Functional characterisation of the regulatory subunit of cyclic AMP-dependent protein kinase A homologue of Giardia lamblia: Differential expression of the regulatory and catalytic subunits during encystation

Unraveling the role of cAMP signaling in Giardia: insights into PKA-mediated regulation of encystation and subcellular interactions

cAMP plays an important role as a second messenger in the stage transition of various protozoan parasites. This signaling pathway relies on multiple effectors, such as protein kinase A (PKA), exchange protein activated by cAMP, and cAMP-response element binding protein transcription factors, to initiate signal transduction in humans. The Giardia genome only contains two adenylate cyclases (ACs), a single phosphodiesterase (PDE) and a single known PKA effector, and the specific functions of these components are not fully understood. In our previous research, we demonstrated the important role of AC2-dependent cAMP signaling in promoting the encystation program. Using the NanoBit technology, we emphasized the significance of AC2-dependent cAMP biosynthesis in regulating the dissociation of the PKA regulatory domain (PKAr) and PKA catalytic domain (PKAc). In this study, our objectives are twofold: first, we used the newly developed Split-Halo to examine subcellular interactions of GlPKAr and GlPKAc in Giardia; and second, we investigated whether PKAc regulates encystation-specific proteins. Our findings revealed distinct subcellular locations where GlPKAr and GlPKAc interacted during the trophozoite stage, including the flagella, basal bodies, and cytoplasm. Upon exposure to encystation stimuli, the interaction shifted from the flagella to the cytosol. Knockdown of GlPKAc resulted in the downregulation of encystation-specific genes, leading to the production of fewer viable and water-resistant cysts indicating a role for PKA in the transcriptional regulation of encystation. These discoveries contribute to a deeper understanding of the cAMP signaling pathway and its important role in governing Giardia's encystation process.

IMPORTANCE

The precise timing of interactions and subcellular compartmentation play crucial roles in signal transduction. The co-immunoprecipitation assay (CO-IP) has long been utilized to validate protein-protein interactions; however, CO-IPs lack spatial and temporal resolutions. Our recent study used the NanoBit assay, which showcased the reversible protein-protein interaction between PKAr and PKAc in response to cAMP analogs and encystation stimuli. Expanding on this groundwork, this study employs the Split-Halo assay to uncover the subcellular compartments where the PKAr and PKAc protein-protein interactions take place and respond to encystation stimuli. Taken together, these molecular tools provide spatiotemporal information on the protein-protein interaction, which will be useful in the field.

Rosmarinic Acid Present in Lepechinia floribunda and Lepechinia meyenii as a Potent Inhibitor of the Adenylyl Cyclase gNC1 from Giardia lamblia

Giardiasis is a parasitosis caused by Giardia lamblia with significant epidemiological and clinical importance due to its high prevalence and pathogenicity. The lack of optimal therapies for treating this parasite makes the development of new effective chemical entities an urgent need. In the search for new inhibitors of the adenylyl cyclase gNC1 obtained from G. lamblia, 14 extracts from Argentinian native plants were screened. Lepechinia floribunda and L. meyenii extracts exhibited the highest gNC1 inhibitory activity, with IC50 values of 9 and 31 µg/mL, respectively. In silico studies showed rosmarinic acid, a hydroxycinnamic acid present in both mentioned species, to be a promising anti-gNC1 compound. This result was confirmed experimentally, with rosmarinic acid showing an IC50 value of 10.1 µM. Theoretical and experimental findings elucidate the molecular-level mechanism of rosmarinic acid, pinpointing the key interactions stabilizing the compound-enzyme complex and the binding site. These results strongly support that rosmarinic acid is a promising scaffold for developing novel compounds with inhibitory activity against gNC1, which could serve as potential therapeutic agents to treat giardiasis.

Encystation stimuli sensing is mediated by adenylate cyclase AC2-dependent cAMP signaling in Giardia

Protozoan parasites use cAMP signaling to precisely regulate the place and time of developmental differentiation, yet it is unclear how this signaling is initiated. Encystation of the intestinal parasite Giardia lamblia can be activated by multiple stimuli, which we hypothesize result in a common physiological change. We demonstrate that bile alters plasma membrane fluidity by reducing cholesterol-rich lipid microdomains, while alkaline pH enhances bile function. Through depletion of the cAMP producing enzyme Adenylate Cyclase 2 (AC2) and the use of a newly developed Giardia-specific cAMP sensor, we show that AC2 is necessary for encystation stimuli-induced cAMP upregulation and activation of downstream signaling. Conversely, over expression of AC2 or exogenous cAMP were sufficient to initiate encystation. Our findings indicate that encystation stimuli induce membrane reorganization, trigger AC2-dependent cAMP upregulation, and initiate encystation-specific gene expression, thereby advancing our understanding of a critical stage in the life cycle of a globally important parasite.

