alpha14-Giardin (annexin E1) is associated with tubulin in trophozoites of Giardia lamblia and forms local slubs in the flagella

Characterization of a new extra-axonemal structure in the Giardia intestinalis flagella

The inner structure of the flagella of Giardia intestinalis is similar to that of other organisms, consisting of nine pairs of outer microtubules and a central pair containing radial spokes. Although the 9+2 axonemal structure is conserved, it is not clear whether subregions, including the transition zone, are present in the flagella of this parasite. Giardia axonemes originate from basal bodies and have a lengthy cytosolic portion before becoming active flagella. The region of the emergence of the flagellum is not accompanied by any membrane specialization, as seen in other protozoa. Although Giardia is an intriguing model of study, few works focused on the ultrastructural analysis of the flagella of this parasite. Here, we analyzed the externalization region of the G. intestinalis flagella using ultra-high resolution scanning microscopy (with electrons and ions), atomic force microscopy in liquid medium, freeze fracture, and electron tomography. Our data show that this region possesses a distinctive morphological feature - it extends outward and takes on a ring-like shape. When the plasma membrane is removed, a structure surrounding the axoneme becomes visible in this region. This new extra-axonemal structure is observed in all pairs of flagella of trophozoites and remains attached to the axoneme even when the interconnections between the axonemal microtubules are disrupted. High-resolution scanning electron microscopy provided insights into the arrangement of this structure, contributing to the characterization of the externalization region of the flagella of this parasite.

Transcriptomic analysis of albendazole resistance in human diarrheal parasite Giardia duodenalis

Benzimidazole-2-carbamates (BZ, e.g., albendazole; ALB), which bind β-tubulin to disrupt microtubule polymerization, are one of two primary compound classes used to treat giardiasis. In most parasitic nematodes and fungi, BZ-resistance is caused by β-tubulin mutations and its molecular mode of action (MOA) is well studied. In contrast, in Giardia duodenalis BZ MOA or resistance is less well understood, may involve target-specific and broader impacts including cellular damage and oxidative stress, and its underlying cause is not clearly determined. Previously, we identified acquisition of a single nucleotide polymorphism, E198K, in β-tubulin in ALB-resistant (ALB-R) G. duodenalis WB-1B relative to ALB-sensitive (ALB-S) parental controls. E198K is linked to BZ-resistance in fungi and its allelic frequency correlated with the magnitude of BZ-resistance in G. duodenalis WB-1B. Here, we undertook detailed transcriptomic comparisons of these ALB-S and ALB-R G. duodenalis WB-1B cultures. The primary transcriptional changes with ALB-R in G. duodenalis WB-1B indicated increased protein degradation and turnover, and up-regulation of tubulin, and related genes, associated with the adhesive disc and basal bodies. These findings are consistent with previous observations noting focused disintegration of the disc and associated structures in Giardia duodenalis upon ALB exposure. We also saw transcriptional changes with ALB-R in G. duodenalis WB-1B consistent with prior observations of a shift from glycolysis to arginine metabolism for ATP production and possible changes to aspects of the vesicular trafficking system that require further investigation. Finally, we saw mixed transcriptional changes associated with DNA repair and oxidative stress responses in the G. duodenalis WB-1B line. These changes may be indicative of a role for H₂O₂ degradation in ALB-R, as has been observed in other G. duodenalis cell cultures. However, they were below the transcriptional fold-change threshold (log₂FC > 1) typically employed in transcriptomic analyses and appear to be contradicted in ALB-R G. duodenalis WB-1B by down-regulation of the NAD scavenging and conversion pathways required to support these stress pathways and up-regulation of many highly oxidation sensitive iron-sulphur (FeS) cluster based metabolic enzymes.

The NLRP3 inflammasome recognizes alpha-2 and alpha-7.3 giardins and decreases the pathogenicity of Giardia duodenalis in mice

BACKGROUND

Giardia duodenalis is a parasitic organism that can cause giardiasis, an intestinal infection, particularly prevalent in young children, with clinical symptoms of diarrhea. We previously reported that extracellular G. duodenalis triggers intracellular nucleotide-binding oligomerization-like receptor 3 (NLRP3) inflammasome activation and regulates the host inflammatory response by secreting extracellular vesicles (EVs). However, the exact pathogen-associated molecular patterns in G. duodenalis EVs (GEVs) involved in this process and the role of the NLRP3 inflammasome in giardiasis remain to be elucidated.

