Variable surface glycoprotein

Variant surface glycoprotein (VSG) is a ~60kDa protein which densely packs the cell surface of protozoan parasites belonging to the genus Trypanosoma. They form a 12-15 nm surface coat and were first isolated from Trypanosoma brucei in 1975 by George Cross. VSG allows the trypanosomatid parasites to evade the mammalian host's immune system by extensive antigenic variation. VSG has no prescribed biochemical activity. Variant surface glycoprotein (VSG) is a ~60kDa protein which densely packs the cell surface of protozoan parasites belonging to the genus Trypanosoma. They form a 12-15 nm surface coat and were first isolated from Trypanosoma brucei in 1975 by George Cross. VSG allows the trypanosomatid parasites to evade the mammalian host's immune system by extensive antigenic variation. VSG has no prescribed biochemical activity. The parasite has a large cellular repertoire of antigenically distinct VSGs (~1500 complete and partial (pseudogenes)) located in telomeric and subtelomeric arrays (on megabase chromosomes or minichromosomes). VSGs are expressed from a bloodstream expression site (BES, ES) in a polycistron by RNA polymerase I (recruited to a ribosomal-type promoter) with other ES-associated genes (ESAGs), of which transferrin receptor (Tfr: ESAG6, ESAG7) is one. Only one VSG gene is expressed at a time, as only one of the ~15 ES are active in a cell. VSG expression is 'switched' by homologous recombination of a silent basic copy gene from an array (directed by homology) into the active telomerically-located expression site. Mosaic VSG genes can be created by homologous recombination of a partial VSG gene from an array. This partial gene may replace any portion of the residing VSG gene, creating a new mosaic VSG. In Trypanosoma brucei, the cell surface is covered by a dense coat of ~5 x 106 VSG dimers, ~90% of all cell surface protein. It also makes up ~10% of total cell protein.