The variant surface glycoprotein as a tool for adaptation in African trypanosomes.

African trypanosomes (prototype: Trypanosoma brucei) are flagellated protozoan parasites that infect a wide variety of mammals, causing nagana in cattle and sleeping sickness in humans. These organisms can cause prolonged chronic infections due to their ability to successively expose different antigenic variants of the variant surface glycoprotein (VSG). The genomic loci where the VSG genes are expressed are telomeric and contain polycistronic transcription units with several genes that are involved in adaptation of the parasite to the host. At least three of these genes, which respectively encode the two subunits of the heterodimeric receptor for transferrin and a protein conferring resistance to the human trypanolytic factor apolipoprotein L-I, share the same origin as the VSG. The high recombination potential of the telomeric VSG expression sites, coupled to their dynamic mono-allelic expression control, provides trypanosomes with a powerful capacity for adaptation to their hosts.