A major surface glycoprotein of trypanosoma brucei is expressed transiently during development and can be regulated post-transcriptionally by glycerol or hypoxia.

Differentiation is a means by which unicellular parasites adapt to different environments. In some cases, the developmental program may be modulated by interactions with the host, but the mechanisms are largely unknown. Trypanosoma brucei is transmitted between mammals by tsetse flies. The development of the procyclic form in the tsetse midgut is marked by the synthesis of a new glycoprotein coat, composed of EP and GPEET procyclins, that is important for survival. Here we demonstrate that the composition of the coat changes in response to extracellular signals in vitro and during development in vivo. EP and GPEET are coinduced when differentiation is initiated. Subsequently, EP expression is maintained, whereas GPEET is repressed after 7-9 days. The timepoint at which GPEET is repressed coincides with the appearance of parasites in a new compartment of the fly midgut. In culture, down-regulation of GPEET can be prevented by exogenous glycerol or accelerated by hypoxia. Regulation is post-transcriptional, and is conferred by the GPEET 3' untranslated region. The same sequence also regulates expression of a reporter gene in the fly. The finding that GPEET is expressed during a defined window during the establishment of infection suggests that it has a specific function in host-parasite interactions rather than a generalized role in shielding underlying membrane molecules.

Survival of Trypanosoma brucei in the tsetse fly is enhanced by the expression of specific forms of procyclin.

African trypanosomes are not passively transmitted, but they undergo several rounds of differentiation and proliferation within their intermediate host, the tsetse fly. At each stage, the survival and successful replication of the parasites improve their chances of continuing the life cycle, but little is known about specific molecules that contribute to these processes. Procyclins are the major surface glycoproteins of the insect forms of Trypanosoma brucei. Six genes encode proteins with extensive glutamic acid-proline dipeptide repeats (EP in the single-letter amino acid code), and two genes encode proteins with an internal pentapeptide repeat (GPEET). To study the function of procyclins, we have generated mutants that have no EP genes and only one copy of GPEET. This last gene could not be replaced by EP procyclins, and could only be deleted once a second GPEET copy was introduced into another locus. The EP knockouts are morphologically indistinguishable from the parental strain, but their ability to establish a heavy infection in the insect midgut is severely compromised; this phenotype can be reversed by the reintroduction of a single, highly expressed EP gene. These results suggest that the two types of procyclin have different roles, and that the EP form, while not required in culture, is important for survival in the fly.