RESUMO
Schistosomiasis is an inflammatory chronic disease that represents a major health problem in tropical and subtropical countries. The drug of choice for treatment, praziquantel, is effective in killing adult worms but fails to kill immature forms and prevent reinfection. One prominent antigen candidate for an anti-schistosomiasis vaccine is the protein Sm21.7 (184 amino-acid residues) from Schistosoma mansoni, a tegumental protein capable of reducing the worm burden in a murine immunization model. In the present work, the Sm21.7 gene was cloned and expressed in Escherichia coli and the full-length protein was purified to homogeneity. Crystals of recombinant Sm21.7 suitable for X-ray diffraction were obtained using PEG monomethyl ether 2000 as a precipitant. X-ray diffraction images of a native crystal (at 2.05â Å resolution) and a quick-cryosoaked NaI derivative (at 1.95â Å resolution) were collected on the W01B-MX2 beamline at the Laboratório Nacional de Luz Síncrotron (LNLS, Brazilian Synchrotron Light Laboratory/MCT). Both crystals belonged to the hexagonal space group P6122, with similar unit-cell parameters a=b=108.5, c=55.8â Å. SIRAS-derived phases were used to generate the first electron-density map, from which a partial three-dimensional model of Sm21.7 (from Gln89 to Asn184) was automatically constructed. Anaysis of dissolved crystals by SDS-PAGE confirmed that the protein was cleaved in the crystallization drop and only the Sm21.7 C-terminal domain was crystallized. The structure of the Sm21.7 C-terminal domain will help in the localization of the epitopes responsible for its protective immune responses, constituting important progress in the development of an anti-schistosomiasis vaccine.
