Two distinct groups of mucin-like genes are differentially expressed in the developmental stages of Trypanosoma cruzi.

Sialic acid acceptors of Trypanosoma cruzi are abundant mucin-like glycoproteins linked to the parasite membrane by a glycosylphosphatidyl inositol (GPI) anchor. They are heterogeneous and variable in different parasite stages. The protein portion of these mucins contains many threonine residues, and is thought to be encoded by a heterogeneous gene family. To investigate whether the high degree of heterogeneity in the mucin gene family is responsible for the diversity of mucins expressed on the parasite surface, we have studied the expression of mucin genes in several developmental stages of T. cruzi. We have found that mucins are expressed in all parasite stages. By using conserved sequences at 3' end of translated sequences of the gene family and the splice leader sequence, we have isolated 120 mucin-like cDNAs by RT-PCR from epimastigote and trypomastigote mRNAs. All transcribed genes contain conserved 5' and 3' regions, which code for the signal peptide, the sequence for GPI anchor addition, and a conserved domain rich in threonine residues. The internal portions of these genes are highly variable in size and sequence, and can be grouped in two major categories. One group contains KP(1-2)T(6-8) repeats, a motif found in mammalian mucins in the central region. This group is expressed preferentially in the trypomastigote forms ready to be released from the infected mammalian cell. The other has highly variable sequences in the central portion, and is expressed in all parasite stages. Because the number of synonymous substitutions is equivalent to the non-synonymous substitutions in the second group, they are probably evolving neutrally. On the other hand, the KP(1-2)T(6-8) containing genes have more synonymous substitutions and are most likely under a strong selective pressure. We propose that the group of KP(1-2)T(6-8) motif corresponds to the highly glycosylated mucins of the trypomastigote stages. In the other group proteolysis may remove the central domain yielding small mucins, such as the mucins found in insect derived stages of T. cruzi.