Title: Proteases in host cell exit and invasion by the malaria parasite
Funder: The Francis Crick Institute

Abstract: Malaria is a devastating disease that causes ~500,000 deaths per annum and extensive morbidity. There is no malaria vaccine and resistance against mainstay antimalarial drugs is widespread, imposing an immense humanitarian and economic burden on much of the developing world and representing a major threat to travellers. Malaria is caused by a protozoan parasite that invades and replicates within red blood cells. My group focuses on the molecular mechanisms by which Plasmodium falciparum, the agent of the most dangerous form of malaria, enters and exits its host cell. Our research has made important contributions to our understanding of invasion and egress. We have shown that a parasite cGMP-dependent kinase called PKG triggers egress by activating the discharge of a parasite protease called SUB1 into the parasitophorous vacuole in which the parasite replicates. SUB1 modifies the parasite surface to ‘prime’ it for egress. In addition, SUB1 activates a set of papain-like proteins (called the SERA family) that mediate downstream roles in egress. Following egress, two further proteases called SUB2 and ROM4 shed proteins from the parasite surface to facilitate invasion of a new red cell. We wish to better understand the molecular functions of these essential enzymes, as well as the role of their various substrates which include several proteins essential for egress and invasion. We will exploit our understanding of egress and invasion to discover drug-like compounds that target key enzymes in the egress/invasion pathway. Our current research plans include the following 5 major objectives: • Regulation of PKG activity. We want to dissect the underlying signals that regulate parasite cGMP levels (and thereby PKG activity) in the moments leading up to egress. These likely involve interplay between an essential phosphodiesterase called PDE-beta and a guanylyl cyclase, which respectively degrade and synthesise cGMP. • Role and regulation of SUB1 protease activity. We are seeking to improve our understanding of how SUB1 activity triggers egress and how the enzyme is regulated. We have compelling recent evidence for an endogenous SUB1 inhibitor with structural similarity to a subtilisin propeptide that is expressed in the parasitophorous vacuole. • Function of the SERA proteins. Of the two SERA family proteins important in P. falciparum blood stages, we know that one (SERA5) regulates the kinetics of egress whilst the other (SERA6) is converted by SUB1 to an active protease that we hypothesise mediates disruption of the host cell cytoskeleton and membrane at egress. We are dissecting the structure and function of both proteins and working to determine the substrate(s) of SERA6. • Function of SUB2 and ROM4. We have recently found that conditional depletion of SUB2 results in a severe invasion defect caused by loss of host red cell membrane sealing at invasion. This leads to rapid lysis of the target red cell and parasite death. We believe that a better understanding of this phenotype will advance our understanding of the membrane fusion event(s) involved in invasion, currently a very poorly characterised element of malarial biology. • Drug discovery. We aim to translate the outcomes of our research into health benefits by seeking drug-like inhibitors of SUB1, SUB2, SERA6, ROM4 and other parasite enzymes involved in egress and invasion, and promote their development as antimalarial drugs. We are currently working with GSK to identify inhibitors of SUB1.


Research activity codes: 2.2 FACTORS RELATING TO PHYSICAL ENVIRONMENT
Health categories: INFECTION