Analysis of Plasmodium falciparum myosin B ATPase activity and structure in complex with the calmodulin-like domain of its light chain MLC-B

Myosin B (MyoB) is a class 14 myosin expressed in all invasive stages of the malaria parasite, Plasmodium falciparum. It is not associated with the glideosome complex that drives motility and invasion of host cells. During red blood cell invasion, MyoB remains at the apical tip of the merozoite but is no longer observed once invasion is completed. MyoB is not essential for parasite survival, but when it is knocked out, merozoites are delayed in the initial stages of red blood cell invasion, giving rise to a growth defect that correlates with reduced invasion success. Therefore, further characterization is needed to understand how MyoB contributes to parasite invasion. Here, we have expressed and purified functional MyoB with the help of parasite-specific chaperones Hsp90 and Unc45, characterized its binding to actin and its known light chain MLC-B using biochemical and biophysical methods and determined its low-resolution structure in solution using small angle X-ray scattering. In addition to MLC-B, we found that four other putative regulatory light chains bind to the MyoB IQ2 motif in vitro. The purified recombinant MyoB adopted the overall shape of a myosin, exhibited actin-activated ATPase activity, and moved actin filaments in vitro. Additionally, we determined that the ADP release rate was faster than the ATP turnover number, and thus, does not appear to be rate limiting. This, together with the observed high affinity to actin and the specific localization of MyoB, may point toward a role in tethering and/or force sensing during early stages of invasion.

A De novo Peptide from a High Throughput Peptide Library Blocks Myosin A -MTIP Complex Formation in Plasmodium falciparum

Apicomplexan parasites, through their motor machinery, produce the required propulsive force critical for host cell-entry. The conserved components of this so-called glideosome machinery are myosin A and myosin A Tail Interacting Protein (MTIP). MTIP tethers myosin A to the inner membrane complex of the parasite through 20 amino acid-long C-terminal end of myosin A that makes direct contacts with MTIP, allowing the invasion of Plasmodium falciparum in erythrocytes. Here, we discovered through screening a peptide library, a de-novo peptide ZA1 that binds the myosin A tail domain. We demonstrated that ZA1 bound strongly to myosin A tail and was able to disrupt the native myosin A tail MTIP complex both in vitro and in vivo. We then showed that a shortened peptide derived from ZA1, named ZA1S, was able to bind myosin A and block parasite invasion. Overall, our study identified a novel anti-malarial peptide that could be used in combination with other antimalarials for blocking the invasion of Plasmodium falciparum.

Systematic analysis of Plasmodium myosins reveals differential expression, localisation, and function in invasive and proliferative parasite stages

The myosin superfamily comprises of actin-dependent eukaryotic molecular motors important in a variety of cellular functions. Although well studied in many systems, knowledge of their functions in Plasmodium, the causative agent of malaria, is restricted. Previously, six myosins were identified in this genus, including three Class XIV myosins found only in Apicomplexa and some Ciliates. The well characterized MyoA is a Class XIV myosin essential for gliding motility and invasion. Here, we characterize all other Plasmodium myosins throughout the parasite life cycle and show that they have very diverse patterns of expression and cellular location. MyoB and MyoE, the other two Class XIV myosins, are expressed in all invasive stages, with apical and basal locations, respectively. Gene deletion revealed that MyoE is involved in sporozoite traversal, MyoF and MyoK are likely essential in the asexual blood stages, and MyoJ and MyoB are not essential. Both MyoB and its essential light chain (MCL-B) are localised at the apical end of ookinetes but expressed at completely different time points. This work provides a better understanding of the role of actomyosin motors in Apicomplexan parasites, particularly in the motile and invasive stages of Plasmodium during sexual and asexual development within the mosquito.

