Crystallization and preliminary X-ray diffraction analysis of HeLp, a heme lipoprotein from the hemolymph of the cattle tick Boophilus microplus

The main protein present in the hemelymph of the cattle tick Boophilus microplus is a lipoprotein able to bind heme (HeLp). It has an apparent molecular weight of 354 000 Da and is composed of two polypeptide chains found in stoichiometric amounts. It contains 33 % lipids. The protein was crystallized using the hanging-drop vapour-diffusion method in the presence of 1,6-hexanediol as a precipitant. X-ray diffraction data were collected to 2.1 Angstrom resolution using a synchrotron-radiation source. The crystal belongs to the triclinic space group P1, with unit-cell parameters a = 90.58, b = 105.50, c = 116.14 Angstrom, alpha = 112.40, beta = 111.64, gamma = 91.35degrees. Owing to the lack of information about the amino-acid sequence, the structure of HeLp will be solved by the use of heavy atoms. Several possible derivatives have been collected and analysis is under way.