Journal Article

Structural basis for ligand and innate immunity factor uptake by the trypanosome haptoglobin-haemoglobin receptor

MacGregor, Paula, Carrington, Mark, Lowe, Edward D, Lane-Serff, Harriet, Higgins, Matthew K

2014

Overview

The haptoglobin-haemoglobin receptor (HpHbR) of African trypanosomes allows acquisition of haem and provides an uptake route for trypanolytic factor-1, a mediator of innate immunity against trypanosome infection. In this study, we report the structure of Trypanosoma brucei HpHbR in complex with human haptoglobin-haemoglobin (HpHb), revealing an elongated ligand-binding site that extends along its membrane distal half. This contacts haptoglobin and the β-subunit of haemoglobin, showing how the receptor selectively binds HpHb over individual components. Lateral mobility of the glycosylphosphatidylinositol-anchored HpHbR, and a ∼50o kink in the receptor, allows two receptors to simultaneously bind one HpHb dimer. Indeed, trypanosomes take up dimeric HpHb at significantly lower concentrations than monomeric HpHb, due to increased ligand avidity that comes from bivalent binding. The structure therefore reveals the molecular basis for ligand and innate immunity factor uptake by trypanosomes and identifies adaptations that allow efficient ligand uptake in the context of the complex trypanosome cell surface. African Trypanosomes are a group of single-celled parasites that are a major concern for livestock farmers in sub-Saharan Africa. They are carried by the tsetse fly and can cause disease in domestic livestock that diminishes productivity through reduced growth, and may ultimately lead to death. The parasites are coated in a dense layer of protein that help them evade the host’s immune system by preventing immune cells from identifying them. Humans have evolved immunity to many trypanosome species by exploiting a weakness in their lifestyle. Trypanosomes need to get haem—a molecule found in the protein haemoglobin—from their host to survive. In blood plasma, haemoglobin is found associated with a carrier protein called haptoglobin. To acquire haem, the parasites have a protein called HpHbR that binds to these haptoglobin-haemoglobin ‘complexes’. However, in humans there are two complexes of proteins called TLFs that contain haemoglobin and a protein related to haptoglobin. The TLFs are also able to bind to HpHbR and are taken into the parasite. Once inside, TLFs cause internal compartments called lysosomes to swell, which leads to the death of the trypanosome. Two subspecies of Trypanosoma brucei are the only trypanosomes that infect humans as they can overcome the TLF1 defense. However, the details of how TLFs cause cell death at the molecular level are not clear. Lane-Serff et al. used a technique called x-ray crystallography to generate 3-D images of the HpHbR protein from T. brucei bound to the haptoglobin-haemoglobin complexes. These images show that HpHbR is elongated so that it only binds to haemoglobin and haptoglobin when they are together as a complex. The images also reveal that the shape of HpHbR enables it to hold apart the proteins in the protective layer that coats the trypanosome. This allows the haptoglobin-haemoglobin complex to bind to HpHbR, but in humans also makes HpHbR more likely to bind to TLF1. These findings may help to guide future efforts to protect humans and livestock from the diseases caused by trypanosomes.

• Publisher: eLife Sciences Publications Ltd,eLife Sciences Publications, Ltd
• Subject: Adaptation / Amino Acid Sequence / Animals / Avidity / Binding Sites / Biochemistry / Biological Transport / Biophysics and Structural Biology / Cell surface / Crystallography, X-Ray / Escherichia coli - genetics / Escherichia coli - metabolism / Gene Expression / Gene Knockout Techniques / Glycosylphosphatidylinositol / Haptoglobin / haptoglobin-haemoglobin receptor / Haptoglobins - chemistry / Haptoglobins - genetics / Haptoglobins - metabolism / Hemoglobin / Hemoglobins - chemistry / Hemoglobins - genetics / Hemoglobins - metabolism / Humans / Immunoglobulins / Innate immunity / Ligands / Lipoproteins, HDL - metabolism / Microbiology and Infectious Disease / Models, Molecular / Molecular Sequence Data / Mutation / Protein Binding / Protein Multimerization / Protein Structure, Tertiary / Proteins / Protozoan Proteins - chemistry / Protozoan Proteins - genetics / Protozoan Proteins - metabolism / Receptors, Cell Surface - chemistry / Receptors, Cell Surface - genetics / Receptors, Cell Surface - metabolism / Recombinant Proteins - chemistry / Recombinant Proteins - genetics / Recombinant Proteins - metabolism / Sequence Alignment / Sf9 Cells / Spodoptera / trypanolytic factor / Trypanosoma brucei brucei - chemistry / Trypanosoma brucei brucei - metabolism / trypanosome