Systematic Comparison of Strategies to Achieve Soluble Expression of Plasmodium falciparum Recombinant Proteins in E. coli

Constructs containing partial coding sequences of myosin A, myosin B, and glideosome-associated protein (50 kDa) of Plasmodium falciparum were used to challenge several strategies designed in order to improve the production and solubility of recombinant proteins in Escherichia coli. Assays were carried out inducing expression in a late log phase culture, optimizing the inductor concentration, reducing the growth temperature for induced cultures, and supplementing additives in the lysis buffer. In addition, recombinant proteins were expressed as fusion proteins with three different tags (6His, GST, and MBP) in four different E. coli strains. We found that the only condition that consistently produced soluble proteins was the use of MBP as a fusion tag, which became a valuable tool for detecting the proteins used in this study and did not caused any interference in protein-protein interaction assays (Far Western Blot). Besides, we found that BL21-pG-KJE8 strain did not improve the solubility of any of the recombinant protein produced, while the BL21-CodonPlus(DE3)-RIL strain improved the expression of some of them independent of the rare codon content. Proteins with rare codons occurring at high frequencies (» 10%) were expressed efficiently in strains that do not supplement tRNAs for these triplets.

In vitro interaction between Plasmodium falciparum myosin B (PfMyoB) and myosin A tail interacting protein (MTIP)

Apicomplexan parasites, including Plasmodium falciparum, are obligate intracellular organisms that utilize a strategy termed "gliding" to move and invade host cells, causing disease. Gliding is carried out by a protein complex known as the glideosome, which includes an actin-myosin motor. To date, six myosins have been identified in P. falciparum (PfMyoA, B, C, D, E, and F), but only the role of PfMyoA, the myosin of the glideosome that is involved in the process of red blood cell and mosquito cell invasion, has been established. Based on previous observations, we speculated that PfMyoA and PfMyoB may have similar or redundant functions. To test this hypothesis, we searched for in vitro interactions between PfMyoB and MTIP (myosin A tail interacting protein), the myosin light chain of PfMyoA. A set of differentially tagged PfMyoA, PfMyoB, and MTIP recombinant proteins was employed to specifically and simultaneously detect each myosin in competition assays and inhibition assays using specific peptides. MTIP potentially acts as the light chain of PfMyoB.

Myosin B of Plasmodium falciparum (PfMyoB): in silico prediction of its three-dimensional structure and its possible interaction with MTIP

The mobility and invasion strategy of Plasmodium falciparum is governed by a protein complex known as the glideosome, which contains an actin-myosin motor. It has been shown that myosin A of the parasite (PfMyoA) is the myosin of the glideosome, and the interaction of PfMyoA with myosin tail domain interacting protein (MTIP) determines its correct location and its ability to function in the complex. Because PfMyoA and myosin B of P. falciparum (PfMyoB) share high sequence identity, are both small proteins without a tail domain, belong to the class XIV myosins, and are expressed in late schizonts and merozoites, we suspect that these myosins may have similar or redundant functions. Therefore, this work examined the structural similarity between PfMyoA and PfMyoB and performed a molecular docking between PfMyoB and MTIP. Three-dimensional (3D) models obtained for PfMyoA and PfMyoB achieved high scores in the structural validation programs used, and their superimposition revealed high structural similarity, supporting the hypothesis of possible similar functions for these two proteins. The 3D interaction models obtained and energy values found suggested that interaction between PfMyoB and MTIP is possible. Given the apparent abundance of PfMyoA relative to PfMyoB in the parasite, we believe that the interaction between PfMyoB and MTIP would only be detectable in specific cellular environments because under normal circumstances, it would be masked by the interaction between PfMyoA and MTIP.

The Plasmodium Class XIV Myosin, MyoB, Has a Distinct Subcellular Location in Invasive and Motile Stages of the Malaria Parasite and an Unusual Light Chain

Myosin B (MyoB) is one of the two short class XIV myosins encoded in the Plasmodium genome. Class XIV myosins are characterized by a catalytic "head," a modified "neck," and the absence of a "tail" region. Myosin A (MyoA), the other class XIV myosin in Plasmodium, has been established as a component of the glideosome complex important in motility and cell invasion, but MyoB is not well characterized. We analyzed the properties of MyoB using three parasite species as follows: Plasmodium falciparum, Plasmodium berghei, and Plasmodium knowlesi. MyoB is expressed in all invasive stages (merozoites, ookinetes, and sporozoites) of the life cycle, and the protein is found in a discrete apical location in these polarized cells. In P. falciparum, MyoB is synthesized very late in schizogony/merogony, and its location in merozoites is distinct from, and anterior to, that of a range of known proteins present in the rhoptries, rhoptry neck or micronemes. Unlike MyoA, MyoB is not associated with glideosome complex proteins, including the MyoA light chain, myosin A tail domain-interacting protein (MTIP). A unique MyoB light chain (MLC-B) was identified that contains a calmodulin-like domain at the C terminus and an extended N-terminal region. MLC-B localizes to the same extreme apical pole in the cell as MyoB, and the two proteins form a complex. We propose that MLC-B is a MyoB-specific light chain, and for the short class XIV myosins that lack a tail region, the atypical myosin light chains may fulfill that role.

