MRC National Institute for Medical Research, London
Science for Health
Shedding of surface protein helps malaria parasite evade antibodies
16 December 2011
Scientists at NIMR have found that shedding of AMA1, a protein involved in red blood cell invasion by the malaria merozoite, is essential and helps the parasite to avoid attack by antibodies. The research is published in PLoS Pathogens.
The life cycle of the human malaria parasite, Plasmodium falciparum, involves repeated rounds of replication within red blood cells, interspersed with host cell rupture which releases merozoites that rapidly invade fresh red blood cells. During invasion several parasite surface proteins, including a protein called AMA1, are efficiently clipped off the parasite. Most of this clipping is performed by a parasite subtilisin-like protease called SUB2, but some also occurs through intramembrane cleavage. The function of this shedding is unknown. Antibodies against AMA1 prevent invasion and can protect against malaria in vivo, so AMA1 is of great interest as a malaria vaccine candidate.
Mike Blackman (pictured), from NIMR’s Division of Parasitology, has investigated the shedding of AMA1 and its inhibition. His study had three primary aims: to establish the importance of intramembrane shedding of AMA1; to interrogate the primary sequence requirements for cleavage of AMA1 by SUB2; and to examine the functional consequences of modifications that block that shedding.
He found that mutagenesis of the intramembrane cleavage site by targeted homologous recombination abolished intramembrane cleavage with no effect on parasite viability in vitro. SUB2 does not recognise a specific amino acid sequence in AMA1, but rather cleaves it at a position determined primarily by its distance from the parasite membrane. Certain mutations at the SUB2 cleavage site prevent shedding, and parasites expressing non-cleavable AMA1 along with unmodified AMA1 grow normally. However, these mutations cannot be introduced into the parasite’s genome, showing that some shedding by SUB2 is essential for parasite survival. Parasites expressing shedding-resistant mutants of AMA1 show enhanced sensitivity to invasion inhibitory antibodies, suggesting that shedding of surface proteins during invasion helps the parasite to evade potentially protective antibodies.
Our results suggest that shedding of AMA1 by SUB2 may perform dual functions, one of which is essential and the other of which enables the parasite to more efficiently evade the host humoral immune response. These observations have obvious implications for shedding of other immunogenic parasite proteins too, including those of several pathogens of major clinical and veterinary importance. Collectively, our findings imply that efficient inhibition of SUB2 activity by suitable drug-like inhibitors would prevent parasite growth, and – importantly – that even partial inhibition of SUB2 activity may enhance the efficacy of invasion inhibitory anti-merozoite antibody responses in vivo.
Mike Blackman
Click image to view at full-size
Three views of the x-ray crystal structure of the AMA1 ectodomain.
Original article
Anna Olivieri, Christine R. Collins, Fiona Hackett, Chrislaine Withers-Martinez, Joshua Marshall, Helen R. Flynn, J. Mark Skehel and Michael J. Blackman (2011)
Juxtamembrane shedding of Plasmodium falciparum AMA1 is sequence independent and essential, and helps evade invasion-inhibitory antibodies
PLoS Pathogens 7(12): e1002448. Article fulltext
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