Differences in erythrocyte receptor specificity of different parts of the Plasmodium falciparum reticulocyte binding protein homologue 2a

The Cellular and Molecular Interaction Between Erythrocytes and Plasmodium falciparum Merozoites

Plasmodium falciparum is the most lethal human malaria parasite, partly due to its genetic variability and ability to use multiple invasion routes via its binding to host cell surface receptors. The parasite extensively modifies infected red blood cell architecture to promote its survival which leads to increased cell membrane rigidity, adhesiveness and permeability. Merozoites are initially released from infected hepatocytes and efficiently enter red blood cells in a well-orchestrated process that involves specific interactions between parasite ligands and erythrocyte receptors; symptoms of the disease occur during the life-cycle’s blood stage due to capillary blockage and massive erythrocyte lysis. Several studies have focused on elucidating molecular merozoite/erythrocyte interactions and host cell modifications; however, further in-depth analysis is required for understanding the parasite’s biology and thus provide the fundamental tools for developing prophylactic or therapeutic alternatives to mitigate or eliminate Plasmodium falciparum-related malaria. This review focuses on the cellular and molecular events during Plasmodium falciparum merozoite invasion of red blood cells and the alterations that occur in an erythrocyte once it has become infected.

Alternative Invasion Mechanisms and Host Immune Response to Plasmodium vivax Malaria: Trends and Future Directions

Plasmodium vivax malaria is a neglected tropical disease, despite being more geographically widespread than any other form of malaria. The documentation of P. vivax infections in different parts of Africa where Duffy-negative individuals are predominant suggested that there are alternative pathways for P. vivax to invade human erythrocytes. Duffy-negative individuals may be just as fit as Duffy-positive individuals and are no longer resistant to P.vivax malaria. In this review, we describe the complexity of P. vivax malaria, characterize pathogenesis and candidate invasion genes of P. vivax, and host immune responses to P. vivax infections. We provide a comprehensive review on parasite ligands in several Plasmodium species that further justify candidate genes in P. vivax. We also summarize previous genomic and transcriptomic studies related to the identification of ligand and receptor proteins in P. vivax erythrocyte invasion. Finally, we identify topics that remain unclear and propose future studies that will greatly contribute to our knowledge of P. vivax.

A processing product of the Plasmodium falciparum reticulocyte binding protein RH1 shows a close association with AMA1 during junction formation

Plasmodium falciparum responsible for the most virulent form of malaria invades human erythrocytes through multiple ligand-receptor interactions. The P. falciparum reticulocyte binding protein homologues (PfRHs) are expressed at the apical end of merozoites and form interactions with distinct erythrocyte surface receptors that are important for invasion. Here using a range of monoclonal antibodies (mAbs) against different regions of PfRH1 we have investigated the role of PfRH processing during merozoite invasion. We show that PfRH1 gets differentially processed during merozoite maturation and invasion and provide evidence that the different PfRH1 processing products have distinct functions during invasion. Using in-situ Proximity Ligation and FRET assays that allow probing of interactions at the nanometre level we show that a subset of PfRH1 products form close association with micronemal proteins Apical Membrane Antigen 1 (AMA1) in the moving junction suggesting a critical role in facilitating junction formation and active invasion. Our data provides evidence that time dependent processing of PfRH proteins is a mechanism by which the parasite is able to regulate distinct functional activities of these large processes. The identification of a specific close association with AMA1 in the junction now may also provide new avenues to target these interactions to prevent merozoite invasion.

Plasmodium falciparum Merozoite Associated Armadillo Protein (PfMAAP) Is Apically Localized in Free Merozoites and Antibodies Are Associated With Reduced Risk of Malaria

Understanding the functional role of proteins expressed by Plasmodium falciparum is an important step toward unlocking potential targets for the development of therapeutic or diagnostic interventions. The armadillo (ARM) repeat protein superfamily is associated with varied functions across the eukaryotes. Therefore, it is important to understand the role of members of this protein family in Plasmodium biology. The Plasmodium falciparum armadillo repeats only (PfARO; Pf3D7_0414900) and P. falciparum merozoite organizing proteins (PfMOP; Pf3D7_0917000) are armadillo-repeat containing proteins previously characterized in P. falciparum. Here, we describe the characterization of another ARM repeat-containing protein in P. falciparum, which we have named the P. falciparum Merozoites-Associated Armadillo repeats protein (PfMAAP). Antibodies raised to three different synthetic peptides of PfMAAP show apical staining of free merozoites and those within the mature infected schizont. We also demonstrate that the antibodies raised to the PfMAAP peptides inhibited invasion of erythrocytes by merozoites from different parasite isolates. In addition, naturally acquired human antibodies to the N- and C- termini of PfMAAP are associated with a reduced risk of malaria in a prospective cohort analysis.

