HIGH RESOLUTION CRYSTAL STRUCTURE OF A HISTAMINE-BINDING PROTEIN FROM THE BROWN EAR TICK, RHIPICEPHALUS APPENDICULATUS.

P.L. Adams¹ , G. C. Paesen² , K. Harlos¹, P. A. Nuttal², D.I. Stuart¹

¹Laboratory of Molecular Biophysics, University of Oxford South Parks Road, Oxford OX1 3QT, UK.
²NERC Institute of Virology and Environmental Microbiology, Mansfield Road, Oxford OX1 3SR, UK

We have discovered a family of histamine-binding proteins (HBP) in salivary glands of the brown ear tick, Rhipicephalus appendiculatus. Unlike the high-affinity histamine receptors found in vertebrates, these proteins do not form part of the cell membranes. They are secreted in the tick saliva as it feeds on the blood of its host. So far we have identified three proteins which have a high degree of sequence identity. Two of the proteins (FS-HBP1 and FS-HBP2) are major, female-specific products, that are only detected in the early stage of adult feeding. The third protein (MS-HBP) is secreted by male ticks throughout their feeding period. FS-HBP2 was crystallised in spacegroup p212121 in the presence of histamine, and its structure was determined to 1.25 resolution. The structure was revealed to be an eight stranded anti-parallel beta-barrel which contains two histamine binding sites. The HBP are new members of the lipocalin super family, a class of proteins which are involved in the transport of small hydrophobic molecules. This is also the first structure for this class of proteins which has been shown to contain two ligand binding sites. Furthermore, soaking experiments have shown that the two binding sites differ in their affinity for histamine. A structure containing histamine in the high affinity site only was determined at 1.3 resolution. The HBP family have several features which set them apart from other lipocalins and fit them to this particular biological role.