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Phosphorylation on tyrosine residues is a key signal transduction mechanism known to regulate intercellular and intracellular communication in multicellular organisms. Despite the lack of conventional tyrosine kinases in the genome of the single cell organism Trypanosoma brucei, phosphorylation on trypanosomal protein tyrosine residues has been reported for this parasite. However, the identities of most of the tyrosine-phosphorylated proteins and their precise site(s) of phosphorylation were unknown. Here, we have applied a phosphotyrosine-specific proteomics approach to identify 34 phosphotyrosine-containing proteins from whole-cell extracts of procyclic form T. brucei. A significant proportion of the phosphotyrosine-containing proteins identified in this study were protein kinases of the CMGC kinase group as well as some proteins of unknown function and proteins involved in energy metabolism, protein synthesis, and RNA metabolism. Interestingly, immunofluorescence microscopy using anti-phosphotyrosine antibodies suggests that there is a concentration of tyrosine-phosphorylated proteins associated with cytoskeletal structures ( basal body and flagellum) and in the nucleolus of the parasite. This localization of tyrosine-phosphorylated proteins supports the idea that the function of signaling molecules is controlled by their precise location in T. brucei, a principle well known from higher eukaryotes.
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