Discussion

African trypanosomes protect themselves from destruction by host defences by covering their surface with a dense coat of a single protein, the variant surface glycoprotein (VSG). The VSG and the coat it forms have been best characterized in Trypanosoma brucei. In this species the protein consists of a larger, elongated N-terminal domain that is exposed and a shorter, more secluded C-terminal domain consisting of one or two structured regions. Sequence variability in the N-terminal domain is fundamental for antigenic variation; how the C-terminal domain supports VSG functionality is however not clear.

Other species such as T. congolense and T. vivax also possess a VSG surface coat which presumably has the same protective function. Though similar, VSGs have evolved to differ from species to species, for instance T. congolense VSGs lack structured regions in their C-terminal domain and this is probably also the case for the T. vivax VSGs.

In order to address the compositional differences in terms of functionality we attempted to generate transgenic T. brucei cells expressing VSGs of either of the other species on their cell surface.  Whereas a T. congolense VSG could readily be expressed as the only VSG on T. brucei cells, this was not the case for the chosen T. vivax VSG. Supporting mutagenesis studies on T. brucei VSGs shed new light on the structure-function relationship in VSGs.