Temporal changes in Plasmodium falciparum reticulocyte binding protein homolog 2b (PfRh2b) in Senegal and The Gambia

BACKGROUND

The Plasmodium falciparum reticulocyte binding protein homolog 2b (PfRh2b) is an important P. falciparum merozoite ligand that mediates invasion of erythrocytes by interacting with a chymotrypsin-sensitive "receptor Z". A large deletion polymorphism is found in the c-terminal ectodomain of this protein in many countries around the world, resulting in a truncated, but expressed protein. The varying frequencies by region suggest that there could be region specific immune selection at this locus. Therefore, this study was designed to determine temporal changes in the PfRh2b deletion polymorphism in infected individuals from Thiès (Senegal) and Western Gambia (The Gambia). It was also sought to determine the selective pressures acting at this locus and whether prevalence of the deletion in isolates genotyped by a 24-SNP molecular barcode is linked to background genotype or whether there might be independent selection acting at this locus.

METHODS

Infected blood samples were sourced from archives of previous studies conducted between 2007 and 2013 at SLAP clinic in Thiès and from 1984 to 2013 in Western Gambia by MRC Unit at LSHTM, The Gambia. A total of 1380 samples were screened for the dimorphic alleles of the PfRh2b using semi-nested Polymerase Chain Reaction PCR. Samples from Thiès were previously barcoded.

RESULTS

In Thiès, a consistent trend of decreasing prevalence of the PfRh2b deletion over time was observed: from 66.54% in 2007 and to 38.1% in 2013. In contrast, in Western Gambia, the frequency of the deletion fluctuated over time; it increased between 1984 and 2005 from (58.04%) to (69.33%) and decreased to 47.47% in 2007. Between 2007 and 2012, the prevalence of this deletion increased significantly from 47.47 to 83.02% and finally declined significantly to 57.94% in 2013. Association between the presence of this deletion and age was found in Thiès, however, not in Western Gambia. For the majority of isolates, the PfRh2b alleles could be tracked with specific 24-SNP barcoded genotype, indicating a lack of independent selection at this locus.

CONCLUSION

PfRh2b deletion was found in the two countries with varying prevalence during the study period. However, these temporal and spatial variations could be an obstacle to the implementation of this protein as a potential vaccine candidate.

A forward genetic screen reveals a primary role for Plasmodium falciparum Reticulocyte Binding Protein Homologue 2a and 2b in determining alternative erythrocyte invasion pathways

Invasion of human erythrocytes is essential for Plasmodium falciparum parasite survival and pathogenesis, and is also a complex phenotype. While some later steps in invasion appear to be invariant and essential, the earlier steps of recognition are controlled by a series of redundant, and only partially understood, receptor-ligand interactions. Reverse genetic analysis of laboratory adapted strains has identified multiple genes that when deleted can alter invasion, but how the relative contributions of each gene translate to the phenotypes of clinical isolates is far from clear. We used a forward genetic approach to identify genes responsible for variable erythrocyte invasion by phenotyping the parents and progeny of previously generated experimental genetic crosses. Linkage analysis using whole genome sequencing data revealed a single major locus was responsible for the majority of phenotypic variation in two invasion pathways. This locus contained the PfRh2a and PfRh2b genes, members of one of the major invasion ligand gene families, but not widely thought to play such a prominent role in specifying invasion phenotypes. Variation in invasion pathways was linked to significant differences in PfRh2a and PfRh2b expression between parasite lines, and their role in specifying alternative invasion was confirmed by CRISPR-Cas9-mediated genome editing. Expansion of the analysis to a large set of clinical P. falciparum isolates revealed common deletions, suggesting that variation at this locus is a major cause of invasion phenotypic variation in the endemic setting. This work has implications for blood-stage vaccine development and will help inform the design and location of future large-scale studies of invasion in clinical isolates.

