Mechanistic insights into Sky1p,a yeast homologue of the mammalian SR protein kinases

The SRPK family is distinguished from typical eukaryotic protein kinases by several unique structural features recently elucidated by X-ray diffraction methods [Nolen et al. (2001) Nat. Struct. Biol. 8, 176-183]. To determine whether these features impart unique catalytic function, the phosphorylation of the physiological Sky1p substrate, Npl3p, was monitored using steady-state and pre-steady-state kinetic techniques. While Sky1p has a low apparent affinity for ATP compared to other protein kinases, it binds Npl3p with very high affinity. The latter is achieved through a combination of local and distal factors in the protein substrate. The phosphoryl donor ATP has access to the nucleotide pocket in the absence or presence of Npl3p, indicating that a large protein substrate does not enforce an ordered addition of ligands. Sky1p binds two Mg(2+)-the first is essential whereas the second further enhances catalysis. While the turnover number is low (0.5 s(-1)), Npl3p is rapidly phosphorylated in the active site (40 s(-1)) based on single turnover experiments. These results indicate that Sky1p employs a catalytic pathway involving fast phosphoryl transfer followed by slow net release of products. These studies represent the first kinetic investigation of a member of the SRPK family and the first pre-steady-state kinetic study of a protein kinase using a natural protein substrate.     

Mutations in the ribonuclease H active site of HIV-RT reveal a role for this site in stabilizing enzyme-primer-template binding

The RNase H activity of HIV-RT is coordinated by a catalytic triad (E478, D443, D498) of acidic residues that bind divalent cations. We examined the effect of RNase H deficient E(478)-->Q and D(549)-->N mutations that do not alter polymerase activity on binding of enzyme to various nucleic acid substrates. Binding of the mutant and wild-type enzymes to various nucleic acid substrates was examined by determining dissociation rate constants (k(off)) by titrating both Mg(2+) and salt concentrations. In agreement with the unaltered polymerase activity of the mutant, the k(off) values for the wild-type and mutant enzymes were essentially identical using DNA-DNA templates in the presence of 6 mM Mg(2+). However, with lower concentrations of Mg(2+) and in the absence of Mg(2+), although both enzymes dissociated more rapidly, the mutant enzymes dissociated several-fold more slowly than the wild type. This was also observed on RNA-DNA templates. These results indicate that alterations in residues essential for Mg(2+) binding have a pronounced positive effect on enzyme-template stability and that the negative residues in the RNase H region of the enzyme have a negative influence on binding in the absence of Mg(2+). In this regard RT is similar to other nucleic acid cleaving enzymes that show enhanced binding upon mutation of active site residues.     

Site-directed mutagenesis of the basic N-terminal cluster of pancreatic bile salt-dependent lipase. Functional significance

Previous studies have postulated the presence of a heparin-binding site on the bile salt-dependent lipase (BSDL), whereas two bile salt-binding sites regulate the enzyme activity. One of these sites may overlap with the tentative heparin-binding site at the level of an N-terminal basic cluster consisting of positive residues Lys(32), Lys(56), Lys(61), Lys(62), and Arg(63). The present study uses specific site-directed mutagenesis to determine the functional significance of this basic cluster. Mutations in this sequence resulted in recombinant enzymes that were able to bind to immobilized and to cell-associated heparin before moving throughout intestinal cells. Recombinant BSDL was fully active on soluble substrate, but mutants were less active on micellar cholesteryl oleate in comparison with the wild-type enzyme. Activation studies by primary (sodium taurocholate) and by secondary (sodium taurodeoxycholate) bile salts revealed that the activation of BSDL by sodium taurocholate at concentrations below the critical micellar concentration, and not that evoked by micellar bile salts, was affected by substitutions, suggesting that this N-terminal basic cluster likely represents the specific bile salt-binding site of BSDL. Substitutions also affected the activation of the enzyme promoted by anionic phospholipids, extending the function of this site to that of a cationic regulatory site susceptible to accommodate anionic ligands.     

Structural evidence of functional divergence in human alkaline phosphatases

The evolution of the alkaline phosphatase (AP) gene family has lead to the existence in humans of one tissue-nonspecific (TNAP) and three tissue-specific isozymes, i.e. intestinal (IAP), germ cell (GCAP), and placental AP (PLAP). To define the structural differences between these isozymes, we have built models of the TNAP, IAP, and GCAP molecules based on the 1.8-structure of PLAP(1) and have performed a comparative structural analysis. We have examined the monomer-monomer interface as this area is crucial for protein stability and enzymatic activity. We found that the interface allows the formation of heterodimers among IAP, GCAP, and PLAP but not between TNAP with any of the three tissue-specific isozymes. Secondly, the active site cleft was mapped into three regions, i.e. the active site itself, the roof of the cleft, and the floor of the cleft. This analysis led to a structural fingerprint of the active site of each AP isozyme that suggests a diversification in substrate specificity for this isozyme family.     

