Structure of a Glycosylphosphatidylinositol-anchored Domain from a Trypanosome Variant Surface Glycoprotein

Abstract

The cell surface of African trypanosomes is covered by a densely packed monolayer of a single protein, the variant surface glycoprotein (VSG). The VSG protects the trypanosome cell surface from effector molecules of the host immune system and is the mediator of antigenic variation. The sequence divergence between VSGs that is necessary for antigenic variation can only occur within the constraints imposed by the structural features necessary to form the monolayer barrier. Here, the structures of the two domains that together comprise the C-terminal di-domain of VSG ILTat1.24 have been determined. The first domain has a structure similar to the single C-terminal domain of VSG MITat1.2 and provides proof of structural conservation in VSG C-terminal domains complementing the conservation of structure present in the N-terminal domain. The second domain, although based on the same fold, is a minimized version missing several structural features. The structure of the second domain contains the C-terminal residue that in the native VSG is attached to a glycosylphosphatidylinositol (GPI) anchor that retains the VSG on the external face of the plasma membrane. The solution structures of this domain and a VSG GPI glycan have been combined to produce the first structure-based model of a GPI-anchored protein. The model suggests that the core glycan of the GPI anchor lies in a groove on the surface of the domain and that there is a close association between the GPI glycan and protein. More widely, the GPI glycan may be an integral part of the structure of other GPI-anchored proteins.

Structure of the trypanosome haptoglobin–hemoglobin receptor and implications for nutrient uptake and innate immunity

“…The only structurally characterized VSGs come from T. brucei, with separate structures available for both N-and C-terminal domains. The VSG N-terminal domain is ∼100 Å long (10), whereas the C-terminal domains are closer to ∼30 Å (35,36) in length, consistent with estimates of 120-to 150-Å thickness for the VSG coat from electron micrographs (37). In T. congolense, VSGs do not contain the C-terminal domain(s) present in T. brucei and contain ∼70 fewer residues.…”

Prospects for vaccination against pathogenic African trypanosomes

“…In contrast to extreme amino acid sequence variability within VSG N‐terminal domains, there is a degree of sequence conservation within the CT domain of T. brucei spp VSGs, with CT Types 1‐4 known to exist for all T. brucei spp VSGs . The distribution of Types among VSGs is approximately 50% for Type 1, 33% for Type 2, 11% for Type 3 and 5% for Type 4 based on an earlier survey of existing trypanosome genomic and proteomic databases (Lepper, BJ, Mansfield, JM, and Paulnock, DM, unpublished).…”

The VSG C-terminal domain is inaccessible to antibodies on live trypanosomes

“…An anti-PFR1 monoclonal was used as a loading control and was also detected as a green signal. The anti-118CTD was raised in rabbit against residues 328–429 (the C-terminus) expressed in E. coli and purified as described for the C-terminal di-domain of VSG ILTat 1.24 [12]. Anti-221CTD was raised against residues 359–433 (the C-terminus) expressed and purified as described [11].…”