Plasmodium parasites cause more than 200 million cases of malaria each year. All the symptoms of malaria are caused after Plasmodium parasites invade human red blood cells. Once inside, they grow, multiply and break open the red blood cells to release new parasites. This cycle is repeated every 48 hours, rapidly amplifying the number of parasites and causing severe anemia and other complications. Plasmodium falciparum, the parasite species responsible for almost all malaria deaths, can use multiple different pathways to invade human red blood cells, but the relative importance of each is not well understood. We tested the invasion pathways used by a collection of closely related parasites and compared their genome sequences to identify the genes responsible. This analysis revealed that expression differences in two neighboring genes of the Reticulocyte Binding Homologue family are responsible for most of the variation in two invasion pathways. P. falciparum may use variation in these genes to avoid the immune system or adapt to specific blood groups, which has important implications for vaccine development against malaria.

Functional Analysis Reveals Geographical Variation in Inhibitory Immune Responses Against a Polymorphic Malaria Antigen

Background

Plasmodium falciparum reticulocyte-binding protein homologue 2b (PfRh2b) is an invasion ligand that is a potential blood-stage vaccine candidate antigen; however, a naturally occurring deletion within an immunogenic domain is present at high frequencies in Africa and has been associated with alternative invasion pathway usage. Standardized tools that provide antigenic specificity in in vitro assays are needed to functionally assess the neutralizing potential of humoral responses against malaria vaccine candidate antigens.

Methods

Transgenic parasite lines were generated to express the PfRh2b deletion. Total immunoglobulin G (IgG) from individuals residing in malaria-endemic regions in Tanzania, Senegal, and Mali were used in growth inhibition assays with transgenic parasite lines.

Results

While the PfRh2b deletion transgenic line showed no change in invasion pathway utilization compared to the wild-type in the absence of specific antibodies, it outgrew wild-type controls in competitive growth experiments. Inhibition differences with total IgG were observed in the different endemic sites, ranging from allele-specific inhibition to allele-independent inhibitory immune responses.

Conclusions

The PfRh2b deletion may allow the parasite to escape neutralizing antibody responses in some regions. This difference in geographical inhibition was revealed using transgenic methodologies, which provide valuable tools for functionally assessing neutralizing antibodies against vaccine-candidate antigens in regions with varying malaria endemicity.

P. falciparum RH5-Basigin interaction induces changes in the cytoskeleton of the host RBC

The successful invasion of Plasmodium is an essential step in their life cycle. The parasite reticulocyte-binding protein homologues (RHs) and erythrocyte-binding like proteins are two families involved in the invasion leading to merozoite-red blood cell (RBC) junction formation. Ca²⁺ signaling has been shown to play a critical role in the invasion. RHs have been linked to Ca²⁺ signaling, which triggers the erythrocyte-binding like proteins release ahead of junction formation, consistent with RHs performing an initial sensing function in identifying suitable RBCs. RH5, the only essential RHs, is a highly promising vaccine candidate. RH5-basigin interaction is essential for merozoite invasion and also important in determining host tropism. Here, we show that RH5 has a distinct function from the other RHs. We show that RH5-Basigin interaction on its own triggers a Ca²⁺ signal in the RBC resulting in changes in RBC cytoskeletal proteins phosphorylation and overall alterations in RBC cytoskeleton architecture. Antibodies targeting RH5 that block the signal prevent invasion before junction formation consistent with the Ca²⁺ signal in the RBC leading to rearrangement of the cytoskeleton required for invasion. This work provides the first time a functional context for the essential role of RH5 and will now open up new avenues to target merozoite invasion.

De novo assembly and transcriptome analysis of Plasmodium gallinaceum identifies the Rh5 interacting protein (ripr), and reveals a lack of EBL and RH gene family diversification

Background

Malaria parasites that infect birds can have narrow or broad host-tropisms. These differences in host specificity make avian malaria a useful model for studying the evolution and transmission of parasite assemblages across geographic ranges. The molecular mechanisms involved in host-specificity and the biology of avian malaria parasites in general are important aspects of malaria pathogenesis that warrant further examination. Here, the transcriptome of the malaria parasite Plasmodium gallinaceum was characterized to investigate the biology and the conservation of genes across various malaria parasite species.

Methods

The P. gallinaceum transcriptome was annotated and KEGG pathway mapping was performed. The ripr gene and orthologous genes that play critical roles in the purine salvage pathway were identified and characterized using bioinformatics and phylogenetic methods.

