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

Schwede A, Macleod OJ, MacGregor P, Carrington M - PLoS Pathog. (2015)

Bottom Line: Epitopes for some VSG monoclonal antibodies are mapped as far as possible from previous experimental data, onto models of VSG structures.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom.

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.

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Structure of VSG221 (MITat1.2).(A) Illustrated model of VSG221 dimer showing the structures of the N-terminal domain, one monomer in blue and one in grey, and the two C-terminal domains in purple (PDB: 1VSG and 1XU6) [1,9]. The N-linked oligosaccharide in the N-terminal domain is shown in red. Three residues are shown that form the core; there are between one and three further residues not shown. The relative positions of the N- and C-terminal domains are not known, and this illustration is a model [1]. (B) Space-filling model of VSG221 viewed from the x-, y-, and z-axes. The maximum width dimensions of the VSG are shown below, and the fit of an ellipse of approximately 28 Å2 is shown around the structure of a VSG viewed from outside the cell.
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ppat.1005259.g001: Structure of VSG221 (MITat1.2).(A) Illustrated model of VSG221 dimer showing the structures of the N-terminal domain, one monomer in blue and one in grey, and the two C-terminal domains in purple (PDB: 1VSG and 1XU6) [1,9]. The N-linked oligosaccharide in the N-terminal domain is shown in red. Three residues are shown that form the core; there are between one and three further residues not shown. The relative positions of the N- and C-terminal domains are not known, and this illustration is a model [1]. (B) Space-filling model of VSG221 viewed from the x-, y-, and z-axes. The maximum width dimensions of the VSG are shown below, and the fit of an ellipse of approximately 28 Å2 is shown around the structure of a VSG viewed from outside the cell.

Mentions: The size of a VSG dimer can be derived from the structure of the N-terminal domain [5,9], and it is assumed that the long axis is perpendicular to the plasma membrane surface (Fig 1). The area taken up by each dimer can be approximated to a circle with an area of 28 nm2 [8], but note that this size estimate does not take into account any coordinated water molecules. This value is remarkably close to the estimates of the area available per VSG dimer as discussed above, strongly supporting the model that the vast majority of the plasma membrane is physically occluded by VSG. The VSG N-terminal domain has a long axis of approximately 10 nm, measured from the structure; allowing for some increase due to the C-terminal domain and GPI anchor, the thickness of the VSG coat is probably 12–15 nm. This value is in agreement with measurements from electron microscopy [10].


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

Schwede A, Macleod OJ, MacGregor P, Carrington M - PLoS Pathog. (2015)

Structure of VSG221 (MITat1.2).(A) Illustrated model of VSG221 dimer showing the structures of the N-terminal domain, one monomer in blue and one in grey, and the two C-terminal domains in purple (PDB: 1VSG and 1XU6) [1,9]. The N-linked oligosaccharide in the N-terminal domain is shown in red. Three residues are shown that form the core; there are between one and three further residues not shown. The relative positions of the N- and C-terminal domains are not known, and this illustration is a model [1]. (B) Space-filling model of VSG221 viewed from the x-, y-, and z-axes. The maximum width dimensions of the VSG are shown below, and the fit of an ellipse of approximately 28 Å2 is shown around the structure of a VSG viewed from outside the cell.
© Copyright Policy
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4697842&req=5

ppat.1005259.g001: Structure of VSG221 (MITat1.2).(A) Illustrated model of VSG221 dimer showing the structures of the N-terminal domain, one monomer in blue and one in grey, and the two C-terminal domains in purple (PDB: 1VSG and 1XU6) [1,9]. The N-linked oligosaccharide in the N-terminal domain is shown in red. Three residues are shown that form the core; there are between one and three further residues not shown. The relative positions of the N- and C-terminal domains are not known, and this illustration is a model [1]. (B) Space-filling model of VSG221 viewed from the x-, y-, and z-axes. The maximum width dimensions of the VSG are shown below, and the fit of an ellipse of approximately 28 Å2 is shown around the structure of a VSG viewed from outside the cell.
Mentions: The size of a VSG dimer can be derived from the structure of the N-terminal domain [5,9], and it is assumed that the long axis is perpendicular to the plasma membrane surface (Fig 1). The area taken up by each dimer can be approximated to a circle with an area of 28 nm2 [8], but note that this size estimate does not take into account any coordinated water molecules. This value is remarkably close to the estimates of the area available per VSG dimer as discussed above, strongly supporting the model that the vast majority of the plasma membrane is physically occluded by VSG. The VSG N-terminal domain has a long axis of approximately 10 nm, measured from the structure; allowing for some increase due to the C-terminal domain and GPI anchor, the thickness of the VSG coat is probably 12–15 nm. This value is in agreement with measurements from electron microscopy [10].

Bottom Line: Epitopes for some VSG monoclonal antibodies are mapped as far as possible from previous experimental data, onto models of VSG structures.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom.

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.

Show MeSH
Related in: MedlinePlus