An Acanthamoeba polyubiquitin gene and application of its promoter to the establishment of a transient transfection system

Indispensable role for the eukaryotic-like ankyrin domains of the ankyrin B effector of Legionella pneumophila within macrophages and amoebae

The Dot/Icm-translocated ankyrin B (AnkB) effector of Legionella pneumophila exhibits molecular mimicry of eukaryotic F-box proteins and is essential for intracellular replication in macrophages and protozoa. In addition to two eukaryotic-like ankyrin (ANK) domains, AnkB harbors a conserved eukaryotic F-box domain, which is involved in polyubiquitination of proteins throughout the eukaryotic kingdom. We have recently shown that the F-box domain of the AnkB effector is essential for decoration of the Legionella-containing vacuole (LCV) with polyubiquitinated proteins within macrophages and protozoan hosts. To decipher the role of the two ANK domains in the function of AnkB, we have constructed in-frame deletion of either or both of the ANK domain-encoding regions (ankB Delta A1, ankB Delta A2, and ankB Delta A1A2) to trans-complement the ankB null mutant. Deletion of the ANK domains results in defects in intracellular proliferation and decoration of the LCV with polyubiquitinated proteins. Export of the truncated variants of AnkB was reduced, and this may account for the observed defects. However, while full-length AnkB ectopically expressed in mammalian cells trans-rescues the ankB null mutant for intracellular proliferation, ectopic expression of AnkB Delta A1, AnkB Delta A2, and AnkB Delta A1A2 fails to trans-rescue the ankB null mutant. Importantly, ectopically expressed full-length AnkB is targeted to the host cell plasma membrane, where it recruits polyubiquitinated proteins. In contrast, AnkB Delta A1, AnkB Delta A2, and AnkB Delta A1A2 are diffusely distributed throughout the cytosol and fail to recruit polyubiquitinated proteins. We conclude that the two eukaryotic-like ANK domains of AnkB are essential for intracellular proliferation, for targeting AnkB to the host membranes, and for decoration of the LCV with polyubiquitinated proteins.

Molecular mimicry by an F-box effector of Legionella pneumophila hijacks a conserved polyubiquitination machinery within macrophages and protozoa

The ability of Legionella pneumophila to proliferate within various protozoa in the aquatic environment and in macrophages indicates a remarkable evolution and microbial exploitation of evolutionarily conserved eukaryotic processes. Ankyrin B (AnkB) of L. pneumophila is a non-canonical F-box-containing protein, and is the only known Dot/Icm-translocated effector of L. pneumophila essential for intra-vacuolar proliferation within both macrophages and protozoan hosts. We show that the F-box domain of AnkB and the ⁹L¹⁰P conserved residues are essential for intracellular bacterial proliferation and for rapid acquisition of polyubiquitinated proteins by the Legionella-containing vacuole (LCV) within macrophages, Dictyostelium discoideum, and Acanthamoeba. Interestingly, translocation of AnkB and recruitment of polyubiquitinated proteins in macrophages and Acanthamoeba is rapidly triggered by extracellular bacteria within 5 min of bacterial attachment. Ectopically expressed AnkB within mammalian cells is localized to the periphery of the cell where it co-localizes with host SKP1 and recruits polyubiquitinated proteins, which results in restoration of intracellular growth to the ankB mutant similar to the parental strain. While an ectopically expressed AnkB-⁹L¹⁰P/AA variant is localized to the cell periphery, it does not recruit polyubiquitinated proteins and fails to trans-rescue the ankB mutant intracellular growth defect. Direct in vivo interaction of AnkB but not the AnkB-⁹L¹⁰P/AA variant with the host SKP1 is demonstrated. Importantly, RNAi-mediated silencing of expression of SKP1 renders the cells non-permissive for intracellular proliferation of L. pneumophila. The role of AnkB in exploitation of the polyubiquitination machinery is essential for intrapulmonary bacterial proliferation in the mouse model of Legionnaires' disease. Therefore, AnkB exhibits a novel molecular and functional mimicry of eukaryotic F-box proteins that exploits conserved polyubiquitination machinery for intracellular proliferation within evolutionarily distant hosts.

Legionella pneumophila is abundantly found in the aquatic environment within various protozoa and can cause a severe pneumonia called Legionnaires' disease when it invades human macrophages in the lung. The ability of L. pneumophila to invade and proliferate within macrophages and protozoa is dependent on the translocation of specific proteins into the invaded cell via a specialized secretory device, and these proteins modulate various host cell processes. Of these translocated proteins, AnkB is indispensable for intracellular growth of L. pneumophila within macrophages and protozoa. Here we show that AnkB is essential for establishing a favorable intracellular replicative niche by promoting the decoration of the Legionella containing vacuole (LCV) with polyubiquitinated proteins. The AnkB effector achieves this by mimicking the action of host cell F-box proteins, a highly conserved component of the SCF ubiquitin ligase complex that is found in both unicellular organisms and mammalian cells. Our study provides new insights into the ability of intracellular pathogens to hijack evolutionarily conserved host cell processes through molecular mimicry to establish a favorable replicative niche within various hosts and to cause disease in mammals.