Results

Analysis of the transcriptome sequence database identified essential genes of the purine salvage pathway in P. gallinaceum that shared high sequence similarity to Plasmodium falciparum when compared to other mammalian Plasmodium spp. However, based on the current sequence data, there was a lack of orthologous genes that belonged to the erythrocyte-binding-like (EBL) and reticulocyte-binding-like homologue (RH) family in P. gallinaceum. In addition, an orthologue of the Rh5 interacting protein (ripr) was identified.

Conclusions

These findings suggest that the pathways involved in parasite red blood cell invasion are significantly different in avian Plasmodium parasites, but critical metabolic pathways are conserved throughout divergent Plasmodium taxa.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0814-0) contains supplementary material, which is available to authorized users.

Triggers of key calcium signals during erythrocyte invasion by Plasmodium falciparum

Invasion of erythrocytes by Plasmodium falciparum merozoites is a complex multi-step process mediated by specific interactions between host receptors and parasite ligands. Reticulocyte-binding protein homologues (RHs) and erythrocyte-binding-like (EBL) proteins are discharged from specialized organelles and used in early steps of invasion. Here we show that monoclonal antibodies against PfRH1 (an RH) block merozoite invasion by specifically inhibiting calcium signalling in the parasite, whereas invasion-inhibiting monoclonal antibodies targeting EBA175 (an EBL protein) have no effect on signalling. We further show that inhibition of this calcium signalling prevents EBA175 discharge and thereby formation of the junction between parasite and host cell. Our results indicate that PfRH1 has an initial sensing as well as signal transduction role that leads to the subsequent release of EBA175. They also provide new insights on how RH-host cell interactions lead to essential downstream signalling events in the parasite, suggesting new targets for malaria intervention.

Complement activation by merozoite antigens of Plasmodium falciparum

Background

Complement (C) is a crucial part of the innate immune system and becomes over activated during malaria, resulting in depletion of C components, especially those for lectin pathway (LP), thereby compromising the host's innate defense. In this study, involvement of P. falciparum antigens in C activation was investigated.

Methods

A highly synchronous culture of the Dd2 clone of P. falciparum was established in a serum free medium. Supernatants harvested from rings, trophozoites and schizonts at various parasite densities were tested for ability to activate C by quantifying amount of C3b deposited on erythrocytes (E). Uninfected sham culture was used as control. Remnants of each C pathway were determined using Wieslab complement System Screenkit (Euro-diagnostica, Sweden). To identify MBL binding antigens of LP, culture supernatants were added to MBL sepharose columns and trapped antigens eluted with increasing concentrations of EDTA (10 mM, 50 mM and 100 mM) and then desalted before being tested for ability to activate C. The EDTA eluate with highest activity was run on a polyacrylamide gel and silver stained proteins analyzed by mass spectroscopy.

Results

Antigens released by P. falciparum growing in culture activated C leading to C3b deposition on E. Maximal activation at 7% parasitemia was associated with schizont stage (36.7%) compared to 22% for rings, 21% for trophozoites and 3% for sham culture. All the three pathways of C were activated, with highest activation being for the alternative pathway (only 6% of C activation potential remained), 65% for classiical and 43% for the LP. Seven MBL binding merozoite proteins were identified by mass spectrometry in the 50 mM EDTA eluate.

Conclusions

MBL binding merozoite adhesins with ability to activate C pathway were identified. The survival advantage for such pronounced C activation is unclear, but opsonisation could facilitate recognition and invasion of E.

Specific phosphorylation of the PfRh2b invasion ligand of Plasmodium falciparum

Red blood cell invasion by the malaria parasite Plasmodium falciparum relies on a complex protein network that uses low and high affinity receptor–ligand interactions. Signal transduction through the action of specific kinases is a control mechanism for the orchestration of this process. In the present study we report on the phosphorylation of the CPD (cytoplasmic domain) of P. falciparum Rh2b (reticulocyte homologue protein 2b). First, we identified Ser³²³³ as the sole phospho-acceptor site in the CPD for in vitro phosphorylation by parasite extract. We provide several lines of evidence that this phosphorylation is mediated by PfCK2 (P. falciparum casein kinase 2): phosphorylation is cAMP independent, utilizes ATP as well as GTP as phosphate donors, is inhibited by heparin and tetrabromocinnamic acid, and is mediated by purified PfCK2. We raised a phospho-specific antibody and showed that Ser³²³³ phosphorylation occurs in the parasite prior to host cell egress. We analysed the spatiotemporal aspects of this phosphorylation using immunoprecipitated endogenous Rh2b and minigenes expressing the CPD either at the plasma or rhoptry membrane. Phosphorylation of Rh2b is not spatially restricted to either the plasma or rhoptry membrane and most probably occurs before Rh2b is translocated from the rhoptry neck to the plasma membrane.

