THE CRYSTAL-STRUCTURE OF CRUZAIN - A THERAPEUTIC TARGET FOR CHAGAS-DISEASE

Trypanosoma cruzi, a protozoan parasite, is the etiologic agent of Ame rican trypanosomiasis or Chagas' disease. Chagas' disease afflicts mor e than 24 million individuals in South and Central America producing a debilitating life-long disease. It is the leading cause of heart fail ure in many Latin American countries. Currently, there is no satisfact ory treatment for this parasitic infection. Cruzain (also known as cru zipain, gp 57/51), the major cysteine protease present in T. cruzi, is critical for the development and survival of the parasite within the host cells, making this enzyme a target for potential trypanocidal dru gs. Here we report the X-ray crystal structure of cruzain complexed wi th the potent inhibitor Z-Phe-Ala-fluoromethyl ketone. The structure w as determined at 2.35 Angstrom (R(cryst) = 0.15) by molecular replacem ent using a modified papain as the search model. The refined structure is compared to papain. Features which distinguish cruzain from papain are discussed since they may aid in the design of specificity inhibit ors. Fluorescence microscopy shows that a biotinylated form of the bou nd inhibitor does not effectively reach host proteases in their lysoso mal compartment, but is selectively taken up by the parasite. The inhi bitor greatly reduces parasitemia in a cell culture system, without ad verse effects to mammalian cells. This biological selectivity can be e xploited, in conjunction with unique active site features revealed by the crystal structure, to develop chemotherapy for Chagas' disease.