Dynamic, variable oligomerization and the trafficking of variant surface glycoproteins of Trypanosoma brucei

• Published in: Traffic (Copenhagen, Denmark) Denmark), 2021-08, Vol.22 (8), p.274-283
• Main Authors: Umaer, Khan, Aresta‐Branco, Francisco, Chandra, Monica, Straaten, Monique, Zeelen, Johan, Lapouge, Karine, Waxman, Brandon, Stebbins, C. Erec, Bangs, James D.
• Format: Article
• Language: English
• Subjects: Antigens; Cell Membrane; Cell surface; Chromatography; Crystal structure; Gel electrophoresis; Glycoproteins; Glycosylphosphatidylinositol; glycosylphosphatidylinositol anchor; Glycosylphosphatidylinositols; Light scattering; Livestock; Membrane Glycoproteins; Monomers; Oligomerization; protein trafficking; Trypanosoma brucei brucei; trypanosome; valence hypothesis; variant surface glycoprotein; Variant Surface Glycoproteins, Trypanosoma - genetics
• ISSN: 1398-9219; 1600-0854
• DOI: 10.1111/tra.12806

African trypanosomes cause disease in humans and livestock, avoiding host immunity by changing the expression of variant surface glycoproteins (VSGs); the major glycosylphosphatidylinositol (GPI) anchored antigens coating the surface of the bloodstream stage. Proper trafficking of VSGs is therefore critical to pathogen survival. The valence model argues that GPI anchors regulate progression and fate in the secretory pathway and that, specifically, a valence of two (VSGs are dimers) is critical for stable cell surface association. However, recent reports that the MITat1.3 (M1.3) VSG N‐terminal domain (NTD) behaves as a monomer in solution and in a crystal structure challenge this model. We now show that the behavior of intact M1.3 VSG in standard in vivo trafficking assays is consistent with an oligomer. Nevertheless, Blue Native Gel electrophoresis and size exclusion chromatography‐multiangle light scattering chromatography of purified full length M1.3 VSG indicates a monomer in vitro. However, studies with additional VSGs show that multiple oligomeric states are possible, and that for some VSGs oligomerization is concentration dependent. These data argue that individual VSG monomers possess different propensities to self‐oligomerize, but that when constrained at high density to the cell surface, oligomeric species predominate. These results resolve the apparent conflict between the valence hypothesis and the M1.3 NTD VSG crystal structure. The valence hypothesis holds that dimeric variant surface glycoproteins (VSG) require two glycosylphosphatidylinositol anchors for stable cell surface trafficking. However, the solution properties of M1.3 VSG and the crystal structure of its N‐terminal domain are monomeric, challenging the hypothesis. In contrast, in vivo trafficking and turnover of M1.3 VSG are consistent with an oligomeric state. To reconcile these two data sets, we propose that in vivo concentrations favor oligomerization. Sizing data with structurally related M1.9 VSG support this model.