Erythrocyte-binding antigens of Plasmodium falciparum are targets of human inhibitory antibodies and function to evade naturally acquired immunity

Abs that inhibit Plasmodium falciparum invasion of erythrocytes form an important component of human immunity against malaria, but key target Ags are largely unknown. Phenotypic variation by P. falciparum mediates the evasion of inhibitory Abs, contributing to the capacity of P. falciparum to cause repeat and chronic infections. However, Ags involved in mediating immune evasion have not been defined, and studies of the function of human Abs are limited. In this study, we used novel approaches to determine the importance of P. falciparum erythrocyte-binding Ags (EBAs), which are important invasion ligands, as targets of human invasion-inhibitory Abs and define their role in contributing to immune evasion through variation in function. We evaluated the invasion-inhibitory activity of acquired Abs from malaria-exposed children and adults from Kenya, using P. falciparum with disruption of genes encoding EBA140, EBA175, and EBA181, either individually or combined as EBA140/EBA175 or EBA175/EBA181 double knockouts. Our findings provide important new evidence that variation in the expression and function of the EBAs plays an important role in evasion of acquired Abs and that a substantial amount of phenotypic diversity results from variation in expression of different EBAs that contributes to immune evasion by P. falciparum. All three EBAs were identified as important targets of naturally acquired inhibitory Abs demonstrated by differential inhibition of parental parasites greater than EBA knockout lines. This knowledge will help to advance malaria vaccine development and suggests that multiple invasion ligands need to be targeted to overcome the capacity of P. falciparum for immune evasion.

Reticulocytes: Plasmodium vivax target cells

Reticulocytes represent the main invasion target for Plasmodium vivax, the second most prevalent parasite species around the world causing malaria in humans. In spite of these cells' importance in research into malaria, biological knowledge related to the nature of the host has been limited, given the technical difficulties present in working with them in the laboratory. Poor reticulocyte recovery from total blood, by different techniques, has hampered continuous in vitro P. vivax cultures being developed, thereby delaying basic investigation in this parasite species. Intense research during the last few years has led to advances being made in developing methodologies orientated towards obtaining enriched reticulocytes from differing sources, thereby providing invaluable information for developing new strategies aimed at preventing infection caused by malaria. This review describes the most recent studies related to obtaining reticulocytes and discusses approaches which could contribute towards knowledge regarding molecular interactions between target cell proteins and their main infective agent, P. vivax.

Identification of a potent combination of key Plasmodium falciparum merozoite antigens that elicit strain-transcending parasite-neutralizing antibodies

Blood-stage malaria vaccines that target single Plasmodium falciparum antigens involved in erythrocyte invasion have not induced optimal protection in field trials. Blood-stage malaria vaccine development has faced two major hurdles, antigenic polymorphisms and molecular redundancy, which have led to an inability to demonstrate potent, strain-transcending, invasion-inhibitory antibodies. Vaccines that target multiple invasion-related parasite proteins may inhibit erythrocyte invasion more efficiently. Our approach is to develop a receptor-blocking blood-stage vaccine against P. falciparum that targets the erythrocyte binding domains of multiple parasite adhesins, blocking their interaction with their receptors and thus inhibiting erythrocyte invasion. However, with numerous invasion ligands, the challenge is to identify combinations that elicit potent strain-transcending invasion inhibition. We evaluated the invasion-inhibitory activities of 20 different triple combinations of antibodies mixed in vitro against a diverse set of six key merozoite ligands, including the novel ligands P. falciparum apical asparagine-rich protein (PfAARP), EBA-175 (PfF2), P. falciparum reticulocyte binding-like homologous protein 1 (PfRH1), PfRH2, PfRH4, and Plasmodium thrombospondin apical merozoite protein (PTRAMP), which are localized in different apical organelles and are translocated to the merozoite surface at different time points during invasion. They bind erythrocytes with different specificities and are thus involved in distinct invasion pathways. The antibody combination of EBA-175 (PfF2), PfRH2, and PfAARP produced the most efficacious strain-transcending inhibition of erythrocyte invasion against diverse P. falciparum clones. This potent antigen combination was selected for coimmunization as a mixture that induced balanced antibody responses against each antigen and inhibited erythrocyte invasion efficiently. We have thus demonstrated a novel two-step screening approach to identify a potent antigen combination that elicits strong strain-transcending invasion inhibition, supporting its development as a receptor-blocking malaria vaccine.

