Balancing diversity with conservation: learning tricks from malaria parasite proteins
A new study sheds light on how the malaria parasite can evade the body's defences whilst retaining the ability to interact with and thrive in its host.
The structure of a complex of the CIDR domain of a PfEMP1 variant (yellow) bound to EPCR (blue). A phenylalanine of the CIDR domain, at the centre of the interface, is shown as pink sticks (Click to enlarge)
Clinton Lau in Matt Higgins' group, working with groups in Denmark and Tanzania, has characterised a family of variant proteins in the malarial parasite Plasmodium falciparum, believed to be responsible for severe childhood malaria.
By identifying conserved features of the protein family that are required to make the infection so deadly, the group can begin to design therapeutics that might prevent this form of malaria.
Many human infective parasites express surface protein families that are highly variable, avoiding immune detection, but that also have exquisitely conserved molecular features enabling them to maintain the necessary interaction with their host targets.
PfEMP1 proteins of Plasmodium falciparum are one such family and are implicated in causing the most severe symptoms of malaria. These proteins are expressed on the surface of infected red blood cells and interact with various human endothelial receptors causing tethering of the cells to blood vessels and preventing clearance by the spleen.
Following their recent identification of the endothelial protein C receptor (EPCR) as the host target for the PfEMP1 proteins that are expressed most commonly in cases of severe childhood malaria, the group decided to focus on the molecular basis of this interaction. The interaction is thought to give rise to cerebral symptoms because EPCR has a role as a receptor for anti-inflammatory ligands in the brain. PfEMP1 blocks binding of an anti-inflammatory molecule to EPCR, most likely interfering with the receptor's normal function.
In their new paper in Cell Host and Microbe (1), the group describes a multipronged approach to understanding how PfEMP1 proteins develop diversity whilst retaining conserved binding features.
They carried out sequence analysis of a domain of PfEMP1 called CIDR that binds to EPCR, and combined this with binding assays and the crystal structures of two CIDR:EPCR complexes.
By analysing the sequence of over 800 CIDR domains from PfEMP1 variants and characterising the binding of a subset to EPCR, they identified the residues which are conserved for EPCR binding. The crystallographic studies led them to the structural features of these regions.
Professor Higgins explains their findings: 'From sequence analysis, we found massive sequence variation in the CIDR domain. Even amongst the nine residues we identified that make direct contact with EPCR, we found that there is considerable variability. However, although there is no sequence conservation, the residues have chemical conservation and the shape of the binding surface is conserved.'
Studies of protein C, an anti-inflammatory ligand of EPCR, demonstrated that its binding site for EPCR overlaps with that of the CIDR domain of PfEMP1.
The next steps, Professor Higgins says, will be to try and target these conserved features of PfEMP1 to prevent binding of PfEMP1 to EPCR. This is a promising strategy against severe malaria since people who have natural immunity against the disease have been found to have antibodies against PfEMP1. Mimicking these naturally acquired antibodies would protect against the most deadly forms of the disease.
Evidence from the new study supports this approach. The group showed that children from a malaria endemic region of Tanzania carry antibodies which can be purified using peptides corresponding to this binding region, and that these block EPCR binding of diverse CIDR variants.
'We will use our knowledge of the chemical basis for this interaction to design immunogens that induce a broadly-neutralising antibody response that blocks many members of the PfEMP1 family,' says Professor Higgins. 'We would test the molecule in animals and then move on to trial in humans.'
The work is important not only because of the longterm goal of designing a vaccine against severe malaria, but also as a proof of principle. The question of whether it is possible to raise antibodies that are effective against large, complex protein families is relevant in many human parasites.
'Our study is not just about how the malaria parasite interacts with EPCR. It also aims to develop general principles of how a parasite surface protein can diversify while retaining binding properties and to see if it is possible to target such an interaction with broadly blocking antibodies,' adds Professor Higgins.