Encystation stimuli sensing mediated by adenylate cyclase AC2-dependent cAMP signaling in Giardia

Protozoan parasites use cAMP signaling to precisely regulate the place and time of developmental differentiation, yet it is unclear how this signaling is initiated. Encystation of the intestinal parasite Giardia lamblia can be activated by multiple stimuli, which we hypothesize result in a common physiological change. We demonstrate that bile alters plasma membrane fluidity by reducing cholesterol-rich lipid microdomains, while alkaline pH enhances bile function. Through depletion of the cAMP producing enzyme Adenylate Cyclase 2 (AC2) and the use of a newly developed Giardia-specific cAMP sensor, we show that AC2 is necessary for encystation stimuli-induced cAMP upregulation and activation of downstream signaling. Conversely, over expression of AC2 or exogenous cAMP were sufficient to initiate encystation. Our findings indicate that encystation stimuli induce membrane reorganization, trigger AC2-dependent cAMP upregulation, and initiate encystation-specific gene expression, thereby advancing our understanding of a critical stage in the life cycle of a globally important parasite.

A Potent N-(piperidin-4-yl)-1H-pyrrole-2-carboxamide Inhibitor of Adenylyl Cyclase of G. lamblia: Biological Evaluation and Molecular Modelling Studies

In this work, we report a derivative of N-(piperidin-4-yl)-1H-pyrrole-2-carboxamide as a new inhibitor for adenylyl cyclase of Giardia lamblia which was obtained from a study using structural data of the nucleotidyl cyclase 1 (gNC1) of this parasite. For such a study, we developed a model for this specific enzyme by using homology techniques, which is the first model reported for gNC1 of G. lamblia. Our studies show that the new inhibitor has a competitive mechanism of action against this enzyme. 2-Hydroxyestradiol was used as the reference compound for comparative studies. Results in this work are important from two points of view. on the one hand, an experimentally corroborated model for gNC1 of G. lamblia obtained by molecular modelling is presented; on the other hand, the new inhibitor obtained is an undoubtedly excellent starting structure for the development of new metabolic inhibitors for G. lamblia.

Early signaling pathways mediating dormant cyst formation in terrestrial unicellular eukaryote Colpoda

Dormant (resting) cyst formation (encystment) in unicellular eukaryotes is the process of a large-scale digestion of vegetative cell structures and reconstruction into the dormant form, which is performed by cell signaling pathways accompanied by up- or down-regulation of protein expression, and by posttranslational modification such as phosphorylation. In this review, the author describes the morphogenetic events during encystment of Colpoda and the early molecular events in the Ca2+/calmodulin-triggered signaling pathways for encystment, based mainly on our research results of the past 10 years; especially, the author discusses the role of c-AMP dependently phosphorylated proteins (ribosomal P0 protein, ribosomal S5 protein, Rieske iron-sulfur protein, actin and histone H4) and encystment-dependently upregulated (EF-1α-HSP60, actin-related protein) and downregulated proteins (ATP synthase β-chain). In addition, the roles of AMPK, a key molecule in the signaling pathways leading to Colpoda encystment, and differentially expressed genes and proteins during encystment of other ciliates are discussed.

Synthesis and degradation of cAMP in Giardia lamblia: possible role and characterization of a nucleotidyl cyclase with a single cyclase homology domain

Despite its importance in the regulation of growth and differentiation processes of a variety of organisms, the mechanism of synthesis and degradation of cAMP (cyclic AMP) has not yet been described in Giardia lamblia In this work, we measured significant quantities of cAMP in trophozoites of G. lamblia incubated in vitro and later detected how it increases during the first hours of encystation, and how it then returns to basal levels at 24 h. Through an analysis of the genome of G. lamblia, we found sequences of three putative enzymes - one phosphodiesterase (gPDE) and two nucleotidyl cyclases (gNC1 and gNC2) - that should be responsible for the regulation of cAMP in G. lamblia Later, an RT-PCR assay confirmed that these three genes are expressed in trophozoites. The bioinformatic analysis indicated that gPDE is a transmembrane protein of 154 kDa, with a single catalytic domain in the C-terminal end; gNC1 is predicted to be a transmembrane protein of 74 kDa, with only one class III cyclase homology domain (CHD) at the C-terminal end; and gNC2 should be a transmembrane protein of 246 kDa, with two class III CHDs. Finally, we cloned and enriched the catalytic domain of gNC1 (gNC1cd) from bacteria. After that, we confirmed that gNC1cd has adenylyl cyclase (AC) activity. This enzymatic activity depends on the presence of Mn²⁺ and Ca²⁺, but no significant activity was displayed in the presence of Mg²⁺ Additionally, the AC activity of gNC1cd is competitively inhibited with GTP, so it is highly possible that gNC1 has guanylyl cyclase activity as well.