The role of the reticulocyte-binding-like protein homologues of Plasmodium in erythrocyte sensing and invasion

Malaria remains a serious public health problem with significant morbidity and mortality accounting for nearly 20% of all childhood deaths in Africa. The cyclical invasion, cytoadherence and destruction of the host's erythrocyte by the parasite are responsible for the observed disease pathology. The invasive form of the parasite, the merozoite, uses a complex set of interactions between parasite ligands and erythrocyte receptors that leads to the formation of a tight junction and ultimately successful erythrocyte invasion. Understanding the molecular mechanism underlying host cell recognition and invasion is crucial for the development of a targeted intervention strategy. Two parasite protein families termed reticulocyte-binding-like protein homologues (RBL) and the erythrocyte-binding-like (EBL) protein family are conserved in all Plasmodium species and have been shown to play an important role in host cell recognition and invasion. Over the last few years significant new insights have been gained in understanding the function of the RBL family and this review attempts to provide an update with a specific focus on the role of RBL in signal transduction pathways during invasion.

Plasmodium falciparum field isolates from South America use an atypical red blood cell invasion pathway associated with invasion ligand polymorphisms

Studies of Plasmodium falciparum invasion pathways in field isolates have been limited. Red blood cell (RBC) invasion is a complex process involving two invasion protein families; Erythrocyte Binding-Like (EBL) and the Reticulocyte Binding-Like (PfRh) proteins, which are polymorphic and not fully characterized in field isolates. To determine the various P. falciparum invasion pathways used by parasite isolates from South America, we studied the invasion phenotypes in three regions: Colombia, Peru and Brazil. Additionally, polymorphisms in three members of the EBL (EBA-181, EBA-175 and EBL-1) and five members of the PfRh (PfRh1, PfRh2a, PfRh2b, PfRh4, PfRh5) families were determined. We found that most P. falciparum field isolates from Colombia and Peru invade RBCs through an atypical invasion pathway phenotypically characterized as resistant to all enzyme treatments (NrTrCr). Moreover, the invasion pathways and the ligand polymorphisms differed substantially among the Colombian and Brazilian isolates while the Peruvian isolates represent an amalgam of those present in the Colombian and Brazilian field isolates. The NrTrCr invasion profile was associated with the presence of the PfRh2a pepC variant, the PfRh5 variant 1 and EBA-181 RVNKN variant. The ebl and Pfrh expression levels in a field isolate displaying the NrTrCr profile also pointed to PfRh2a, PfRh5 and EBA-181 as being possibly the major players in this invasion pathway. Notably, our studies demonstrate the uniqueness of the Peruvian P. falciparum field isolates in terms of their invasion profiles and ligand polymorphisms, and present a unique opportunity for studying the ability of P. falciparum parasites to expand their invasion repertoire after being reintroduced to human populations. The present study is directly relevant to asexual blood stage vaccine design focused on invasion pathway proteins, suggesting that regional invasion variants and global geographical variation are likely to preclude a simple one size fits all type of vaccine.

Molecular interactions and signaling mechanisms during erythrocyte invasion by malaria parasites

Invasion of erythrocytes by Plasmodium merozoites is a complex process that is mediated by specific molecular interactions. Here, we review recent studies on interactions between erythrocyte binding antigens (EBA) and PfRH proteins from the parasite and erythrocyte receptors involved in invasion. The timely release of these parasite ligands from internal organelles such as micronemes and rhoptries to the merozoite surface is critical for receptor-engagement leading to successful invasion. We review information on signaling mechanisms that control the regulated secretion of parasite proteins during invasion. Erythrocyte invasion involves the formation and movement of a junction between the invading merozoite and host erythrocyte. We review recent studies on the molecular composition of the junction and the molecular motor that drives movement of the junction.