Interaction between the N-terminal domain of human DNA topoisomerase I and the arginine-serine domain of its substrate determines phosphorylation of SF2/ASF splicing factor.

Human DNA topoisomerase I, known for its DNA-relaxing activity, is possibly one of the kinases phosphorylating members of the SR protein family of splicing factors, in vivo. Little is known about the mechanism of action of this novel kinase. Using the prototypical SR protein SF2/ASF (SRp30a) as model substrate, we demonstrate that serine residues phosphorylated by topo I/kinase exclusively located within the most extended arginine-serine repeats of the SF2/ASF RS domain. Unlike other kinases such as cdc2 and SRPK1, which also phosphorylated serines at the RS domain, topo I/kinase required several SR dipeptide repeats. These repeats possibly contribute to a versatile structure in the RS domain thereby facilitating phosphorylation. Furthermore, far-western, fluorescence spectroscopy and kinase assays using the SF2/ASF mutants, demonstrated that kinase activity and binding were tightly coupled. Since the deletion of N-terminal 174 amino acids of Topo I destroys SF2/ASF binding and kinase activity but not ATP binding, we conclude that at least two distinct domains of Topo I are necessary for kinase activity: one in the C-terminal region contributing to the ATP binding site and the other one in the N-terminal region that allows binding of SF2/ASF.     

L’éléphantiasis pénoscrotal, diagnostic et prise en charge: à propos de trois cas

Penoscrotal elephantiasis is a rare disease outside areas where filariasis is endemic. It is a benign disease but can become disabling in that it can make sexual relations difficult and sometimes even affect urination. We report three cases of primitive penoscrotal elephantiasis treated with complete surgical resection of pathological tissue and penoscrotal reconstruction, with good functional and aesthetic results. We update, through our own experience and a review of the literature, aspects of the diagnostic and therapeutic care of penoscrotal elephantiasis.     

Alternative splicing and disease

Almost all protein-coding genes are spliced and their majority is alternatively spliced. Alternative splicing is a key element in eukaryotic gene expression that increases the coding capacity of the human genome and an increasing number of examples illustrates that the selection of wrong splice sites causes human disease. A fine-tuned balance of factors regulates splice site selection. Here, we discuss well-studied examples that show how a disturbance of this balance can cause human disease. The rapidly emerging knowledge of splicing regulation now allows the development of treatment options.     

Identification of two human nuclear proteins that recognise the cytosine-rich strand of human telomeres in vitro

Most studies on the structure of DNA in telomeres have been dedicated to the double-stranded region or the guanosine-rich strand and consequently little is known about the factors that may bind to the telomere cytosine-rich (C-rich) strand. This led us to investigate whether proteins exist that can recognise C-rich sequences. We have isolated several nuclear factors from human cell extracts that specifically bind the C-rich strand of vertebrate telomeres [namely a d(CCCTAA)_n repeat] with high affinity and bind double-stranded telomeric DNA with a 100× reduced affinity. A biochemical assay allowed us to characterise four proteins of apparent molecular weights 66–64, 45 and 35 kDa, respectively. To identify these polypeptides we screened a λgt11-based cDNA expression library, obtained from human HeLa cells using a radiolabelled telomeric oligonucleotide as a probe. Two clones were purified and sequenced: the first corresponded to the hnRNP K protein and the second to the ASF/SF2 splicing factor. Confirmation of the screening results was obtained with recombinant proteins, both of which bind to the human telomeric C-rich strand in vitro.     

Distinctive features of Drosophila alternative splicing factor RS domain: implication for specific phosphorylation, shuttling, and splicing activation

The human splicing factor 2, also called human alternative splicing factor (hASF), is the prototype of the highly conserved SR protein family involved in constitutive and regulated splicing of metazoan mRNA precursors. Here we report that the Drosophila homologue of hASF (dASF) lacks eight repeating arginine-serine dipeptides at its carboxyl-terminal region (RS domain), previously shown to be important for both localization and splicing activity of hASF. While this difference has no effect on dASF localization, it impedes its capacity to shuttle between the nucleus and cytoplasm and abolishes its phosphorylation by SR protein kinase 1 (SRPK1). dASF also has an altered splicing activity. While being competent for the regulation of 5' alternative splice site choice and activation of specific splicing enhancers, dASF fails to complement S100-cytoplasmic splicing-deficient extracts. Moreover, targeted overexpression of dASF in transgenic flies leads to higher deleterious developmental defects than hASF overexpression, supporting the notion that the distinctive structural features at the RS domain between the two proteins are likely to be functionally relevant in vivo.     