METHODS

Recombinant eukaryotic expression plasmids of pcDNA3.1(+)-alpha-2 and alpha-7.3 giardins in GEVs were constructed, transfected into primary mouse peritoneal macrophages and screened by measuring the expression levels of the inflammasome target molecule caspase-1 p20. The preliminary identification of G. duodenalis alpha-2 and alpha-7.3 giardins was further verified by measuring the protein expression levels of key molecules of the NLRP3 inflammasome (NLRP3, pro-interleukin-1 beta [IL-1β], pro-caspase-1, and caspase-1 p20), the secretion levels of IL-1β, the level of apoptosis speck-like protein (ASC) oligomerization and the immunofluorescence localization of NLRP3 and ASC. The roles of the NLRP3 inflammasome in G. duodenalis pathogenicity were then evaluated using mice in which NLRP3 activation was blocked (NLRP3-blocked mice), and body weight, parasite burden in the duodenum and histopathological changes in the duodenum were monitored. In addition, we explored whether alpha-2 and alpha-7.3 giardins triggered IL-1β secretion in vivo through the NLRP3 inflammasome and determined the roles of these molecules in G. duodenalis pathogenicity in mice.

RESULTS

Alpha-2 and alpha-7.3 giardins triggered NLRP3 inflammasome activation in vitro. This led to caspase-1 p20 activation, upregulation of the protein expression levels of NLRP3, pro-IL-1β and pro-caspase-1, significant enhancement of IL-1β secretion, ASC speck formation in the cytoplasm and also induction of ASC oligomerization. Deletion of the NLRP3 inflammasome aggravated G. duodenalis pathogenicity in mice. Compared to wild-type mice gavaged with cysts, mice gavaged with cysts in NLRP3-blocked mice displayed increased trophozoite loads and severe duodenal villus damage, characterized by necrotic crypts with atrophy and branching. In vivo assays revealed that alpha-2 and alpha-7.3 giardins could induce IL-1β secretion through the NLRP3 inflammasome and that immunization with alpha-2 and alpha-7.3 giardins decreased G. duodenalis pathogenicity in mice.

CONCLUSIONS

Overall, the results of the present study revealed that alpha-2 and alpha-7.3 giardins trigger host NLRP3 inflammasome activation and decrease G. duodenalis infection ability in mice, which are promising targets for the prevention of giardiasis.

The structural organization of Giardia intestinalis cytoskeleton

Giardia intestinalis, the causative agent of giardiasis, has complex cytoskeleton organization with structures involved in motility, adhesion, cell division, and cell differentiation. Microtubules are key components of the cytoskeleton and are the main elements of the ventral disc, median body, funis, in addition to four pairs of flagella. These cytoskeletal elements are basically stable microtubule arrangements. Although tubulins are the main proteins of these elements, molecular and biochemical analyses of Giardia trophozoites have revealed the presence of several new and not yet characterized proteins in these structures, which may contribute to their nanoarchitecture (mainly in the ventral disc). Despite these findings, morphological data are still required for understanding the organization and biogenesis of the cytoskeletal structures. In the study of this complex and specialized network of filaments in Giardia, two distinct and complementary approaches have been used in recent years: (a) transmission electron microscopy tomography of conventionally processed as well as cryo-fixed samples and (b) high-resolution scanning electron microscopy and helium ion microscopy in combination with new plasma membrane extraction protocols. In this review we include the most recent studies that have allowed better understanding of new Giardia components and their association with other filamentous structures of this parasite, thus providing new insights in the role of the cytoskeletal structures and their function in Giardia trophozoites.

