How Does the VSG Coat of Bloodstream Form African Trypanosomes Interact with External Proteins?

Abstract

Variations on the statement “the variant surface glycoprotein (VSG) coat that covers the external face of the mammalian bloodstream form of Trypanosoma brucei acts a physical barrier” appear regularly in research articles and reviews. The concept of the impenetrable VSG coat is an attractive one, as it provides a clear model for understanding how a trypanosome population persists; each successive VSG protects the plasma membrane and is immunologically distinct from previous VSGs. What is the evidence that the VSG coat is an impenetrable barrier, and how do antibodies and other extracellular proteins interact with it? In this review, the nature of the extracellular surface of the bloodstream form trypanosome is described, and past experiments that investigated binding of antibodies and lectins to trypanosomes are analysed using knowledge of VSG sequence and structure that was unavailable when the experiments were performed. Epitopes for some VSG monoclonal antibodies are mapped as far as possible from previous experimental data, onto models of VSG structures. The binding of lectins to some, but not to other, VSGs is revisited with more recent knowledge of the location and nature of N-linked oligosaccharides. The conclusions are: (i) Much of the variation observed in earlier experiments can be explained by the identity of the individual VSGs. (ii) Much of an individual VSG is accessible to antibodies, and the barrier that prevents access to the cell surface is probably at the base of the VSG N-terminal domain, approximately 5 nm from the plasma membrane. This second conclusion highlights a gap in our understanding of how the VSG coat works, as several plasma membrane proteins with large extracellular domains are very unlikely to be hidden from host antibodies by VSG.

Expression of a specific variant surface glycoprotein has a major impact on suramin sensitivity and endocytosis inTrypanosoma brucei

“…To test whether VSG Sur expression has a general impact on endocytosis, we investigated endocytosis of VSG Sur and VSG 221 . Concanavalin A (ConA) is a lectin that binds to glycosylated surface proteins, in case of trypanosomes mainly to VSG . It is endocytosed together with the VSG and can therefore be used to measure VSG endocytosis rates.…”

Blocking Synthesis of the Variant Surface Glycoprotein Coat in Trypanosoma brucei Leads to an Increase in Macrophage Phagocytosis Due to Reduced Clearance of Surface Coat Antibodies

“…VSG is the most abundant protein in bloodstream form T . brucei (~10 7 molecules) and the VSG coat is an extremely dense layer of tightly packed VSG molecules [4, 9, 10]. The VSG coat has been modelled with VSG approaching maximal densities [11].…”

“…Concurrently, a global translation arrest is induced, resulting in stalled cells which do not grow, again preventing catastrophic dilution of the protective VSG surface [14]. Despite its dense nature, the VSG layer forms a semi-permeable barrier blocking deposition of host complement proteins and antibodies on the trypanosome surface membrane [9, 15–17], yet allowing smaller nutrient molecules to reach surface receptors [18]. As VSG proteins are attached to the cell surface through glycosylphosphatidylinositol (GPI) anchors, VSG molecules can move freely on the cell surface forming an unusually fluid protective surface [19].…”

Evaluation of Antigens for Development of a Serological Test for Human African Trypanosomiasis

“…In particular, VSG LiTat 1.3 and VSG LiTat 1.5, which belong to the group of VSGs that cover the external surface of the bloodstream form of T . brucei [39], were found to be the two most reactive antigens against gambiense samples, and among the three most reactive antigens against rhodesiense samples. Noteworthy, T .…”