Protein targeting to the malaria parasite plastid

The relict plastid, or apicoplast, of the malaria parasite Plasmodium falciparum is an essential organelle and a promising drug target. Most apicoplast proteins are nuclear encoded and post-translationally targeted into the organelle using a bipartite N-terminal extension, consisting of a typical endomembrane signal peptide and a plant-like transit peptide. Apicoplast protein targeting commences through the parasite's secretory pathway. We review recent experimental evidence suggesting that the apicoplast resides in the mainstream endomembrane system proximal to the Golgi. Further, we explore possible mechanisms for translocation of nuclear-encoded apicoplast proteins across the four bounding membranes. Recent insights into the composition of the transit peptide and how it is cleaved and degraded after use are also examined. Characterization of apicoplast targeting has not only shed light on how this group of parasites mediate intracellular protein trafficking events but also it has helped identify new targets for therapeutics. The distinctive leader sequences of apicoplast proteins make them readily identifiable, allowing assembly of a virtual organelle metabolome from the genome. Such analysis has lead to the identification of several biochemical pathways that are absent from the human host and thus represent novel therapeutic targets for parasitic infection.     

Autonomous Judges: African Ordeals as Dramas of Power

Judicial ordeals subject accused persons to fire, poison etc. and mark or kill those whom they divine as guilty, while the innocent are unscathed. They presume a person-directed universe, may share features with non-divinatory processes and can be used politically, but they are trusted as autonomous, unlike fallible human judgments. African examples are comparable in form and often in context to those of medieval Europe. The article focuses on sasswood poison ordeals in Southeastern Liberia. It is argued that they change over time and may convince individual participants differently. They are also considered as persuasive dramaturgical events.     

The multifunctional role of the pallilysin‐associated Treponema pallidum protein,Tp0750, in promoting fibrinolysis and extracellular matrix component degradation

The mechanisms that facilitate dissemination of the highly invasive spirochaete, Treponema pallidum, are incompletely understood. Previous studies showed the treponemal metalloprotease pallilysin (Tp0751) possesses fibrin clot degradation capability, suggesting a role in treponemal dissemination. In the current study we report characterization of the functionally linked protein Tp0750. Structural modelling predicts Tp0750 contains a von Willebrand factor type A (vWFA) domain, a protein‐protein interaction domain commonly observed in extracellular matrix (ECM)‐binding proteins. We report Tp0750 is a serine protease that degrades the major clot components fibrinogen and fibronectin. We also demonstrate Tp0750 cleaves a matrix metalloprotease (MMP) peptide substrate that is targeted by several MMPs, enzymes central to ECM remodelling. Through proteomic analyses we show Tp0750 binds the endothelial fibrinolytic receptor, annexin A2, in a specific and dose‐dependent manner. These results suggest Tp0750 constitutes a multifunctional protein that is able to (1) degrade infection‐limiting clots by both inhibiting clot formation through degradation of host coagulation cascade proteins and promoting clot dissolution by complexing with host proteins involved in the fibrinolytic cascade and (2) facilitate ECM degradation via MMP‐like proteolysis of host components. We propose that through these activities Tp0750 functions in concert with pallilysin to enable T. pallidum dissemination.     

Endothelial protein C receptor-associated invasiveness of rheumatoid synovial fibroblasts is likely driven by group V secretory phospholipase A2

Introduction

Rheumatoid synovial fibroblasts (RASFs) mediate joint inflammation and destruction in rheumatoid arthritis (RA). Endothelial protein C receptor (EPCR) is a specific receptor for the natural anticoagulant activated protein C (APC). It mediates the cytoprotective properties of APC and is expressed in rheumatoid synovial tissue. A recent report shows that group V secretory phospholipase A2 (sPLA₂V) prevents APC from binding to EPCR in endothelium and inhibits EPCR/APC function. The aim of this study was to investigate the expression and function of EPCR on RASFs.

Methods

Human synovial fibroblasts (SFs) were isolated from RA or osteoarthritis (OA) synovial tissues and treated with control, EPCR, or sPLA₂V small interfering RNA (siRNA); recombinant human APC, tumor necrosis factor-alpha (TNF-α), or sPLA₂V. RASF viability and migration/invasion were measured by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) and collagen gel migration/invasion assays, respectively, and cartilage degradation by 1,9-dimethylmethylene blue (DMMB) assay in the presence of human OA articular cartilage explants. The expression or activation of cytokines, EPCR, cadherin-11, mitogen-activated protein (MAP) kinases, and nuclear factor-kappa-B (NF-κB) or both were detected by enzyme-linked immunosorbent assay, Western blotting, or immunostaining.

