As part of our ongoing project of identification of actin-binding proteins implicated in the cell transition (flagellate to amoeboid/adherent) of Trichomonas vaginalis, we have characterized an alpha-actinin-related protein in this parasite. The protein (P100) has a molecular mass of 100 kDa and an isoelectric point of 5.5. A monoclonal antibody raised against this protein co-localizes with the actin network. P100 gene transcripts are co-expressed with actin throughout the cell cycle. Analysis of the deduced protein sequence reveals three domains: an N-terminal actin-binding region; a central region rich in alpha-helix; and a C-terminal domain with Ca(2+)-binding capacity. Whereas the N- and C-terminal regions are well-conserved as compared to other alpha-actinins, we observe in the central region an atypical distribution of residues in five repeats. The sequence of the repeats does not show any homology with the rod domain of the other alpha-actinins, except for the first repeat which shows some similarity. The four other repeats of T. vaginalis P100 appear to result from a duplication event which is not detectable in the other sequences.