Microtubule organelles in Giardia

Giardia lamblia is a widespread parasitic protist with a complex MT cytoskeleton that is critical for motility, attachment, mitosis and cell division, and transitions between its two life cycle stages-the infectious cyst and flagellated trophozoite. Giardia trophozoites have both highly dynamic and highly stable MT organelles, including the ventral disc, eight flagella, the median body and the funis. The ventral disc, an elaborate MT organelle, is essential for the parasite's attachment to the intestinal villi to avoid peristalsis. Giardia's four flagellar pairs enable swimming motility and may also promote attachment. They are maintained at different equilibrium lengths and are distinguished by their long cytoplasmic regions and novel extra-axonemal structures. The functions of the median body and funis, MT organelles unique to Giardia, remain less understood. In addition to conserved MT-associated proteins, the genome is enriched in ankyrins, NEKs, and novel hypothetical proteins that also associate with the MT cytoskeleton. High-resolution ultrastructural imaging and a current inventory of more than 300 proteins associated with Giardia's MT cytoskeleton lay the groundwork for future mechanistic analyses of parasite attachment to the host, motility, cell division, and encystation/excystation. Giardia's unique MT organelles exemplify the capacity of MT polymers to generate intricate structures that are diverse in both form and function. Thus, beyond its relevance to pathogenesis, the study of Giardia's MT cytoskeleton informs basic cytoskeletal biology and cellular evolution. With the availability of new molecular genetic tools to disrupt gene function, we anticipate a new era of cytoskeletal discovery in Giardia.

Proximity Staining Using Enzymatic Protein Tagging in Diplomonads

The function of many proteins is intrinsically related to their cellular location. Novel methods for ascertainment of the ultrastructural location of proteins have been introduced in recent years, but their implementation in protists has so far not been readily realized. Here, we present an optimized proximity labeling protocol using the APEX system in the salmon pathogen Spironucleus salmonicida. This protocol was also applicable to the human pathogen Giardia intestinalis. Both organisms required extraneous addition of hemin to the growth medium to enable detectable peroxidase activity. Further, we saw no inherent limitation in labeling efficiency coupled to the cellular compartment, as evident with some other proximity labeling systems. We anticipate that the APEX proximity labeling system might offer a great resource to establish the ultrastructural localization of proteins across genetically tractable protists but might require organism-specific labeling conditions.

The diplomonads are a group of understudied eukaryotic flagellates whose most prominent member is the human pathogen Giardia intestinalis. Methods commonly used in other eukaryotic model systems often require special optimization in diplomonads due to the highly derived character of their cell biology. We have optimized a proximity labeling protocol using pea ascorbate peroxidase (APEX) as a reporter for transmission electron microscopy (TEM) to enable the study of ultrastructural cellular details in diplomonads. Currently available TEM-compatible tags require light-induced activation (1, 2) or are inactive in many cellular compartments (3), while ascorbate peroxidase has not been shown to have those limitations. Here, we have optimized the in vivo activities of two versions of pea ascorbate peroxidase (APX^W41F and APEX) using the diplomonad fish parasite Spironucleus salmonicida, a relative of G. intestinalis. We exploited the well-known peroxidase substrates, Amplex UltraRed and 3,3′-diaminobenzidine (DAB), to validate the activity of the two tags and argue that APEX is the most stable version to use in Spironucleus salmonicida. Next, we fused APEX to proteins with established localization to evaluate the activity of APEX in different cellular compartments of the diplomonad cell and used Amplex UltraRed as well as antibodies along with superresolution microscopy to confirm the protein-APEX localization. The ultrastructural details of protein-APEX fusions were determined by TEM, and we observed marker activity in all cellular compartments tested when using the DAB substrate. Finally, we show that the optimized conditions established for S. salmonicida can be used in the related diplomonad G. intestinalis.

IMPORTANCE The function of many proteins is intrinsically related to their cellular location. Novel methods for ascertainment of the ultrastructural location of proteins have been introduced in recent years, but their implementation in protists has so far not been readily realized. Here, we present an optimized proximity labeling protocol using the APEX system in the salmon pathogen Spironucleus salmonicida. This protocol was also applicable to the human pathogen Giardia intestinalis. Both organisms required extraneous addition of hemin to the growth medium to enable detectable peroxidase activity. Further, we saw no inherent limitation in labeling efficiency coupled to the cellular compartment, as evident with some other proximity labeling systems. We anticipate that the APEX proximity labeling system might offer a great resource to establish the ultrastructural localization of proteins across genetically tractable protists but might require organism-specific labeling conditions.

