CHARACTERIZATION OF THE LIGAND-BINDING SITE OF THE TRANSFERRIN RECEPTOR IN TRYPANOSOMA-BRUCEI DEMONSTRATES A STRUCTURAL RELATIONSHIP WITH THE N-TERMINAL DOMAIN OF THE VARIANT SURFACE GLYCOPROTEIN

The Trypanosoma brucei transferrin (Tf) receptor is a heterodimer enco ded by ESAG7 and ESAG6, two genes contained in the different polycistr onic transcription units of the variant surface glycoprotein (VSG) gen e, The sequence of ESAG7/6 differs slightly between different units, s o that receptors with different affinities for Tf are expressed altern atively following transcriptional snitching of VSG expression sites du ring antigenic variation of the parasite, Based on the sequence homolo gy between pESAG7/6 and the N-terminal domain of VSGs, it can be predi cted that the four blocks containing the major sequence differences be tween pESAG7 and pESAG6 form surface-exposed loops and generate the li gand-binding site, The exchange of a few amino acids in this region be tween pESAG6s encoded by different VSG units greatly increased the aff inity for bovine Tf, Similar changes in other regions were ineffective , while mutations predicted to alter the VSG-like structure abolished the binding, Chimeric proteins containing the N-terminal dimerization domain of VSG and the C-terminal half of either pESAG7 or pESAG6, whic h contains the ligand-binding domain, can form heterodimers that bind Tf, Taken together, these data provided evidence that the T.brucei Tf receptor is structurally related to the N-terminal domain of the VSG a nd that the ligand-binding site corresponds to the exposed surface loo ps of the protein.