Results

EPCR was expressed by both OASFs and RASFs but was markedly increased in RASFs. When EPCR was suppressed by siRNA or blocking antibody cell viability, cell invasion and cartilage degradation were reduced by more than 30%. Inflammatory mediators interleukin-1-beta (IL-1β), cadherin-11, and NF-κB were significantly reduced by EPCR suppression under control or TNF-α-stimulated conditions. The expression or activation (or both) of MAP kinases ERK, p38, and JNK were also markedly decreased in cells transfected with EPCR siRNA. Further analysis revealed that sPLA₂V co-localized with EPCR on RASFs. Suppression of sPLA₂V reduced cell viability and cartilage degradation and increased APC binding to RASFs. Conversely, recombinant sPLA₂V increased cartilage degradation, blocked APC binding to RASFs, and could not rescue the effects induced by EPCR suppression.

Conclusions

Our results demonstrate that EPCR is overexpressed by RASFs and mediates the aggressive behavior of RASFs. This function of EPCR is contrary to its cytoprotective role in other settings and is likely driven by sPLA₂V.     

Protein targeting to destinations of the secretory pathway in the malaria parasite Plasmodium falciparum

The secretory pathway in the malaria parasite Plasmodium falciparum has many unique aspects in terms of protein destinations and trafficking mechanisms. Recently, several exciting insights into protein trafficking within this intracellular parasite have been unveiled: these include signals that are required for targeting of proteins to the red blood cell and the relict plastid (known as the apicoplast); and the elucidation of the pathways of the haemoglobin proteases targeted to the food vacuole. Protein-targeting to the apical organelles in P. falciparum, however, is still not very well understood, but available research offers a tantalising glimpse of the system.     

Evidence for Golgi-independent transport from the early secretory pathway to the plastid in malaria parasites

The malaria parasite Plasmodium falciparum harbours a relict plastid (termed the apicoplast) that has evolved by secondary endosymbiosis. The apicoplast is surrounded by four membranes, the outermost of which is believed to be part of the endomembrane system. Nuclear-encoded apicoplast proteins have a two-part N-terminal extension that is necessary and sufficient for translocation across these four membranes. The first domain of this N-terminal extension resembles a classical signal peptide and mediates translocation into the secretory pathway, whereas the second domain is homologous to plant chloroplast transit peptides and is required for the remaining steps of apicoplast targeting. We explored the initial, secretory pathway component of this targeting process using green fluorescent reporter protein constructs with modified leaders. We exchanged the apicoplast signal peptide with signal peptides from other secretory proteins and observed correct targeting, demonstrating that apicoplast targeting is initiated at the general secretory pathway of P. falciparum. Furthermore, we demonstrate by immunofluorescent labelling that the apicoplast resides on a small extension of the endoplasmic reticulum (ER) that is separate from the cis-Golgi. To define the position of the apicoplast in the endomembrane pathway in relation to the Golgi we tracked apicoplast protein targeting in the presence of the secretory inhibitor Brefeldin A (BFA), which blocks traffic between the ER and Golgi. We observe apicoplast targeting in the presence of BFA despite clear perturbation of ER to Golgi traffic by the inhibitor, which suggests that the apicoplast resides upstream of the cis-Golgi in the parasite's endomembrane system. The addition of an ER retrieval signal (SDEL) - a sequence recognized by the cis-Golgi protein ERD2 - to the C-terminus of an apicoplast-targeted protein did not markedly affect apicoplast targeting, further demonstrating that the apicoplast is upstream of the Golgi. Apicoplast transit peptides are thus dominant over an ER retention signal. However, when the transit peptide is rendered non-functional (by two point mutations or by complete deletion) SDEL-specific ER retrieval takes over, and the fusion protein is localized to the ER. We speculate either that the apicoplast in P. falciparum resides within the ER directly in the path of the general secretory pathway, or that vesicular trafficking to the apicoplast directly exits the ER.     

A Novel Family of Apicomplexan Glideosome-associated Proteins with an Inner Membrane-anchoring Role

