title: An ion-channel modulator from the saliva of the brown ear tick has a highly modified Kunitz/BPTI structure.
abstract: 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that much of the loop structure of classical Kunitz domains, including the protruding protease-binding loop, has been replaced by beta-strands.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.
authors.affiliation: CEH Oxford, Mansfield Road, Oxford OX1 3SR, UK. gcp@ceh.ac.uk
authors.forename: Guido C
authors.initials: GC
authors.lastname: Paesen
authors.forename: Christian
authors.initials: C
authors.lastname: Siebold
authors.forename: Mark L
authors.initials: ML
authors.lastname: Dallas
authors.forename: Chris
authors.initials: C
authors.lastname: Peers
authors.forename: Karl
authors.initials: K
authors.lastname: Harlos
authors.forename: Patricia A
authors.initials: PA
authors.lastname: Nuttall
authors.forename: Miles A
authors.initials: MA
authors.lastname: Nunn
authors.forename: David I
authors.initials: DI
authors.lastname: Stuart
authors.forename: Robert M
authors.initials: RM
authors.lastname: Esnouf
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D007611:Aprotinin / Q000737:chemistry* / Q000145:classification / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D018360:Crystallography, X-Ray; D019281:Dimerization; D019143:Evolution, Molecular; D006801:Humans; D007473:Ion Channels / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D018408:Patch-Clamp Techniques; D010802:Phylogeny; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D048494:Rhipicephalus / Q000737:chemistry*; D012463:Saliva / Q000737:chemistry*; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
chemical_list: D007473:Ion Channels; D007611:Aprotinin