Structure of the trypanosome transferrin receptor and insights into ligand binding and therapeutic strategies

African trypanosomes are extracellular protozoan pathogens that have evolved a complex and sophisticated interface with hosts to acquire nutrients and protect against the host immune system. The variant surface glycoprotein (VSG) forms a dense and dynamic protective coat, which serves to defend against the immunoglobulin-mediated host response. In addition, a unique set of receptors mediate uptake of host macromolecules and must comply with recognising ligands from a broad host range whilst also avoiding immune detection. Host transferrin (Tf) is acquired by trypanosomes through the expression of a transferrin receptor (TfR), which does not share homology with the host TfR. Multiple diverse copies of the TfR are present in the trypanosome genome but only one is expressed at a time. Several hypotheses revolve around the evolutionary advantage of multiplication and diversification of the receptor, falling into two main categories. Either the TfR repertoire serves to accommodate different Tfs from the wide vertebrate host range, or variation has arisen as part of an immune evasion strategy. To elucidate the complexities surrounding the receptor, two different TfRs were expressed as recombinant proteins to study receptor-ligand interactions at the molecular level. Biophysical analyses of binding interactions provided no evidence to support the hypothesis that the receptor repertoire had evolved to favour host promiscuity. Instead, a single receptor was capable of binding Tf from multiple different mammals, highlighting the lack of requisite to evolve species-specific receptors. Structural determination of the trypanosome TfR in conjunction with sequence variation mapping provided further evidence. The structure revealed a predominantly conserved Tf binding site, while variation and N-linked glycans resided in the surrounding accessible regions at the apical surface of the receptor, indicating that receptor evolution was likely driven by an immune evasion strategy. The findings presented in this thesis have served to further our understanding of the molecular architecture of the trypanosome cell surface by providing structural and biophysical data to define a complex host-parasite interaction. Therapeutic strategies have also been adapted through advances in our knowledge of the relationship between trypanosome and host. Finally, the trypanosome TfR is a remarkable example of evolutionary drift within a receptor, propagated by the advantage conferred by immune avoidance, and restricted by functional requirements of ligand uptake. Thus, the TfR repertoire has likely derived from an antigenic variation strategy to promote long-term persistence in the host