Expression of a polypeptide containing a dipeptide repeat is confined to the insect stage of Trypanosoma brucei

The protozoan parasite Trypanosoma brucei is transmitted between mammalian hosts by the tsetse fly (Glossina spp.). Trypanosomes ingested by the fly undergo a number of changes in the insect midgut during differentiation to procyclic forms. These include the loss of the variant specific glycoprotein (VSG) coat and the appearance of a common set of procyclic surface antigens. In order to investigate genes other than VSG genes which are expressed only at certain stages of the life cycle, the first cDNA specific to procyclic culture form trypanosomes (equivalent to the stage found in the insect midgut) has been characterized. The encoded polypeptide shows several characteristics of membrane proteins, but its most striking feature is the presence of a repetitive amino-acid sequence in which there are 22 tandem repeats of the dipeptide-Glu-Pro-. Related genes are also found in other trypanosome species and in leishmania. This gene shows many similarities to a number of surface antigen genes described in malaria and, more recently, Trypanosoma cruzi. This is the first example of a repetitive sequence in a parasite protein which is present only in the insect vector, and which therefore cannot be implicated in the mammalian host immune response.