Involvement of actin and myosins in Plasmodium berghei ookinete motility

Ookinetes of the genus Plasmodium are motile, invasive cells that develop in the mosquito midgut following ingestion of a parasite-infected blood meal. We show here that ookinetes display gliding motility on glass slides in the presence of insect cells. Moreover, in addition to stationary "flexing" and "twirling" of the cells, two distinct types of movements occur: productive forward translocational motility in straight segment that progresses with an average speed of approximately 6mum/min and rotational motility, which does not lead to forward translocation. Locomotion is reduced by treatment with butanedione monoxime, an inhibitor of myosin ATPase, and by three different actin inhibitors. We also studied the expression during ookinete development of genes encoding actin and two small class XIV myosins, PbMyoA, and PbMyoB. Western immunoblots revealed that PbMyoA is only present in fully mature ookinetes, whilst the other two proteins are additionally expressed in gametocytes and zygotes. Immunofluorescence experiments reveal that MyoA and actin co-localize in the apical tip of the parasite whereas MyoB displays a punctate pattern of expression around the entire cell periphery. Following treatment with jasplakinolide, the apparent level of detectable actin appears to substantially increase and becomes concentrated in a discrete area in the basal pole of the ookinete.

Cellular and Molecular Mechanics of Gliding Locomotion in Eukaryotes

Gliding is a form of substrate-dependent cell locomotion exploited by a variety of disparate cell types. Cells may glide at rates well in excess of 1 microm/sec and do so without the gross distortion of cellular form typical of amoeboid crawling. In the absence of a discrete locomotory organelle, gliding depends upon an assemblage of molecules that links cytoplasmic motor proteins to the cell membrane and thence to the appropriate substrate. Gliding has been most thoroughly studied in the apicomplexan parasites, including Plasmodium and Toxoplasma, which employ a unique assortment of proteins dubbed the glideosome, at the heart of which is a class XIV myosin motor. Actin and myosin also drive the gliding locomotion of raphid diatoms (Bacillariophyceae) as well as the intriguing form of gliding displayed by the spindle-shaped cells of the primitive colonial protist Labyrinthula. Chlamydomonas and other flagellated protists are also able to abandon their more familiar swimming locomotion for gliding, during which time they recruit a motility apparatus independent of that driving flagellar beating.

Plasmodium falciparum myosins: transcription and translation during asexual parasite development

Six myosins genes are now annotated in the Plasmodium falciparum Genome Project. Malaria myosins have been named alphabetically; accordingly, we refer to the two latest additions as Pfmyo-E and Pfmyo-F. Both new myosins contain regions characteristic of the functional motor domain of "true" myosins and, unusually for P. falciparum myosins, Pfmyo-F encodes two consensus IQ light chain-binding motifs. Phylogenetic analysis of the 17 currently known apicomplexan myosins together with one representative of each myosin class clusters all but one of the apicomplexan sequences together in Class XIV. This refines the earlier definition of the Class XIV Subclasses XIVa and XIVb. RT-PCR on blood stage parasite mRNA amplifies a specific product for all six myosins and each shows developmentally regulated transcription. Thus: Pfmyo-A and Pfmyo-B genes are transcribed throughout development; Pfmyo-C is predominant in trophozoites; Pfmyo-D occurs in trophozoites and schizonts; Pfmyo-E though barely present in earlier stages is abundant in schizonts; Pfmyo-F increases steadily throughout development and maturation. It is known that Pfmyo-A and Pfmyo-B are synthesised during late schizogony and we now show that Pfmyo-D expression is also temporally regulated to late trophozoites and schizonts where it distributes close to segregating nuclei. Thus, in asexual stages myosin synthesis does not always parallel transcript accumulation, showing that translation is also regulated. The implication is that the mRNAs are either subjected to turnover, synthesised and degraded, or that they are sequestered in an inactivate form until required for protein synthesis.