ESAG-6 AND ESAG-7 PRODUCTS OF TRYPANOSOMA-BRUCEI FORM A TRANSFERRIN-BINDING PROTEIN COMPLEX

In Trypanosoma brucei, the gene for the expressed variant surface glyc oprotein (VSG) is preceded by a series of open reading frames designat ed expression site associated genes (ESAGs), which to gether with the VSG gene form a polycistronic transcription unit. It is shown that the products derived from two ESAGs (ESAG 6 and 7 in the nomenclature of Pays, E., et al. Cell 57, 835-845 (1989)) form a complex, which binds transferrin with high affinity. Transferrin affinity chromatography yi elds heterodimers or higher order heterooligomers composed of the prod ucts of ESAG 6 and ESAG 7. The former is a heterogeneously glycosylate d protein of 50 to 60 kDa modified by a glycosylphosphatidyl inositol membrane anchor at the COOH-terminus, while the latter is the previous ly identified 42kDa glycoprotein carrying an unmodified COOH-terminus (Schell, D., et al. EMBO J. 10, 1061-1066 (1991) and Schell, D., et al . EMBO J. 12, 2990 (1993)).When isolated from trypanosomes grown in ro dents, the complex is in part free and in part associated with transfe rrin. Also, the complex is present both in the membrane fraction and t he soluble fraction of cell lysates. As shown by immunoelectron micros copy, both transferrin and ESAG 6/7 derived proteins can be demonstrat ed in the lumen of the flagellar pocket, an invagination of the plasma membrane serving as the sole site for endocytotic uptake of macromole cular nutrients. Weak labeling is also obtained on the flagellar pocke t membrane and in intracellular vesicles. The possibilty that the bind ing protein complex serves as a receptor for the uptake of transferrin in T. brucei is discussed.