The single cyclic nucleotide-specific phosphodiesterase of the intestinal parasite Giardia lamblia represents a potential drug target

Background

Giardiasis is an intestinal infection correlated with poverty and poor drinking water quality, and treatment options are limited. According to the Center for Disease Control and Prevention, Giardia infections afflict nearly 33% of people in developing countries, and 2% of the adult population in the developed world. This study describes the single cyclic nucleotide-specific phosphodiesterase (PDE) of G. lamblia and assesses PDE inhibitors as a new generation of anti-giardial drugs.

Methods

An extensive search of the Giardia genome database identified a single gene coding for a class I PDE, GlPDE. The predicted protein sequence was analyzed in-silico to characterize its domain structure and catalytic domain. Enzymatic activity of GlPDE was established by complementation of a PDE-deficient Saccharomyces cerevisiae strain, and enzyme kinetics were characterized in soluble yeast lysates. The potency of known PDE inhibitors was tested against the activity of recombinant GlPDE expressed in yeast and against proliferating Giardia trophozoites. Finally, the localization of epitope-tagged and ectopically expressed GlPDE in Giardia cells was investigated.

Results

Giardia encodes a class I PDE. Catalytically important residues are fully conserved between GlPDE and human PDEs, but sequence differences between their catalytic domains suggest that designing Giardia-specific inhibitors is feasible. Recombinant GlPDE hydrolyzes cAMP with a K_m of 408 μM, and cGMP is not accepted as a substrate. A number of drugs exhibit a high degree of correlation between their potency against the recombinant enzyme and their inhibition of trophozoite proliferation in culture. Epitope-tagged GlPDE localizes as dots in a pattern reminiscent of mitosomes and to the perinuclear region in Giardia.

Conclusions

Our data strongly suggest that inhibition of G. lamblia PDE activity leads to a profound inhibition of parasite proliferation and that GlPDE is a promising target for developing novel anti-giardial drugs.

Cellular signaling by the cyclic nucleotides cAMP and cGMP is ubiquitously found in organisms from human to unicellular parasites. Cyclic nucleotide-specific phosphodiesterases (PDEs) are pivotal regulators of these signaling processes and these enzymes represent important drug targets for a variety of diseases. Eleven PDE families are distinguished in humans and selective inhibition of a single human PDE family without targeting others is feasible. In parasites, interference in the signaling mechanism by PDE inhibition may be fatal. The diarrhea-causing parasite Giardia lamblia contains only one single PDE, named GlPDE. GlPDE activity is highly impaired by a range of PDE inhibitors, which also suppress parasite proliferation in vitro. Thus, there is a good agreement between PDE inhibition and parasite drug susceptibility. We demonstrate molecular differences between human PDEs and GlPDE that can be exploited for the development of GlPDE-selective inhibitors. Finally, our data may suggest localization of GlPDE to mitosome organelles, which are absent in human cells and thus are in the focus as possible targets for the treatment of giardiasis. This may add to the notion that GlPDE represents a potential target for the development of novel anti-giardial drugs.

Targeted overexpression of cyclic AMP-dependent protein kinase subunit in Toxoplasma gondii promotes replication and virulence in host cells

