Multifunctional class I transcription in Trypanosoma brucei depends on a novel protein complex.

The vector-borne, protistan parasite Trypanosoma brucei is the only known eukaryote with a multifunctional RNA polymerase I that, in addition to ribosomal genes, transcribes genes encoding the parasite's major cell-surface proteins-the variant surface glycoprotein (VSG) and procyclin. In the mammalian bloodstream, antigenic variation of the VSG coat is the parasite's means to evade the immune response, while procyclin is necessary for effective establishment of trypanosome infection in the fly. Moreover, the exceptionally high efficiency of mono-allelic VSG expression is essential to bloodstream trypanosomes since its silencing caused rapid cell-cycle arrest in vitro and clearance of parasites from infected mice. Here we describe a novel protein complex that recognizes class I promoters and is indispensable for class I transcription; it consists of a dynein light chain and six polypeptides that are conserved only among trypanosomatid parasites. In accordance with an essential transcriptional function of the complex, silencing the expression of a key subunit was lethal to bloodstream trypanosomes and specifically affected the abundance of rRNA and VSG mRNA. The complex was dubbed class I transcription factor A.

A TFIIB-like protein is indispensable for spliced leader RNA gene transcription in Trypanosoma brucei.

The lack of general class II transcription factors was a hallmark of the genomic sequences of the human parasites Trypanosoma brucei, Trypanosoma cruzi and Leishmania major. However, the recent identification of TFIIA as part of a protein complex essential for RNA polymerase II-mediated transcription of SLRNA genes, which encode the trans splicing-specific spliced leader RNA, suggests that trypanosomatids assemble a highly divergent set of these factors at the SLRNA promoter. Here we report the identification of a trypanosomatid TFIIB-like (TFIIB(like)) protein which has limited overall sequence homology to eukaryotic TFIIB and archaeal TFB but harbors conserved residues within the N-terminal zinc ribbon domain, the B finger and cyclin repeat I. In accordance with the function of TFIIB, T.brucei TFIIB(like) is encoded by an essential gene, localizes to the nucleus, specifically binds to the SLRNA promoter, interacts with RNA polymerase II, and is absolutely required for SLRNA transcription.

Antimalarial activity of the anticancer and proteasome inhibitor bortezomib and its analog ZL3B.

BACKGROUND: The high rate of mortality due to malaria and the worldwide distribution of parasite resistance to the commonly used antimalarial drugs chloroquine and pyrimethamine emphasize the urgent need for the development of new antimalarial drugs. An alternative approach to the long and uncertain process of designing and developing new compounds is to identify among the armamentarium of drugs already approved for clinical treatment of various human diseases those that may have strong antimalarial activity. METHODS: Proteasome inhibitor bortezomib (Velcade: [(1R)-3-methyl-1-[[(2S)-1-oxo-3-phenyl-2-[(pyrazinylcarbonyl) amino]propyl]amino]butyl] boronic acid), which has been approved for treatment of patients with multiple myeloma, and a second boronate analog Z-Leu-Leu-Leu-B(OH)2 (ZL3B), were tested against four different strains of P. falciparum (3D7, HB3, W2 and Dd2) that are either sensitive or have different levels of resistance to the antimalarial drugs pyrimethamine and chloroquine. RESULTS: Bortezomib and ZL3B are equally effective against drug-sensitive and -resistant parasites and block intraerythrocytic development prior to DNA synthesis, but have no effect on parasite egress or invasion. CONCLUSION: The identification of bortezomib and its analog as potent antimalarial drugs will set the stage for the advancement of this class of compounds, either alone or in combination therapy, for treatment of malaria, and emphasize the need for large-scale screens to identify new antimalarials within the library of clinically approved compounds.

Tissue-specific regulation of canine intestinal and hepatic phenol and morphine UDP-glucuronosyltransferases by beta-naphthoflavone in comparison with humans.

UDP-glucuronosyltransferases (UGTs) are regulated in a species- and tissue-dependent manner by endogenous and environmental factors. The present study was undertaken to further our knowledge about regulation of UGTs in dogs, a species widely used in preclinical safety evaluation. beta-Naphthoflavone (BNF) was selected as a known aryl hydrocarbon receptor agonist and antioxidant-type inducer. The latter group of inducers is intensively investigated as dietary chemoprotectants against colon cancer. Dog UGTs were investigated in comparison with related human UGTs by examples, (i) expression of dog UGT1A6, the first sequenced dog phenol UGT, and (ii) morphine UGT activities, responsible for intestinal and hepatic first-pass metabolism of morphine. The following results were obtained: (i) dog UGT1A6 was found to be constitutively expressed in liver and marginally increased by BNF treatment. Expression was low in small intestine but ca. 6-fold higher in colon than for example in jejunum. Conjugation of 4-methylumbelliferone, one of the substrates of dog UGT1A6, was also enhanced 7-fold in colonic compared to jejunal microsomes. (ii) Compared to the corresponding human tissues, canine 3-O- and 6-O-morphine UGT activities were found to be >10-fold higher in dog liver and ca. 10-fold lower in small intestinal microsomes. Small intestinal morphine and 4-hydroxybiphenyl UGT activities appeared to be moderately (2- to 3-fold) induced by oral treatment with BNF. (iii) In contrast to dogs, morphine UGT activities were found to be similar in homogenates from human enterocytes and liver. The results suggest marked differences in tissue-specific regulation of canine vs. human hepatic and intestinal phenol or morphine UGTs.