Naegleria fowleri: Functional expression of the Nfa1 protein in transfected Naegleria gruberi by promoter modification

To establish a transient transfection system in a Naegleria, we constructed three nfa1-pEGFP-N1 vectors by the promoter replacement and insertion of a nfa1 gene and transfected the DNAs into Naegleria gruberi using a lipid reagent. The transfection efficiency and usefulness of the three modified vectors were estimated by identifying the expressions of the EGFP and Nfa1 protein from N. gruberi. After transfection, the Nfa1 protein was functionally expressed on pseudopodia of N. gruberi. The strong GFP fluorescence was observed in N. gruberi transfected with the actin-nfa1-pEGFP-N1 vector, of which the CMV promoter region in the expression vector was replaced with the actin 5' UTR region. Additionally, when transgenic N. gruberi trophozoites were co-cultured with CHO target cells, the Nfa1 protein was also located on cytoplasm and pseudopodia, especially on a food cup that was formed in contact with target cells as it shown in pathogenic N. fowleri.

Stable transfection of Acanthamoeba castellanii

A simple method for stable transfection of Acanthamoeba castellanii using plasmids which confer resistance to neomycin G418 is described. Expression of neomycin phosphotransferase is driven by the Acanthamoeba TBP gene promoter, and can be monitored by cell growth in the presence of neomycin G418 or by Western blot analysis. Transfected cells can be passaged in the same manner as control cells and can be induced to differentiate into cysts, in which form they maintain resistance to neomycin G418 for at least several weeks, although expression of neomycin phosphotransferase is repressed during encystment. Expression of EGFP or an HA-tagged EGFP-TBP fusion can be driven from the same plasmid, using an additional copy of the Acanthamoeba TBP gene promoter or a deletion mutant. The TBP-EGFP fusion is localized to the nucleus, except in a small proportion of presumptive pre-mitotic cells. EGFP expression can also be driven by the cyst-specific CSP21 gene promoter, which is completely repressed in growing cells but strongly induced in differentiating cells. Transfected cells maintain their phenotype for several weeks, even in the absence of neomycin G418, suggesting that transfected genes are stably integrated within the genome. These results demonstrate the utility of the neomycin resistance based plasmids for stable transfection of Acanthamoeba, and may assist a number of investigations.

Intracellular localization and dynamics of myosin-II and myosin-IC in live Acanthamoeba by transient transfection of EGFP fusion proteins

We developed a reliable method for transient transfection of Acanthamoeba using Superfect (Qiagen) and a vector with the Acanthamoeba ubiquitin promoter and enhanced green fluorescent protein (EGFP) as the reporter gene. The transfection efficiency was 3% for profilin-I-EGFP and EGFP-myosin-II tail, and less than 0.5% for larger constructs such as full length myosin-II or myosin-IC. Profilin-I-EGFP was distributed throughout the cytoplasm as observed previously with rhodamine-labeled profilin, while EGFP alone accumulated in the nucleus. EGFP fused to full length myosin-II or to the C-terminal 256 residues of the myosin-II tail concentrated in fluorescent spots similar to thick filaments and minifilaments identified previously in fixed cells with fluorescent antibodies. Thick filaments were located in the dorsal cytoplasm and along the lateral margins of the back half of the cell. Thick filaments formed behind the leading edge and moved continuously towards the rear of the cell, where they disassembled. If phosphorylation of the myosin-II heavy chain was prevented by mutation of all three phosphorylated serines to alanine, thick filaments of unphosphorylated myosin-II accumulated around vesicles of various sizes. EGFP-myosin-IC was spread throughout the cytoplasm but concentrated transiently around contractile vacuoles and macropinocytosis cups providing that the construct included both the head and a tail with the SH3 domain.

Structure and expression of a cyst specific protein of Acanthamoeba castellanii

The life cycle of Acanthamoeba is divided into a growth-division phase and two distinctive processes of cellular differentiation, termed encystment and excystment. Polyacrylamide gel electrophoresis revealed that a specific protein of 21 kDa in molecular weight occurs in the cyst, but not in the trophozoite stages of A. castellanii Neff strain. This cyst-specific protein, designated as CSP21, was purified from guanidine-HCl extract of cyst wall and anti-CSP21 antibody was produced. Immunoblotting of proteins extracted from a variety of species of Acanthamoeba genus suggested that the antibody is specific for group II amoebae, therefore, providing a useful tool for Acanthamoebae taxonomy. A cDNA clone for A. castellanii CSP21 was isolated by immunoscreening of a cDNA expression library constructed from mRNA of amoebae at encysting stage. The deduced primary structure indicated that CSP21 is a hydrophilic protein showing no significant homology with peptides thus far published. RNA blot analysis showed that the expression of CSP21 mRNA was restricted within early stages of encystment, suggesting that the biosynthesis of CSP21 is regulated at mRNA level.