Toxoplasma gondii (T. gondii) is one of the most common parasite that can infect almost any warm-blooded animals including humans. The cyclic nucleotide-dependent protein kinase (PKA) regulates a spectrum of intracellular signal pathways in many organisms. Protein kinase catalytic subunit (PKAC) is the core of the whole protein, and plays an important role in the life cycle of T.gondii. Here, T.gondii PKAC (TgPKAC) overexpression strain (TgPKAC-OE) was constructed. The growth of the TgPKAC-OE, RH△Ku80, and TgPKAC inhibition strains (TgPKAC-H89) were analysed by SYBR-green real-time PCR, and the ultrastructure was observed by transmission electron microscopy. The survival rate in mice was also recorded to analyse the virulence of the parasites. We also investigated the subcellular localization of TgPKAC in Vero cells by laser scanning microscope. We found that TgPKAC-OE strain exhibited obviously increased growth rate in Vero cells in vitro, and infected mice survived for a shorter time compared to wild type strain. Ultrastructural analysis found more autophagosomes-like structures in TgPKAC-H89 parasite compared to RH△Ku80 strain, and the relative expression level of Toxoplasma gondii autophagy-related protein (ATG8) in TgPKAC-H89 parasite was higher than wild type parasite. Laser confocal results showed that TgPKAC was mainly expressed in the cytoplasm of Vero cells. In conclusion, we hypothesized that inhibition of TgPKAC could cause autophagy of Toxoplasma gondii and then influence the replication of the parasite. TgPKAC plays an important role in parasite virulence in vivo, and the subcellular localization was successfully detected in Vero cells. Our data will provide a basis for further study of TgPKAC function and help screen drug targets of T. gondii.

Enzyme Activity Assays for Protein Kinases: Strategies to Identify Active Substrates

Protein kinases are an important class of enzymes and drug targets. New opportunities to discover medicines for neglected diseases can be leveraged by the extensive kinase tools and knowledge created in targeting human kinases. A valuable tool for kinase drug discovery is an enzyme assay that measures catalytic function. The functional assay can be used to identify inhibitors, estimate affinity, characterize molecular mechanisms of action (MMOAs) and evaluate selectivity. However, establishing an enzyme assay for a new kinases requires identification of a suitable substrate. Identification of a new kinase's endogenous physiologic substrate and function can be extremely costly and time consuming. Fortunately, most kinases are promiscuous and will catalyze the phosphotransfer from ATP to alternative substrates with differing degrees of catalytic efficiency. In this manuscript we review strategies and successes in the identification of alternative substrates for kinases from organisms responsible for many of the neglected tropical diseases (NTDs) towards the goal of informing strategies to identify substrates for new kinases. Approaches for establishing a functional kinase assay include measuring auto-activation and use of generic substrates and peptides. The most commonly used generic substrates are casein, myelin basic protein, and histone. Sequence homology modeling can provide insights into the potential substrates and the requirement for activation. Empirical approaches that can identify substrates include screening of lysates (which may also help identify native substrates) and use of peptide arrays. All of these approaches have been used with a varying degree of success to identify alternative substrates.

Identification of a protein kinase A regulatory subunit from Leishmania having importance in metacyclogenesis through induction of autophagy

cAMP-mediated responses act as modulators of environmental sensing and cellular differentiation of many kinetoplastidae parasites including Leishmania. Although cAMP synthesizing (adenylate cyclase) and degrading (phosphodiesterase) enzymes have been cloned and characterized from Leishmania, no cAMP-binding effector molecule has yet been identified from this parasite. In this study, a regulatory subunit of cAMP-dependent protein kinase (Ldpkar1), homologous to mammalian class I cAMP-dependent protein kinase regulatory subunit, has been identified from L. donovani. Further characterization suggested possible interaction of LdPKAR1 with PKA catalytic subunits and inhibition of PKA activity. This PKA regulatory subunit is expressed in all life cycle stages and its expression attained maximum level in stationary phase promastigotes, which are biochemically similar to the infective metacyclic promastigotes. Starvation condition, the trigger for metacyclogenesis in the parasite, elevates LdPKAR1 expression and under starvation condition promastigotes overexpressing Ldpkar1 attained metacyclic features earlier than normal cells. Furthermore, Ldpkar1 overexpression accelerates autophagy, a starvation-induced cytological event necessary for metacyclogenesis and amastigote formation. Conditional silencing of Ldpkar1 delays the induction of autophagy in the parasite. The study, for the first time, reports the identification of a functional cAMP-binding effector molecule from Leishmania that may modulate important cytological events affecting metacyclogenesis.

The minimal kinome of Giardia lamblia illuminates early kinase evolution and unique parasite biology

Background

The major human intestinal pathogen Giardia lamblia is a very early branching eukaryote with a minimal genome of broad evolutionary and biological interest.

