Mammals have two twinfilin isoforms whose subcellular localizations and tissue distributions are differentially regulated

Twinfilin is a highly conserved actin monomer-binding protein that regulates cytoskeletal dynamics in organisms from yeast to mammals. In addition to the previously characterized mammalian twinfilin-1, a second protein with approximately 65% sequence identity to twinfilin-1 exists in mouse and humans. However, previous studies failed to identify any actin binding activity in this protein (Rohwer, A., Kittstein, W., Marks, F., and Gschwendt, M. (1999) Eur. J. Biochem. 263, 518-525). Here we show that this protein, which we named twinfilin-2, is indeed an actin monomer-binding protein. Similar to twinfilin-1, mouse twinfilin-2 binds ADP-G-actin with a higher affinity (KD = 0.12 microM) than ATP-G-actin (KD = 1.96 microM) and efficiently inhibits actin filament assembly in vitro. Both mouse twinfilins inhibit the nucleotide exchange on actin monomers and directly interact with capping protein. Furthermore, the actin interactions of mouse twinfilin-1 and twinfilin-2 are inhibited by phosphatidylinositol (4,5)-bisphosphate. Although biochemically very similar, our Northern blots and in situ hybridizations show that these two proteins display distinct expression patterns. Twinfilin-1 is the major isoform in embryos and in most adult mouse non-muscle cell-types, whereas twinfilin-2 is the predominant isoform of adult heart and skeletal muscles. Studies with isoform-specific antibodies demonstrated that although the two proteins show similar localizations in unstimulated cells, they are regulated by different mechanisms. The small GTPases Rac1 and Cdc42 induce the redistribution of twinfilin-1 to membrane ruffles and cell-cell contacts, respectively, but do not affect the localization of twinfilin-2. Taken together, these data show that mammals have two twinfilin isoforms, which are differentially expressed and regulated through distinct cellular signaling pathways.     

Green fluorescent protein (GFP) fusion constructs in gene therapy research

The history of green fluorescent protein (GFP) as a marker is less than 10 years old, but it has already made a major impact on many areas of natural sciences, especially on cell biology and histochemistry. GFP can be detected in living cells without selection or staining and it can be fused to other proteins to yield fluorescent chimeras. The potential of GFP has also been recognised by gene therapy researchers and various GFP-tagged therapeutic proteins have been constructed. These chimeric proteins have been used to determine the expression level, site and time course of the therapeutic gene, or the correlation between gene transfer rate and therapeutic outcome. This review summarises the status of the applications of GFP fusions in gene therapy research.     

Trypanosoma brucei: in vitro slender-to-stumpy differentiation of culture-adapted,monomorphic bloodstream forms

Pleomorphic Trypanosoma brucei strains are characterized by their ability to differentiate from replicating long slender forms into non-dividing short stumpy forms in the mammalian host. The differentiation process can be efficiently induced in vitro by treatment with the membrane-permeable cAMP derivative 8-(4-chlorophenylthio)-cAMP (pCPTcAMP). In contrast, monomorphic T. brucei strains do not differentiate to stumpy forms in the host. Here, we show that exposure of monomorphic, culture-adapted T. brucei bloodstream forms to pCPTcAMP allowed their subsequent differentiation into short stumpy forms. The stumpy nature of pCPTcAMP-treated parasites was confirmed by (1) morphological change, (2) inhibition of growth and DNA synthesis, (3) cell cycle arrest in the G(1)/G(0) phase, (4) expression of NADH diaphorase activity and dihydrolipoamide dehydrogenase, (5) disappearance of the small subunit of ribonucleotide reductase, (6) up-regulation of the major lysosomal membrane protein, and (7) efficient transformation into replicating procyclic insect forms after induction with citrate/cis-aconitate. Our results indicate that the inability of monomorphic T. brucei bloodstream forms to differentiate into short stumpy forms in the host may be due to a failure in the signalling pathway rather than in the differentiation process itself. Treatment of monomorphic bloodstream trypanosomes with pCPTcAMP could be a useful method for identifying the genes involved in the slender-to-stumpy differentiation process.     

The ubiquitous chromatin protein DEK alters the structure of DNA by introducing positive supercoils

We have investigated the molecular mechanism by which the proto-oncogene protein DEK, an abundant chromatin-associated protein, changes the topology of DNA in chromatin in vitro. Band-shift assays and electron microscopy revealed that DEK induces both intra- and intermolecular interactions between DNA molecules. Binding of the DEK protein introduces constrained positive supercoils both into protein-free DNA and into DNA in chromatin. The induced change in topology is reversible after removal of the DEK protein. As shown by sedimentation analysis and electron microscopy, the DEK-induced positive supercoiling causes distinct structural changes of DNA and chromatin. The observed direct effects of DEK on chromatin folding help to understand the function that this major chromatin protein performs in the nucleus.     

Functional analysis by site-directed mutagenesis of the NAD(+)-reducing hydrogenase from Ralstonia eutropha

The tetrameric cytoplasmic [NiFe] hydrogenase (SH) of Ralstonia eutropha couples the oxidation of hydrogen to the reduction of NAD(+) under aerobic conditions. In the catalytic subunit HoxH, all six conserved motifs surrounding the [NiFe] site are present. Five of these motifs were altered by site-directed mutagenesis in order to dissect the molecular mechanism of hydrogen activation. Based on phenotypic characterizations, 27 mutants were grouped into four different classes. Mutants of the major class, class I, failed to grow on hydrogen and were devoid of H(2)-oxidizing activity. In one of these isolates (HoxH I64A), H(2) binding was impaired. Class II mutants revealed a high D(2)/H(+) exchange rate relative to a low H(2)-oxidizing activity. A representative (HoxH H16L) displayed D(2)/H(+) exchange but had lost electron acceptor-reducing activity. Both activities were equally affected in class III mutants. Mutants forming class IV showed a particularly interesting phenotype. They displayed O(2)-sensitive growth on hydrogen due to an O(2)-sensitive SH protein.