The identification, purification, and characterization of two invariant surface glycoproteins located beneath the surface coat barrier of bloodstream forms of Trypanosoma brucei.

Two new polypeptides, termed ISG70 and ISG64, have been found in Trypanosoma brucei, using enzyme-catalyzed radioiodination techniques. Both are externally disposed integral membrane glycoproteins, containing N-linked carbohydrate chains. No structural homology was detected between ISG70, ISG64, or the variant surface glycoprotein (VSG) when assessed by 1) comparative peptide mapping, 2) immunoprecipitation analysis, and 3) lectin affinity chromatography. ISG70 occurred in 5.1 x 10(4) copies/cell and has been purified 880-fold from detergent extracts of plasma membranes by a procedure that includes gel filtration, lectin affinity chromatography, and preparative SDS-polyacrylamide gel electrophoresis. ISG70 was present only in bloodstream forms and was specifically detected in six different cloned variants from the Molteno Institute trypanosomal antigen type (MITat) serodeme of T. brucei and from the single cloned variant of the International Laboratory for Research on Animal Diseases trypanosomal antigen type (ILTat) serodeme that was examined. Rabbits with chronic infections of T. brucei displayed circulating antibodies against ISG70. Both the immunogenicity of ISG70 and its invariant nature suggest that it may be useful in the development of an effective serodiagnostic test. Furthermore, its stage-specific location combined with its invariant nature implies that its function is strictly related to a physiological role required for the parasite's residence in its mammalian host.