Divergent Transcriptional Responses to Physiological and Xenobiotic Stress in Giardia duodenalis

Understanding how parasites respond to stress can help to identify essential biological processes. Giardia duodenalis is a parasitic protist that infects the human gastrointestinal tract and causes 200 to 300 million cases of diarrhea annually. Metronidazole, a major antigiardial drug, is thought to cause oxidative damage within the infective trophozoite form. However, treatment efficacy is suboptimal, due partly to metronidazole-resistant infections. To elucidate conserved and stress-specific responses, we calibrated sublethal metronidazole, hydrogen peroxide, and thermal stresses to exert approximately equal pressure on trophozoite growth and compared transcriptional responses after 24 h of exposure. We identified 252 genes that were differentially transcribed in response to all three stressors, including glycolytic and DNA repair enzymes, a mitogen-activated protein (MAP) kinase, high-cysteine membrane proteins, flavin adenine dinucleotide (FAD) synthetase, and histone modification enzymes. Transcriptional responses appeared to diverge according to physiological or xenobiotic stress. Downregulation of the antioxidant system and α-giardins was observed only under metronidazole-induced stress, whereas upregulation of GARP-like transcription factors and their subordinate genes was observed in response to hydrogen peroxide and thermal stressors. Limited evidence was found in support of stress-specific response elements upstream of differentially transcribed genes; however, antisense derepression and differential regulation of RNA interference machinery suggest multiple epigenetic mechanisms of transcriptional control.

Immunolocalization of α18- and α12-giardin in Giardia lamblia trophozoites

To study subcellular localization of α18- and α12-giardin in Giardia lamblia trophozoites, the α18- and α12-giardin genes were amplified from G. lamblia assemblage A, respectively. The PCR products were cloned into the prokaryotic expression vector pET-28a(+), and the positive recombinant plasmids were transformed into E. coli Rosetta (DE3) strain for the expression, and expressed α18- and α12-giardin fusion protein were purified by Ni-Agarose resin, respectively. Mice were immunized with purified fusion proteins for preparation of polyclonal antibody, and then the subcellular localization of α18- and α12-giardin was determined by fluorescence immunoassay. Results showed that the concentrations of purified α18- and α12-giardin fusion proteins were 1.20 and 0.86 mg/ml, respectively. The titers of anti-α18- and anti-α12-giardin polyclonal antibody were both as high as 1:25600 dilutions. Immunofluorescent analysis showed that α18- and α12-giardin proteins were mainly localized at four pairs of flagella and the cytoplasm of G. lamblia trophozoites, suggesting that α18- and α12-giardin are the flagella and cytoplasm-associated proteins, respectively. The above information would lay the foundation for research about the crystal structure and biological function of α18- and α12-giardin.

Annexins family: insights into their functions and potential role in pathogenesis of sarcoidosis

Annexins are Ca²⁺-regulated phospholipid-binding proteins that play an important role in the cell life cycle, exocytosis, and apoptosis. Annexin A11 is one of the oldest vertebrate annexins that has a crucial role in sarcoidosis pathogenesis. The mechanism of effect in sarcoidosis granuloma cells may be due to alterations in apoptosis. Immune cells with a specific mutation at protein location 230 are resistant to apoptosis and consequently have continued effects on inflammation and progression of sarcoidosis. The mechanism of action of annexin A11 may be based upon alterations in delivering calcium to two different apoptosis pathways (caspase and P53).

Comparative Cell Biology and Evolution of Annexins in Diplomonads

Annexins are proteins that associate with phospholipids in a Ca²⁺-dependent fashion. These proteins have been intensely studied in animals and plants because of their importance in diverse cellular processes, yet very little is known about annexins in single-celled eukaryotes, which represent the largest diversity of organisms. The human intestinal parasite Giardia intestinalis is known to have more annexins than humans, and they contribute to its pathogenic potential. In this study, we investigated the annexin complement in the salmon pathogen Spironucleus salmonicida, a relative of G. intestinalis. We found that S. salmonicida has a large repertoire of annexins and that the gene family has expanded separately across diplomonads, with members showing sequence diversity similar to that seen across kingdom-level groups such as plants and animals. S. salmonicida annexins are prominent components of the cytoskeleton and membrane. Two annexins are associated with a previously unrecognized structure in the anterior of the cell.