The phylum Apicomplexa are a group of obligate intracellular parasites responsible for a wide range of important diseases. Central to the lifecycle of these unicellular parasites is their ability to migrate through animal tissue and invade target host cells. Apicomplexan movement is generated by a unique system of gliding motility in which substrate adhesins and invasion-related proteins are pulled across the plasma membrane by an underlying actin-myosin motor. The myosins of this motor are inserted into a dual membrane layer called the inner membrane complex (IMC) that is sandwiched between the plasma membrane and an underlying cytoskeletal basket. Central to our understanding of gliding motility is the characterization of proteins residing within the IMC, but to date only a few proteins are known. We report here a novel family of six-pass transmembrane proteins, termed the GAPM family, which are highly conserved and specific to Apicomplexa. In Plasmodium falciparum and Toxoplasma gondii the GAPMs localize to the IMC where they form highly SDS-resistant oligomeric complexes. The GAPMs co-purify with the cytoskeletal alveolin proteins and also to some degree with the actin-myosin motor itself. Hence, these proteins are strong candidates for an IMC-anchoring role, either directly or indirectly tethering the motor to the cytoskeleton.Apicomplexan parasites cause a multitude of illnesses through infection of both human and livestock hosts. Members of this phylum include the opportunistic human parasites Toxoplasma gondii and Cryptosporidium parvum, pathogens of livestock, including Theileria annulata and Eimeria tenalla, and most notably the Plasmodium species, the causative agents of malaria in humans. Infection with P. falciparum results in ∼1–3 million deaths and a further 500 million infections annually (1).During various stages of the Apicomplexan lifecycle the parasites require motility to migrate through their insect and vertebrate hosts and to invade and internalize themselves within targeted host cells (2–4). The parasite''s unique mechanism of gliding motility is powered by an Apicomplexan-specific motor complex termed the actin-myosin motor (5), which resides between the outer plasma membrane and inner membrane complex (IMC)⁴ (6). The IMC is a continuous patchwork of flattened vesicular cisternae located directly beneath the plasma membrane and overlying the cytoskeletal network (7, 8). The IMC appears to arise from Golgi-associated vesicles flattened during parasite maturation to form large membranous sheets, which envelope the parasite and leave only a small gap at the extreme parasite apex (9).The myosin component of the actin-myosin motor has previously been defined as a tetrameric complex consisting of a class XIV myosin termed Myo-A (10), a myosin tail interacting protein (also called myosin light chain) (7) and the two glideosome-associated proteins GAP45 and GAP50 (11). These motor components are linked to the outer IMC membrane via the membrane proteins GAP45/50 (11). Between the plasma membrane and the IMC are actin filaments held in place through aldolase-mediated contact with the C-terminal tails of plasma membrane-spanning adhesive proteins whose ectodomains bind substrate and host cells (2). To power the forward movement of apicomplexan zoite stages, myosin pulls the actin filaments and their attached adhesins rearward. For this to succeed the GAP-myosin complex must presumably be fixed to the IMC, possibly via interactions with unidentified proteins linking the motor to the underlying cytoskeleton. Studies of fluorescently tagged GAP50 confirm it is relatively immobile within the IMC, however attempts to identify potential anchoring proteins have not been successful and have instead indicated that GAP50 may be immobilized by the lipid-raft like properties of the IMC membranes (12).The actin-myosin complex is confined to the outer IMC membrane while the opposing innermost IMC membrane is studded with 9 nm intramembranous particles, revealed by electron microscopy of freeze fractured Toxoplasma tachyzoites and Plasmodium ookinetes (13, 14). The size of these particles suggests that the proteins involved are likely to form high molecular weight complexes that overlay the parasite''s cytoskeletal network and possibly anchor the IMC to the cytoskeleton (12–15). Due to the close apposition of the inner and outer IMC membranes (14, 16), it is possible that the intramembranous particles could bridge the IMC lumen and interact with the GAP-myosin complex contributing to its stabilization within the IMC.To identify putative proteins that might be components of the intramembranous particles, we examined data from the detergent-resistant membrane (DRM) proteome of schizont-stage P. falciparum parasites containing developing merozoites (17, 18). DRMs, or lipid-rafts, were of considerable interest, because they appeared to harbor proteins involved in host cell invasion such as glycosylphosphatidylinositol (GPI)-anchored merozoite surface proteins. Our data also indicated that P. falciparum schizont-stage DRMs contained the IMC proteins PfGAP45/50 (17), and recent studies in T. gondii have also suggested that the IMC is enriched in DRMs (12). Another study indicated that when P. falciparum DRM protein complexes were separated by blue native gel electrophoresis, a band was produced containing PfGAP45/50 and PfMyo-A as well as a novel six-pass transmembrane protein (PlasmoDB: PFD1110w, GenBank^TM: {"type":"entrez-protein","attrs":{"text":"CAD49269","term_id":"23498297","term_text":"CAD49269"}}CAD49269) (18). This protein was related to another six-pass transmembrane DRM protein (PlasmoDB: MAL13P1.130, GenBank^TM: {"type":"entrez-protein","attrs":{"text":"CAD52385","term_id":"23615394","term_text":"CAD52385"}}CAD52385) we had previously identified in P. falciparum schizont-stage DRMs (17).We show here that MAL13P1.130 and PFD1110w, termed PfGAPM1 and PfGAPM2 (glideosome-associated protein with multiple-membrane spans), respectively, belong to a family of proteins specific to the Apicomplexa and demonstrate that P. falciparum GAPM proteins, and their orthologues in T. gondii, localize to the parasite IMC. The GAPMs form high molecular weight complexes that are resistant to dissociation and solubilization by a variety of common detergents and could therefore be components of the intramembranous particles seen in electron microscopy. When isolated by immunoprecipitation, the GAPM complexes co-purify with components of the actin-myosin motor and particularly the parasite cytoskeletal network suggesting GAPMs could anchor the IMC to the cytoskeleton and perhaps even play a role in tethering the motor to cytoskeleton.