Spemannstrasse 34, D7400 Tubingen and 'Max-Planck-Institut fiir Biochemie, D8033 Martinsried, FRG Communicated by P.Overath A transferrin-binding protein (TFBP) with an apparent molecular weight of 42 kd was purified from detergentsoluble membrane proteins of bloodstream forms of Trypanosoma brucei. The protein is not expressed in the insect-borne stage of the parasite's life-cycle. Purified TFBP can be converted from an amphiphilic to a hydrophilic form by cleavage with T.brucei glycosylphosphatidylinositol (GPI)-specific phospholipase C, demonstrating that the C-terminus is modified by a GPImembrane anchor. The TFBP is encoded by an expression-site-associated gene [ESAG 6 in the nomenclature of Pays et al. (1989) Cell, 57,[835][836][837][838][839][840][841][842][843][844][845] which is under the control of the promoter transcribing the expressed variant surface glycoprotein gene. The possible function of TFBP as a receptor for the uptake of transferrin in bloodstream forms is discussed.
A method for the selective depletion of transferrin from bovine serum is presented. Bloodstream forms of Trypanosoma brucei cannot grow in medium containing transferrin-deficient serum, whereas reconstitution with transferrin restores normal growth. We conclude that transferrin is an essential growth factor for the mammalian stage of the parasite.
In search for invariant surface proteins in Trypanosoma brucei bloodstream forms, acid phosphatase was investigated. Earlier work had shown that part of the cellular phosphatase activity is associated with the flagellar pocket of the parasite. It is demonstrated that T. brucei contains at least two membrane‐bound enzymes, one is sensitive to the inhibitor L‐(+)‐tartrate while the other is resistant. The tartrate‐sensitive phosphatase was purified to homogeneity by monoclonal antibody affinity chromatography and shown to be a glycoprotein of low abundance (13,000 molecules/ cell). It has an apparent molecular weight of 70,000 Da. The usefulness of acid phosphatase as a marker for characterizing the membrane lining the flagellar pocket is discussed.
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