Annexins are multifunctional, calcium-binding proteins found in organisms across all kingdoms. Most studies of annexins from single-celled eukaryotes have focused on the alpha-giardins, proteins assigned to the group E annexins, expressed by the diplomonad Giardia intestinalis. We have characterized the annexin gene family in another diplomonad parasite, Spironucleus salmonicida, by phylogenetic and experimental approaches. We constructed a comprehensive phylogeny of the diplomonad group E annexins and found that they are abundant across the group with frequent gene duplications and losses. The annexins of S. salmonicida were found to be related to alpha-giardins but with better-preserved type II Ca²⁺ coordination sites. Two annexins were confirmed to bind phospholipids in a Ca²⁺-dependent fashion but with different specificities. Superresolution and confocal microscopy of epitope-tagged S. salmonicida annexins revealed localization to distinct parts of the cytoskeleton and membrane. The ultrastructural details of the localization of several annexins were determined by proximity labeling and transmission electron microscopy. Two annexins localize to a novel cytoskeletal structure in the anterior of the cell. Our results show that the annexin gene family is expanded in diplomonads and that these group E annexins are associated mostly with cytoskeletal and membrane structures.

IMPORTANCE Annexins are proteins that associate with phospholipids in a Ca²⁺-dependent fashion. These proteins have been intensely studied in animals and plants because of their importance in diverse cellular processes, yet very little is known about annexins in single-celled eukaryotes, which represent the largest diversity of organisms. The human intestinal parasite Giardia intestinalis is known to have more annexins than humans, and they contribute to its pathogenic potential. In this study, we investigated the annexin complement in the salmon pathogen Spironucleus salmonicida, a relative of G. intestinalis. We found that S. salmonicida has a large repertoire of annexins and that the gene family has expanded separately across diplomonads, with members showing sequence diversity similar to that seen across kingdom-level groups such as plants and animals. S. salmonicida annexins are prominent components of the cytoskeleton and membrane. Two annexins are associated with a previously unrecognized structure in the anterior of the cell.

Sequence analysis and prokaryotic expression of Giardia lamblia α-18 giardin gene

To study the genetic variation and prokaryotic expression of α18 giardin gene of Giardia lamblia zoonotic assemblage A and host-specific assemblage F, the α18 genes were amplified from G. lamblia assemblages A and F by PCR and sequenced. The PCR product was cloned into the prokaryotic expression vector pET-28a(+) and the positive recombinant plasmid was transformed into Escherichia coli Rosetta (DE3) strain for the expression. The expressed α18 giardin fusion protein was validated by SDS-PAGE and Western blot analysis, and purified by Ni-Agarose resin. The putative sequence of α18 giardin amino acid was analyzed by bioinformatics software. Results showed that the α18 giardin gene was 861 bp in length, encoding 286 amino acids; it was 100% homologous between human-derived and dog-derived G. lamblia assemblage A, but it was 86.8% homologous with G. lamblia assemblage F (cat-derived). Giardin α18 was about 36 kDa in molecular weight, with good reactivity. Prediction based on in silico analyses: it had hydrophobicity, without signal peptide and transmembrane domain, and contained 11 alpha regions, 13 beta sheets, 1 beta turn and 7 random coils in secondary structure. The above information would lay the foundation for research about the subcellular localization and biological function of α18 giardin in G. lamblia.

Identification of α-11 giardin as a flagellar and surface component of Giardia lamblia

Giardia lamblia is a protozoan pathogen with distinct cytoskeletal structures, including median bodies and eight flagella. In this study, we examined components comprising G. lamblia flagella. Crude flagellar extracts were prepared from G. lamblia trophozoites, and analyzed by two-dimensional (2-D) gel electrophoresis. The 19 protein spots were analyzed by MALDI-TOF mass spectrometry, identifying ten metabolic enzymes, six distinct giardins, Giardia trophozoite antigen 1, translational initiation factor eIF-4A, and an extracellular signal-regulated kinase 2. Among the identified proteins, we studied α-11 giardin which belongs to a group of cytoskeletal proteins specific to Giardia. Western blot analysis and real-time PCR indicated that expression of α-11 giardin is not significantly increased during encystation of G. lamblia. Immunofluorescence assays using anti-α-11 giardin antibodies revealed that α-11 giardin protein mainly localized to the plasma membranes and basal bodies of the anterior flagella of G. lamblia trophozoites, suggesting that α-11 giardin is a genuine component of the G. lamblia cytoskeleton.

