Host/parasite interaction mediated by carbohydrate/ lectin recognition results in the attachment to and invasion of host cells and immunoregulation, enabling parasite replication and establishment of infection. Trypanosoma cruzi, the protozoan responsible for Chagas disease, expresses on its surface a family of enzymatically active and inactive trans-sialidases. The parasite uses the active trans-sialidase for glycoprotein sialylation in an unusual trans-glycosylation reaction. Inactive trans-sialidase is a sialic acid-binding lectin that costimulates host T cells through leucosialin (CD43) engagement. The co-mitogenic effect of trans-sialidase can be selectively abrogated by N-acetyllactosamine, suggesting the presence of an additional carbohydrate binding domain for galactosides, in addition to that for sialic acid. Here we investigated the interaction of inactive trans-sialidase in the presence of -galactosides. By using NMR spectroscopy, we demonstrate that inactive trans-sialidase has a -galactoside recognition site formed following a conformational switch induced by sialoside binding. Thus prior positioning of a sialyl residue is required for the -galactoside interaction. When an appropriate sialic acid-containing molecule is available, both sialoside and -galactoside are simultaneously accommodated in the inactive trans-sialidase binding pocket. This is the first report of a lectin recognizing two distinct ligands by a sequential ordered mechanism. This uncommon binding behavior may play an important role in several biological aspects of T. cruzi/host cell interaction and could shed more light into the catalytic mechanism of the sialic acid transfer reaction of enzymatically active trans-sialidase.Trypanosoma cruzi is the etiologic agent of Chagas disease or American trypanosomiasis that affects ϳ18 million people in Central and South America. Another 100 million people are at risk of infection (1). Mammalian cell invasion is crucial for T. cruzi survival (2). Elucidation of molecular components regulating the initiation of the parasitic infection is critical for understanding the pathogenesis of Chagas disease and will enable the development of novel, effective, and selective treatments.Initial communication between T. cruzi trypomastigotes and mammalian cells requires contact of soluble or membranebound parasite molecules with host ligands. T. cruzi expresses on its surface a family of glycosylphosphatidylinositol-anchored active and inactive trans-sialidase (TS) 1 proteins (3-5), which contain a Tyr or a His residue, respectively, at position 342 (4). The parasite uses active TS to sialylate its surface glycoproteins by a trans-sialidase reaction (6). Thus, TS activity is capable of extensively remodeling the T. cruzi cell surface by using host glycoconjugates as sialyl donor. Alternatively, the enzyme may sialylate host cell glycomolecules to generate receptors used by the trypanosome for adherence to and penetration of target cells. Results with sialic acid-deficient mutants of Chinese hamster ov...