An ion-channel modulator from the saliva of the brown ear tick has a highly modified Kunitz/BPTI structure

Ra-KLP, a 75 amino acid protein secreted by the salivary gland of the brown ear tick Rhipicephalus appendiculatus has a sequence resembling those of Kunitz/BPTI proteins. We report the detection, purification and characterization of the function of Ra-KLP. In addition, determination of the three-dimensional crystal structure of Ra-KLP at 1.6 A resolution using sulphur single-wavelength anomalous dispersion reveals that much of the loop structure of classical Kunitz domains, including the protruding protease-binding loop, has been replaced by beta-strands. Even more unusually, the N-terminal portion of the polypeptide chain is pinned to the "Kunitz head" by two disulphide bridges not found in classical Kunitz/BPTI proteins. The disulphide bond pattern has been further altered by the loss of the bridge that normally stabilizes the protease-binding loop. Consistent with the conversion of this loop into a beta-strand, Ra-KLP shows no significant anti-protease activity; however, it activates maxiK channels in an in vitro system, suggesting a potential mechanism for regulating host blood supply during feeding.