Giardia flagellar motility is not directly required to maintain attachment to surfaces

Giardia trophozoites attach to the intestinal microvilli (or inert surfaces) using an undefined “suction-based” mechanism, and remain attached during cell division to avoid peristalsis. Flagellar motility is a key factor in Giardia's pathogenesis and colonization of the host small intestine. Specifically, the beating of the ventral flagella, one of four pairs of motile flagella, has been proposed to generate a hydrodynamic force that results in suction-based attachment via the adjacent ventral disc. We aimed to test this prevailing “hydrodynamic model” of attachment mediated by flagellar motility. We defined four distinct stages of attachment by assessing surface contacts of the trophozoite with the substrate during attachment using TIRF microscopy (TIRFM). The lateral crest of the ventral disc forms a continuous perimeter seal with the substrate, a cytological indication that trophozoites are fully attached. Using trophozoites with two types of molecularly engineered defects in flagellar beating, we determined that neither ventral flagellar beating, nor any flagellar beating, is necessary for the maintenance of attachment. Following a morpholino-based knockdown of PF16, a central pair protein, both the beating and morphology of flagella were defective, but trophozoites could still initiate proper surface contacts as seen using TIRFM and could maintain attachment in several biophysical assays. Trophozoites with impaired motility were able to attach as well as motile cells. We also generated a strain with defects in the ventral flagellar waveform by overexpressing a dominant negative form of alpha2-annexin::GFP (D122A, D275A). This dominant negative alpha2-annexin strain could initiate attachment and had only a slight decrease in the ability to withstand normal and shear forces. The time needed for attachment did increase in trophozoites with overall defective flagellar beating, however. Thus while not directly required for attachment, flagellar motility is important for positioning and orienting trophozoites prior to attachment. Drugs affecting flagellar motility may result in lower levels of attachment by indirectly limiting the number of parasites that can position the ventral disc properly against a surface and against peristaltic flow.

Giardia is a widespread, single-celled, intestinal parasite that infects millions of people and animals each year. Colonization of the small intestine is a critical part of Giardia's life cycle in any host. This colonization is initiated when cells attach to the intestinal wall via a specialized suction cup-like structure, the ventral disc. In the host, Giardia moves by beating four pairs of flagella; movement of the ventral pair has been implicated in attachment. This study shows that the beating of the flagella is not important for attachment, but rather for positioning Giardia close to the intestinal wall prior to attachment, and thus disproves the commonly held model of giardial attachment. This work implies that drugs targeting Giardia motility could prevent or slow attachment, leading to lower rates of infection.

Life with eight flagella: flagellar assembly and division in Giardia

Flagellar movement in Giardia, a common intestinal parasitic protist, is crucial to its survival in the host. Each axoneme is unique in possessing a long, cytoplasmic portion as well as a membrane-bound portion. Intraflagellar transport (IFT) is required for the assembly of membrane-bound regions, yet the cytoplasmic regions may be assembled by IFT-independent mechanisms. Steady-state axoneme length is maintained by IFT and by intrinsic and active microtubule dynamics. Following mitosis and before their segregation, giardial flagella undergo a multigenerational division cycle in which the parental eight flagella migrate and reposition to different cellular locations; eight new flagella are assembled de novo. Each daughter cell thus inherits four mature and four newly synthesized flagella.

Dual acylation accounts for the localization of {alpha}19-giardin in the ventral flagellum pair of Giardia lamblia

A Giardia-specific protein family denominated as alpha-giardins, represents the major protein component, besides tubulin, of the cytoskeleton of the human pathogenic parasite Giardia lamblia. One of its members, alpha19-giardin, carries an N-terminal sequence extension of MGCXXS, which in many proteins serves as a target for dual lipid conjugation: myristoylation at the glycine residue after removal of the methionine and palmitoylation at the cysteine residue. As the first experimental evidence of a lipid modification, we found alpha19-giardin to be associated with the membrane fraction of disrupted trophozoites. After heterologous coexpression of alpha19-giardin with giardial N-myristoyltransferase (NMT) in Escherichia coli, we found the protein in a myristoylated form. Additionally, after heterologous expression together with the palmitoyl transferase Pfa3 in Saccharomyces cerevisiae, alpha19-giardin associates with the membrane of the main vacuole. Immunocytochemical colocalization studies on wild-type Giardia trophozoites with tubulin provide evidence that alpha19-giardin exclusively localizes to the ventral pair of the giardial flagella. A mutant in which the putatively myristoylated N-terminal glycine residue was replaced by alanine lost this specific localization. Our findings suggest that the dual lipidation of alpha19-giardin is responsible for its specific flagellar localization.