Adenosine phosphorothioates (ATP alpha S and ATP tau S) differentially affect the two steps of mammalian pre-mRNA splicing.

We have investigated the function of ATP hydrolysis in mammalian pre-mRNA in vitro splicing using adenosine phosphorothioates (ATP alpha S and ATP tau S) known to affect the activity of a number of ATP-requiring enzymes. Spliceosome assembly, but neither one of the two transesterification reactions involved in splicing, occurs with ATP alpha S suggesting that at least two types of ATP-requiring factors are brought into play. ATP alpha S has no effect in the presence of normal ATP and, therefore, spliceosomes assembled in the presence of ATP alpha S remain competent for splicing when supplied with normal ATP. ATP tau S noticeably and irreversibly inhibits the second transesterification reaction, i.e. at a time when most of the analog has been hydrolyzed and regenerated to normal ATP by creatine phosphate. This indicates that the inhibition results from an earlier event, most likely the thiophosphorylation of spliceosomal proteins. Under this assumption, the inhibition could be due to the failure of the thiophosphorylated proteins to be dephosphorylated. Indeed, okadaic acid, a potent inhibitor of protein phosphatases, inhibits the second step of a reaction in the presence of normal ATP. We propose that some splicing factors undergo phosphorylation-dephosphorylation cycles during spliceosome assembly and splicing, while others that could be the mammalian equivalents of the RNA helicase-like proteins recently discovered in yeast most likely bind and hydrolyze ATP.     

Purified U5 small nuclear ribonucleoprotein can relieve the inhibition of spliceosome assembly and splicing by snRNP-free nuclear proteins.

As demonstrated by RNase T1 protection assays at 0 degrees C without ATP, U1 and U5 snRNPs purified by isopycnic centrifugation in cesium chloride bind to the 5' and 3' splice sites of human beta-globin pre-mRNA, respectively. We also devised a saturation-complementation assay and have found that this purified U5 snRNP, unlike U1, successfully competes with snRNP-free fractions of nuclear proteins which inhibit spliceosome assembly and splicing. Restoration of activity requires intact U5 snRNA and correlates with the presence of the 100 Kd intron binding protein (IBP) which we have previously characterized (Tazi et al., 1986, Cell 47, 755-766). Our results are compatible with a model in which the recognition of the 3' splice site by IBP-U5 snRNP is one of the earliest events of the spliceosome assembly. It could organize the structure of the 3' splice site region of the human beta-globin like pre-mRNAs. However, on the basis of results showing that beta-globin and major late adenovirus seem to have different requirements with respect to IBP-U5 snRNP, it appears that some pre-mRNAs could have a native structure that necessitates less if at all IBP-U5.     

Eye development under the control of SRp55/B52-mediated alternative splicing of eyeless

The genetic programs specifying eye development are highly conserved during evolution and involve the vertebrate Pax-6 gene and its Drosophila melanogaster homolog eyeless (ey). Here we report that the SR protein B52/SRp55 controls a novel developmentally regulated splicing event of eyeless that is crucial for eye growth and specification in Drosophila. B52/SRp55 generates two isoforms of eyeless differing by an alternative exon encoding a 60-amino-acid insert at the beginning of the paired domain. The long isoform has impaired ability to trigger formation of ectopic eyes and to bind efficiently Eyeless target DNA sequences in vitro. When over-produced in the eye imaginal disc, this isoform induces a small eye phenotype, whereas the isoform lacking the alternative exon triggers eye over-growth and strong disorganization. Our results suggest that B52/SRp55 splicing activity is used during normal eye development to control eye organogenesis and size through regulation of eyeless alternative splicing.