Results

To explore early kinase evolution and regulation of Giardia biology, we cataloged the kinomes of three sequenced strains. Comparison with published kinomes and those of the excavates Trichomonas vaginalis and Leishmania major shows that Giardia's 80 core kinases constitute the smallest known core kinome of any eukaryote that can be grown in pure culture, reflecting both its early origin and secondary gene loss. Kinase losses in DNA repair, mitochondrial function, transcription, splicing, and stress response reflect this reduced genome, while the presence of other kinases helps define the kinome of the last common eukaryotic ancestor. Immunofluorescence analysis shows abundant phospho-staining in trophozoites, with phosphotyrosine abundant in the nuclei and phosphothreonine and phosphoserine in distinct cytoskeletal organelles. The Nek kinase family has been massively expanded, accounting for 198 of the 278 protein kinases in Giardia. Most Neks are catalytically inactive, have very divergent sequences and undergo extensive duplication and loss between strains. Many Neks are highly induced during development. We localized four catalytically active Neks to distinct parts of the cytoskeleton and one inactive Nek to the cytoplasm.

Conclusions

The reduced kinome of Giardia sheds new light on early kinase evolution, and its highly divergent sequences add to the definition of individual kinase families as well as offering specific drug targets. Giardia's massive Nek expansion may reflect its distinctive lifestyle, biphasic life cycle and complex cytoskeleton.

Mining the Giardia genome and proteome for conserved and unique basal body proteins

Giardia lamblia is a flagellated protozoan parasite and a major cause of diarrhoea in humans. Its microtubular cytoskeleton mediates trophozoite motility, attachment and cytokinesis, and is characterised by an attachment disk and eight flagella that are each nucleated in a basal body. To date, only 10 giardial basal body proteins have been identified, including universal signalling proteins that are important for regulating mitosis or differentiation. In this study, we have exploited bioinformatics and proteomic approaches to identify new Giardia basal body proteins and confocal microscopy to confirm their localisation in interphase trophozoites. This approach identified 75 homologs of conserved basal body proteins in the genome including 65 not previously known to be associated with Giardia basal bodies. Thirteen proteins were confirmed to co-localise with centrin to the Giardia basal bodies. We also demonstrate that most basal body proteins localise to additional cytoskeletal structures in interphase trophozoites. This might help to explain the roles of the four pairs of flagella and Giardia-specific organelles in motility and differentiation. A deeper understanding of the composition of the Giardia basal bodies will contribute insights into the complex signalling pathways that regulate its unique cytoskeleton and the biological divergence of these conserved organelles.

cAMP-dependent protein kinase from Plasmodium falciparum: an update

One of the most important public health problems in the world today is the emergence and dissemination of drug-resistant malaria parasites. Plasmodium falciparum is the causative agent of the most lethal form of human malaria. New anti-malarial strategies are urgently required, and their design and development require the identification of potential therapeutic targets. However, the molecular mechanisms controlling the life cycle of the malaria parasite are still poorly understood. The published genome sequence of P. falciparum and previous studies have revealed that several homologues of eukaryotic signalling proteins, such as protein kinases, are relatively conserved. Protein kinases are now widely recognized as important drug targets in protozoan parasites. Cyclic AMP-dependent protein kinase (PKA) is implicated in numerous processes in mammalian cells, and the regulatory mechanisms of the cAMP pathway have been characterized. P. falciparum cAMP-dependent protein kinase plays an important role in the parasite's life cycle and thus represents an attractive target for the development of anti-malarial drugs. In this review, we focus on the P. falciparum cAMP/PKA pathway to provide new insights and an improved understanding of this signalling cascade.

Encystation of Giardia lamblia: a model for other parasites

To colonize the human small intestine, Giardia lamblia monitors a dynamic environment. Trophozoites attach to enterocytes that mature and die. The parasites must 'decide' whether to re-attach or differentiate into cysts that survive in the environment and re-activate when ingested. Other intestinal parasites face similar challenges. Study of these parasites is limited because they do not encyst in vitro. Giardia trophozoites were persuaded to encyst in vitro by mimicking physiological stimuli. Cysts are dormant, yet 'spring-loaded for action' to excyst upon ingestion. Giardial encystation has been studied from morphological, cell biological, biochemical, and molecular viewpoints. Yet important gaps remain and the mechanisms that co-ordinate responses to external signals remain enigmatic.

