Cloning and heterologous expression of Entamoeba histolytica adenylate kinase and uridylate/cytidylate kinase

In silico analysis of EST and genomic sequences allowed the prediction of cis-regulatory elements for Entamoeba histolytica mRNA polyadenylation

In most eukaryotic cells, the poly(A) tail at the 3'-end of messenger RNA (mRNA) is essential for nuclear export, translatability, stability and transcription termination. Poly(A) tail formation involves multi-protein complexes that interact with specific sequences in 3'-untranslated region (3'-UTR) of precursor mRNA (pre-mRNA). Here we have performed a computational analysis of a large EST and genomic sequences collection from Entamoeba histolytica, the protozoan parasite responsible for human amoebiasis, to identify conserved elements that could be involved in pre-mRNA polyadenylation. Results evidenced the presence of an AU-rich domain corresponding to the consensus UA(A/U)UU polyadenylation signal or variants, the cleavage and polyadenylation site that is generally denoted by U residue and flanked by two U-rich tracts, and a novel A-rich element. This predicted array was validated through the analysis of genomic sequences and predicted mRNA folding of genes with known polyadenylation site. The molecular organization of pre-mRNA 3'-UTR cis-regulatory elements appears to be roughly conserved through evolutionary scale, whereas the polyadenylation signal seems to be species-specific in protozoan parasites and the novel A-rich element is unique for the primitive eukaryote E. histolytica. To our knowledge, this paper is the first work about the identification of potential pre-mRNA 3'-UTR cis-regulatory sequences through in silico analysis of large sets of cDNA and genomic sequences in a protozoan parasite.

Protein targeting of an unusual, evolutionarily conserved adenylate kinase to a eukaryotic flagellum

The eukaryotic flagellum is a large structure into which specific constituent proteins must be targeted, transported and assembled after their synthesis in the cytoplasm. Using Trypanosoma brucei and a proteomic approach, we have identified and characterized a novel set of adenylate kinase proteins that are localized to the flagellum. These proteins represent unique isoforms that are targeted to the flagellum by an N-terminal extension to the protein and are incorporated into an extraaxonemal structure (the paraflagellar rod). We show that the N-terminal extension is both necessary for isoform location in the flagellum and sufficient for targeting of a green fluorescent protein reporter protein to the flagellum. Moreover, these N-terminal extension sequences are conserved in evolution and we find that they allow the identification of novel adenylate kinases in the genomes of humans and worms. Given the existence of specific isoforms of certain central metabolic enzymes, and targeting sequences for these isoforms, we suggest that these isoforms form part of a complex, "solid-phase" metabolic capability that is built into the eukaryotic flagellum.

Molecular and functional characterization of adenylate kinase 2 gene from Leishmania donovani

ATP-regenerating enzymes may have an important role in maintaining ATP levels in mitochondria-like kinetoplast organelle and glycosomes in parasitic protozoa. Adenylate kinase (AK) (ATP:AMP phosphotransferase) catalyses the reversible transfer of the gamma-phosphate group from ATP to AMP, releasing two molecules of ADP. This study describes cloning and functional characterization of the gene encoding AK2 from a genomic library of Leishmania donovani and also its expression in leishmania promastigote cultures. AK2 was localized on an approximately 1.9-Mb chromosomal band as a single copy gene. L. donovani AK2 gene is expressed as a single 1.9-kb mRNA transcript that is developmentally regulated and accumulated during the early log phase. The overexpression of L. donovani AKgene in Escherichia coli yielded a 26-kDa polypeptide that could be refolded to a functional protein with AK activity. The recombinant protein was purified to apparent homogeneity. Kinetic analysis of purified L. donovani AK showed hyperbolic behaviour for both ATP and AMP, with Km values of 104 and 74 microM, respectively. The maximum enzyme activity (Vmax) was 0.18 micromol.min(-1).mg(-1) protein. P1,P5-(bis adenosine)-5'-pentaphosphate (Ap5A), the specific inhibitor of AK, competitively inhibited activity of the recombinant enzymes with estimated Ki values of 190 nM and 160 nM for ATP and AMP, respectively. Ap5A also inhibited the growth of L. donovani promastigotes in vitro which could be only partially reversed by the addition of ADP. Thus, presence of a highly regulated AK2, which may have role in maintenance of ADP/ATP levels in L. donovani, has been demonstrated.

Cryptosporidium parvum Cpn60 targets a relict organelle

Chaperonin 60 (Cpn60) is a well-established marker protein for eukaryotic mitochondria and plastids. In order to determine whether the small double-membrane-bounded organelle posterior to the nucleus in the apicomplexan Cryptosporidium parvum is a mitochondrion, the Cpn60 gene of C. parvum sporozoites ( CpCpn60) was analyzed and antibodies were generated for localization of the peptide. Sequence and phylogenetic analyses indicated that CpCpn60 is a mitochondrial isotype and that antibodies against it localize to the rough endoplasmic reticulum-enveloped remnant organelle of C. parvum sporozoites. These data show this organelle is of mitochondrial origin.