A SINGLE TYROSINE DIFFERENTIATES ACTIVE AND INACTIVE TRYPANOSOMA-CRUZI TRANS-SIALIDASES

Several genes encode members of the Trypanosoma cruzi (Tc) trans-sialidase (TS) family. These proteins contain an enzymatic domain on the N terminus, the only one required for TS activity, and an antigenic domain (SAPA (shed acute phase antigen) amino acid (aa) repeats) on the C terminus. Only some members of this glycoprotein family are enzymatically active. The complete sequence of two clones encoding the enzymatic domain of active and inactive protein from each of two Tc strains has now been obtained. Comparison of these sequences showed a limited divergence among them: 20 out of the 642 deduced aa in the enzymatic domain were found to differ. From these 20 aa, only one was found to be essential for enzymatic activity. A Tyr(342) residue is deduced in both active proteins while a His(342) is present in both inactive ones. This naturally occurring Tyr(342) --> His substitution completely abolished the TS activity. In addition to Tyr(342), second deduced aa, pro(231), was found to be necessary for full enzymatic TS activity; a pro(231) --> Ala change rendered the TS protein partially active. Fourteen aa residues, including Tyr(342), out of the 16 aa in the active site of asialidase from Salmonella typhimurium are present at the same or very similar positions in the Tc TS.