In this work, we report the cloning and characterization of the first cell surface
casein kinase II (CKII) substrate (Tc-1) of Trypanosoma cruzi, the causative agent of
Chagas' disease. Analysis of the gene sequence revealed a 1,653-bp open reading frame coding for 550
amino acid residues. Northern blot analysis showed a 4.5-kb transcript that is expressed in invasive trypomastigotes but not in noninvasive epimastigote forms of T. cruzi. Southern blot analysis indicates that Tc-1 is a single-copy gene. At the
amino acid level, Tc-1 displayed 95% and 99% identity to two hypothetical
proteins recently reported by the T. cruzi genome project. Analysis of the translated amino acid sequence indicates that the Tc-1 gene has a putative transmembrane domain with multiple cytoplasmic and extracellular CKII phosphosites. Exogenous human CKII was able to phosphorylate
serine residues on both recombinant Tc-1 and Tc-1 of intact trypomastigotes. This phosphorylation was inhibited by the CKII inhibitors
heparin and 4,5,6,7,-tetrabromo-2-azabenzimidazole. Immunoblots of solubilized trypomastigotes, epimastigotes, and amastigotes probed with anti-recombinant Tc-1
immunoglobulin G revealed a 62-kDa
protein that is expressed only in infective trypomastigotes. Immunoprecipitation of labeled
surface proteins of trypomastigotes indicated that the 62-kDa
protein is a
surface protein, and we found that the
protein is uniformly distributed on the surface of trypomastigotes by direct immunofluorescence.
Antibodies to Tc-1 effectively blocked trypomastigote invasion of host cells and consequently reduced parasite load. Preincubation of either trypomastigotes or myoblasts with CKII inhibitors blocked T. cruzi
infection. Thus, for the first time, we describe a cell surface CKII substrate of a protozoan parasite that is phosphorylated by human CKII and that is involved in cellular
infection.