Structural studies on the glycophospholipid membrane anchor of Trypanosoma brucei variant surface glycoprotein

In common with other African trypanosomes Trypanosoma brucei evades the mammalian host immune response by antigenic variation (reviewed recently by Boothroyd 1985). Antigenic variation involves the sequential expression of individual genes encoding immunologically distinct variant surface glycoproteins (VSGs) which form a dense cell surface coat. The current model of coat architecture is of a densely packed monolayer of VSG molecules covering the entire plamsa membrane of the organism. From a number of observations it would appear that the VSG coat acts as a permeability barrier to macromolecules, protecting the organism both from attack by the alternative complement pathway and from lysis mediated by antibodies directed against invariant membrane components. The survival of the parasite in the mammalian host is therefore entirely dependant on the integrity of the VSG cell surface coat. The coat is an extraordinarily specialized structure in molecular terms; all of the different VSGs must serve the same protective function whilst differing markedly in primary amino acid sequence to remain immunologically distinct. The only site of cross-reaction resides in a common carbohydrate epitope (Holder and Cross 1981) which forms part of the VSG membrane anchor.