhibitor, soybean trypsin inhibitor and trypsin, or trypsin. Bars represent the mean ± SD from three experiments.