T. cruzi does not synthesize sialic acid, but the infective trypomastigotes contain an unusual trans-sialidase (TS) on their surface membranes. Within seconds after leaving host cells to enter the blood stream, the trypomastigote surface glycoproteins are sialylated. This reaction leads to the assembly of the Ssp-3 epitope, which plays a essential role in the adhesion and subsequent invasion of other host cells. The TS does not employ CMP-NeuAc, the normally used donor for sialic acid transfer reactions. Such a TS has not been described in mammalian cells, making the enzyme a potential target for chemotherapy. Independently of the possible role of TS in the biology of T. cruzi and pathology of Chagas' disease, the Ts is of particular interest to carbohydrate biochemists. In this proposal we describe experimental approaches: 1) to clarify the structure and function of the TS, and its relationship to a previously described T. cruzi sialidase. We will: a) clone and express the DNA coding for the TS gene(s) and assay the activity of the product(s) for TS and sialidase activities; b) titrate TS and sialidade in extracts of various strains of T. cruzi, to determine whether the two enzymatic activities are closely correlated, or vary independently of each other; c) isolate the two activities from crude trypomastigote extracts, and verify whether they co-purify; d) study some properties of the isolated enzyme(s). 2) to determine the role of the sialic acid-containing molecules in target cell invasion, and in the protecting trypomastigotes from lysis by complement. 3) to verify whether the sialylated molecules can be ligands for members of the LECCAM family of host receptors, and play a role in the parasite migration in the mammalian host's tissues. 4) to characterize the structure of the oligosaccharides which are sialylated by the TS, and specifically, of L)those oligosaccharides of the Ssp-3-bearing surface molecules.
