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.