Crumbs interacts with moesin and beta(Heavy)-spectrin in the apical membrane skeleton of Drosophila

The apical transmembrane protein Crumbs is necessary for both cell polarization and the assembly of the zonula adherens (ZA) in Drosophila epithelia. The apical spectrin-based membrane skeleton (SBMS) is a protein network that is essential for epithelial morphogenesis and ZA integrity, and exhibits close colocalization with Crumbs and the ZA in fly epithelia. These observations suggest that Crumbs may stabilize the ZA by recruiting the SBMS to the junctional region. Consistent with this hypothesis, we report that Crumbs is necessary for the organization of the apical SBMS in embryos and Schneider 2 cells, whereas the localization of Crumbs is not affected in karst mutants that eliminate the apical SBMS. Our data indicate that it is specifically the 4.1 protein/ezrin/radixin/moesin (FERM) domain binding consensus, and in particular, an arginine at position 7 in the cytoplasmic tail of Crumbs that is essential to efficiently recruit both the apical SBMS and the FERM domain protein, DMoesin. Crumbs, Discs lost, betaHeavy-spectrin, and DMoesin are all coimmunoprecipitated from embryos, confirming the existence of a multimolecular complex. We propose that Crumbs stabilizes the apical SBMS via DMoesin and actin, leading to reinforcement of the ZA and effectively coupling epithelial morphogenesis and cell polarity.     

Efficacité et innocuité du prélèvement percutané de sperme testiculaire dans la prise en charge de l’infertilité masculine

Objective

To assess the efficacy and safety of percutaneous testicular biopsy to provide sperm cells for ICSI in male patients with azoospermia not amenable to surgical treatment.

Materials and methods

From October 1995 to December 2001, 175 biopsies were performed in men with azoospermia to provide material for intracytoplasmic sperm injection. Azoospermia was obstructive (OA) in 41 cases and non-obstructive (NOA) in 134 cases. Open biopsy was performed in the first 15 patients in the series and percutaneous biopsy was performed on an outpatient basis, under local anesthesia, with a Biopty Gun® (14G needle), in the subsequent patients as the first step in management. Open surgical biopsies were performed in another 15 patients following a sperm cell-negative percutaneous biopsy.

Results

All biopsies performed for OA were positive, but only 51/134 biopsies (38%) were positive in the NOA group. The material provided by percutaneous biopsy, when positive for sperm cells, was always sufficient to perform ICSI. When percutaneous biopsy was negative, open surgical biopsy failed to give better results. Five men developed minor complications (acute hematocele) following percutaneous biopsies requiring reoperation for hemostasis (3.12%). No major complications were observed. Results were comparable in terms of fertilization and pregnancy rates whether fresh or frozen-thawed sperm was used.

Conclusion

Percutaneous testicular sperm extraction is a safe, well-tolerated and cost-effective procedure in the management of male-factor infertility related to azoospermia.     

Dopamine receptors for every species: Gene duplications and functional diversification in Craniates

The neuromodulatory effects of dopamine on the central nervous system of craniates are mediated by two classes of G protein-coupled receptors (D1 and D2), each comprising several subtypes. A systematic isolation and characterization of the D1 and D2-like receptors was carried out in most of the Craniate groups. It revealed that two events of gene duplications took place during vertebrate evolution, before or simultaneously to the emergence of Gnathostomes. It led to the conservation of two-to-four paralogous receptors (subtypes), depending on the species. Additional duplication of dopamine receptor gene occurred independently in the teleost fish lineage. Duplicated genes were maintained in most of the vertebrate groups, certainly by the acquisition of a few functional characters, specific of each subtypes, as well as by discrete changes in their expression territories in the brain. The evolutionary scenario elaborated from these data suggests that receptor gene duplications were the necessary conditions for the expansion of vertebrate forebrain to occur, allowing dopamine systems to exert their fundamental role as modulator of the adaptive capabilities acquired by vertebrate species.     

Active versus passive anti-cytokine antibody therapy against cytokine-associated chronic diseases

Current therapeutic vaccine trials in major chronic diseases including AIDS, cancer, allergy and autoimmunity, target antigenic pathogens but not the pathogenic stromal cytokines which can be major sources of histopathologic processes. Considering that the limited efficacy of these vaccines has been ascribed to local pathogen-induced cytokine dysfunction, we propose to antagonize pathogenic cytokine(s) by high affinity neutralizing auto-Abs triggered by specific anti-cytokine vaccines. As anticipated by theoretical considerations, animal experiments and initial clinical trials showed that anti-cytokine immunization was safe, well tolerated and triggered transient high titers Abs neutralizing pathogenic cytokines but, in contrast to conventional vaccines, no relevant cellular response was observed. Advantages of active versus passive anti-cytokine Ab therapy, particularly for long-term treatments, as those required in AIDS, cancer, allergy and autoimmunity include greater ease of maintaining high Ab titers, lack of anti-antibody responses and low cost.