Magnetic-field induced crossover of superconducting percolation regimes in the layered organic Mott system kappa-(BEDT-TTF)2Cu[N(CN) 2]Cl.

Fluctuation spectroscopy is used to investigate the organic bandwidth-controlled Mott system kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl. We find evidence for percolative-type superconductivity in the spatially inhomogeneous coexistence region of antiferromagnetic insulating and superconducting states. When the superconducting transition is driven by a magnetic field, percolation seems to be dominated by instable superconducting clusters upon approaching T(c)(B) from above, before a "classical" type of percolation is resumed at low fields, dominated by the fractional change of superconducting clusters. The 1/f noise is resolved into Lorentzian spectra in the crossover region, where the action of an individual fluctuator is enhanced, pointing to a mesoscopic phase separation.

Specificity of human cathepsin S determined by processing of peptide substrates and MHC class II-associated invariant chain.

Cathepsin S (CatS) is a lysosomal cysteine protease of the papain family, the members of which possess relatively broad substrate specificities. It has distinct roles in major histocompatibility complex (MHC) class II-associated peptide loading and in antigen processing in both the MHC class I and class II pathways. It may therefore represent a target for interference with antigen presentation, which could be of value in the therapy of (auto)immune diseases. To obtain more detailed information on the specificity of CatS, we mapped its cleavage site preferences at subsites S3-S1' by in vitro processing of a peptide library. Only five amino acid residues at the substrate's P2 position allowed for cleavage by CatS under time-limited conditions. Preferences for groups of amino acid residues were also observed at positions P3, P1 and P1'. Based on these results, we developed highly CatS-sensitive peptides. After processing of MHC class II-associated invariant chain (Ii), a natural protein substrate of CatS, we identified CatS cleavage sites in Ii of which a majority matched the amino acid residue preference data obtained with peptides. These observed cleavage sites in Ii might be of relevance for its in vivo processing by CatS.

Autophagy promotes MHC class II presentation of peptides from intracellular source proteins.

MHC-peptide complexes mediate key functions in adaptive immunity. In a classical view, MHC-I molecules present peptides from intracellular source proteins, whereas MHC-II molecules present antigenic peptides from exogenous and membrane proteins. Nevertheless, substantial crosstalk between these two pathways has been observed. We investigated the influence of autophagy on the MHC-II ligandome and demonstrated that peptide presentation is altered considerably upon induction of autophagy. The presentation of peptides from intracellular and lysosomal source proteins was strongly increased on MHC-II in contrast with peptides from membrane and secreted proteins. In addition, autophagy influenced the MHC-II antigen-processing machinery. Our study illustrates a profound influence of autophagy on the class II peptide repertoire and suggests that this finding has implications for the regulation of CD4(+) T cell-mediated processes.

Characterization of legumain.

The mammalian legumain, also called asparaginyl endopeptidase (AEP), is critically involved in the processing of bacterial antigens for MHC class II presentation. In order to investigate the substrate specificity of AEP in the P1' position, we created a peptide library and digested it with purified pig kidney AEP. Digestion was less efficient only when proline was in the P1' position. Maximum AEP activity was found in lysosomal fractions of different types of antigen presenting cells (APC). When the multiple sclerosis-associated autoantigen myelin basic protein (MBP) was digested with AEP, the immunodominant epitope 83-99 was destroyed. Myoglobin as an alternative substrate was AEP resistant. These results suggest an important, but not necessarily critical role for AEP in lysosomal antigen degradation.

Cathepsin G, and not the asparagine-specific endoprotease, controls the processing of myelin basic protein in lysosomes from human B lymphocytes.

The asparagine-specific endoprotease (AEP) controls lysosomal processing of the potential autoantigen myelin basic protein (MBP) by human B lymphoblastoid cells, a feature implicated in the immunopathogenesis of multiple sclerosis. In this study, we demonstrate that freshly isolated human B lymphocytes lack significant AEP activity and that cleavage by AEP is dispensable for proteolytic processing of MBP in this type of cell. Instead, cathepsin (Cat) G, a serine protease that is not endogenously synthesized by B lymphocytes, is internalized from the plasma membrane and present in lysosomes from human B cells where it represents a major functional constituent of the proteolytic machinery. CatG initialized and dominated the destruction of intact MBP by B cell-derived lysosomal extracts, degrading the immunodominant MBP epitope and eliminating both its binding to MHC class II and a MBP-specific T cell response. Degradation of intact MBP by CatG was not restricted to a lysosomal environment, but was also performed by soluble CatG. Thus, the abundant protease CatG might participate in eliminating the immunodominant determinant of MBP. Internalization of exogenous CatG represents a novel mechanism of professional APC to acquire functionally dominant proteolytic activity that complements the panel of endogenous lysosomal enzymes.