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that much of the loop structure of classical Kunitz domains, including the protruding protease-binding loop, has been replaced by beta-strands.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.

Original publication




Journal article


Journal of molecular biology

Publication Date





734 - 747


CEH Oxford, Mansfield Road, Oxford OX1 3SR, UK. gcp@ceh.ac.uk


Cell Line, Saliva, Animals, Humans, Rhipicephalus, Aprotinin, Ion Channels, Crystallography, X-Ray, Patch-Clamp Techniques, Sequence Alignment, Evolution, Molecular, Phylogeny, Amino Acid Sequence, Protein Structure, Secondary, Protein Structure, Tertiary, Sequence Homology, Amino Acid, Dimerization, Models, Molecular, Molecular Sequence Data