Structure–function analysis defines the minimal functional C-terminal domain of the variant surface glycoprotein of Trypanosoma brucei

In their mammalian hosts, African trypanosomes abundantly express GPI-anchored variant surface glycoproteins (VSGs) on their cell surfaces. These provide a protective surface coat that has been studied best in Trypanosoma brucei. The genome of this single-celled parasite contains more than 2000 VSG genes and pseudogenes, a rich foundation based on which only one functional VSG is expressed at any given time. This allows coat exchange by antigenic variation which is an elegant means of repeatedly evading the immune response of the mammalian host. All proteins of the VSG family are composed of a larger, elongated N-terminal domain that is most exposed and a smaller C-terminal domain that is sandwiched between the N-terminal domain and the GPI-anchor, which connects the protein to the outer leaflet of the plasma membrane. While sequence variability in the N-terminal domain of different members of the VSG family is essential for antigenic variation, the role of the C-terminal domain remains less clear. Additionally, other species, such as Trypanosoma congolense and Trypanosoma vivax, do not possess a similarly structured C-terminal domain in their VSGs. Here, we systematically mutated the C-terminal domain of selected T. brucei VSGs and defined a minimal domain required for VSG function. We show that the size of the minimal C-terminal domain resembles that of T. congolense VSGs, and structured regions are not essential. We further propose that the evolutionary pressure to conserve the build of the C-terminal domain is related to functions beyond protein structure.