Giardia lamblia aurora kinase: a regulator of mitosis in a binucleate parasite

Giardia lamblia is a major cause of diarrhoeal disease worldwide. Since it has no known toxin, the ability of trophozoites to colonise the human small intestine is required for its pathogenesis. Mitosis in this protozoan parasite is a unique challenge because its two equivalent nuclei and complex cytoskeleton must be duplicated and segregated accurately. Giardial mitosis is a complex and rapid event that is poorly understood at the cellular and molecular levels. Higher eukaryotes have one to three members of the highly conserved Ser/Thr aurora kinase (AK) family that regulate key aspects of mitosis and cytokinesis. Giardia has a single AK orthologue (gAK) with 61% similarity to human AK A. In addition to the conserved active site residues, activation loop and destruction-box motifs characteristic of AKs, gAK contains a unique insert near the active site region. We epitope-tagged gAK at its C-terminus and expressed it under its own promoter. During interphase, gAK localises exclusively to the nuclei, but is not phosphorylated as shown by lack of staining with an antibody specific to phosphorylated AK A (pAK). In contrast, during mitosis pAK localises to the basal bodies/centrosomes and co-localises with tubulin to the spindle. During specific stages of mitosis, giardial pAK also localised dynamically to cytoskeletal structures unique to Giardia: the paraflagellar dense rods of the anterior flagella and the median body, whose functions are unknown, as well as to the parent attachment disc. Two AK inhibitors significantly decreased giardial growth and increased the numbers of cells arrested in cytokinesis. These inhibitors appeared to increase microtubule nucleation and cell-ploidy. Our data show that gAK is phosphorylated in mitosis and suggest that it plays an important role in the Giardia cell cycle. The pleiotropic localisation of AK suggests that it may co-ordinate the reorganisation and segregation of tubulin-containing structures in mitosis. We believe this is the first report of a signalling protein regulating cell division in Giardia.

Protein phosphatase 2A plays a crucial role in Giardia lamblia differentiation

The ability of Giardia lamblia to undergo two distinct differentiations in response to physiologic stimuli is central to its pathogenesis. The giardial cytoskeleton changes drastically during encystation and excystation. However, the signal transduction pathways mediating these transformations are poorly understood. We tested the hypothesis that PP2A, a highly conserved serine/threonine protein phosphatase, might be important in giardial differentiation. We found that in vegetatively growing trophozoites, gPP2A-C protein localizes to basal bodies/centrosomes, and to cytoskeletal structures unique to Giardia: the ventral disk, and the dense rods of the anterior, posterior-lateral, and caudal flagella. During encystation, gPP2A-C protein disappears from only the anterior flagellar dense rods. During excystation, gPP2A-C localizes to the cyst wall in excysting cysts but is not found in the wall of cysts with emerging excyzoites. Transcriptome and immunoblot analyses indicated that gPP2A-C mRNA and protein are upregulated in mature cysts and during the early stage of excystation that models passage through the host stomach. Stable expression of gPP2A-C antisense RNA did not affect vegetative growth, but strongly inhibited the formation of encystation secretory vesicles (ESV) and water-resistant cysts. Moreover, the few cysts that formed were highly defective in excystation. Thus, gPP2A-C localizes to universal cytoskeletal structures and to structures unique to Giardia. It is also important for encystation and excystation, crucial giardial transformations that entail entry into and exit from dormancy.

Protein kinase C isoforms from Giardia duodenalis: identification and functional characterization of a β-like molecule during encystment

Protein kinase C (PKC) is a family of serine/threonine kinases that regulate many different cellular processes such as cell growth and differentiation in eukaryotic cells. Using specific polyclonal antibodies raised against mammalian PKC isoforms, it was demonstrated here for the first time that Giardia duodenalis expresses several PKC isoforms (beta, delta, epsilon, theta and zeta). All PKC isoforms detected showed changes in their expression pattern during encystment induction. In addition, selective PKC inhibitors blocked the encystment in a dose-dependent manner, suggesting that PKC isozymes may play important roles during this differentiation process. We have characterized here the only conventional-type PKC member found so far in Giardia, which showed an increased expression and changes in its intracellular localization pattern during cyst formation. The purified protein obtained by chromatography on DEAE-cellulose followed by size-exclusion chromatography, displayed in vitro kinase activity using histone HI-IIIS as substrate, which was dependent on cofactors required by conventional PKCs, i.e., phospholipids and calcium. An open reading frame in the Giardia Genome Database that encodes a homolog of PKCbeta catalytic domain was identified and cloned. The expressed recombinant protein was also recognized by a mammalian anti-PKCbeta antibody and was referred as giardial PKCbeta on the basis of all these experimental evidence.