Phosphorylation of p40AUF1 regulates binding to A + U-rich mRNA-destabilizing elements and protein-induced changes in ribonucleoprotein structure

Messenger RNA turnover directed by A + U-rich elements (AREs) involves selected ARE-binding proteins. Whereas several signaling systems may modulate ARE-directed mRNA decay and/or post-translationally modify specific trans-acting factors, it is unclear how these mechanisms are linked. In THP-1 monocytic leukemia cells, phorbol ester-induced stabilization of some mRNAs containing AREs was accompanied by dephosphorylation of Ser83 and Ser87 of polysome-associated p40AUF1. Here, we report that phosphorylation of p40AUF1 influences its ARE-binding affinity as well as the RNA conformational dynamics and global structure of the p40AUF1-ARE ribonucleoprotein complex. Most notably, association of unphosphorylated p40AUF1 induces a condensed RNA conformation upon ARE substrates. By contrast, phosphorylation of p40AUF1 at Ser83 and Ser87 inhibits this RNA structural transition. These data indicate that selective AUF1 phosphorylation may regulate ARE-directed mRNA turnover by remodeling local RNA structures, thus potentially altering the presentation of RNA and/or protein determinants involved in subsequent trans-factor recruitment.     

Underproduction of sigma 70 mimics a stringent response. A proteome approach

When Escherichia coli cells enter stationary phase due to carbon starvation the synthesis of ribosomal proteins is rapidly repressed. In a DeltarelA DeltaspoT mutant, defective in the production of the alarmone guanosine tetraphosphate (ppGpp), this regulation of the levels of the protein synthesizing system is abolished. Using a proteomic approach we demonstrate that the production of the vast majority of detected E. coli proteins are decontrolled during carbon starvation in the DeltarelA DeltaspoT strain and that the starved cells behave as if they were growing exponentially. In addition we show that the inhibition of ribosome synthesis by the stringent response can be qualitatively mimicked by artificially lowering the levels of the housekeeping sigma factor, sigma(70). In other words, genes encoding the protein-synthesizing system are especially sensitive to reduced availability of sigma(70) programmed RNA polymerase. This effect is not dependent on ppGpp since lowering the levels of sigma(70) gives a similar but less pronounced effect in a ppGpp(0) strain. The data is discussed in view of the models advocating for a passive control of gene expression during stringency based on alterations in RNA polymerase availability.     

Saccharomyces cerevisiae RRM3, a 5' to 3' DNA helicase,physically interacts with proliferating cell nuclear antigen

Proliferating cell nuclear antigen (PCNA) plays an essential role in eukaryotic DNA replication, and numerous DNA replication proteins have been found to interact with PCNA through a conserved eight-amino acid motif called the PIP-box. We have searched the genome of the yeast Saccharomyces cerevisiae for open reading frames that encode proteins with putative PIP-boxes and initiated testing of 135 novel candidates for their ability to interact with PCNA-conjugated agarose beads. The first new PCNA-binding protein identified in this manner is the 5' to 3' DNA helicase RRM3. Yeast two-hybrid tests show that N-terminal deletions of RRM3, which remove the PIP-box but leave the helicase motifs intact, abolish the interaction with PCNA. In addition, mutating the two phenylalanine residues in the PIP-box to alanine or aspartic acid reduces binding to PCNA, confirming that the PIP-box in RRM3 is responsible for interaction with PCNA. The results presented here suggest that the RRM3 helicase functions at the replication fork.     

ATR enforces the topoisomerase II-dependent G2 checkpoint through inhibition of Plk1 kinase

An ATR-dependent G(2) checkpoint responds to inhibition of topoisomerase II and delays entry into mitosis by sustaining nuclear exclusion of cyclin B1-Cdk1 complexes. Here we report that induction of this checkpoint with ICRF-193, a topoisomerase II catalytic inhibitor that does not cause DNA damage, was associated with an ATR-dependent inhibition of polo-like kinase 1 (Plk1) kinase activity and a decrease in cyclin B1 phosphorylation. Expression of constitutively active Plk1 but not wild type Plk1 reversed ICRF-193-induced mitotic delay in HeLa cells, suggesting that Plk1 kinase activity is important for the checkpoint response to ICRF-193. G(2)/M synchronized normal human fibroblasts, when treated with ICRF-193, showed a decrease in cyclin B1 phosphorylation and Plk1 kinase activity despite high cyclin B1-Cdk1 kinase activity. G(2) fibroblasts that were treated with caffeine to override the checkpoint response to ICRF-193 displayed a high incidence of chromosomal aberrations. Taken together, these results suggest that ATR-dependent inhibition of Plk1 kinase activity may be one mechanism to regulate cyclin B1 phosphorylation and sustain nuclear exclusion during the G(2) checkpoint response to topoisomerase II inhibition. Moreover, the results demonstrate an important role for the topoisomerase II-dependent G(2) checkpoint in the preservation of human genomic stability.     

Facile hydrogen-deuterium exchange at the 5'-position of an analogue of S-adenosyl-l-methionine

S-3',4'-anhydroadenosyl-l-methionine is an analogue of the S-adenosyl-l-methionine coenzyme. Here we report on a rapid solvent exchange of the methylene protons at the 5'-position of this analogue. The rate of H/D exchange was measured by nuclear magnetic resonance spectroscopy under buffered conditions in deuterium oxide. The reaction is specific base catalyzed and displays a second-order rate constant of 2 x 10(4) M(-1) s(-1), which corresponds to a rate enhancement of 10(12) compared to solvent exchange of alpha-methylene protons in acyclic, aliphatic sulfonium ions. No other carbon bonded hydrogens in the molecule exchange with solvent under the experimental conditions. Allylic stabilization of a carbanionic-like transition state for the solvent exchange process can account for these results. Solvent exchange under these mild conditions provides a simple way to prepare a 5'-2H-labeled form of the coenzyme analogue.     

Phagocytosis and phagosome maturation are regulated by calcium in J774 macrophages interacting with unopsonized prey

Phagocytosis by neutrophils, macrophages, and other professional phagocytes requires rapid remodeling of actin. Early phagosomes are surrounded by a rim of F-actin that is disassembled during phagosomoal maturation. Breakdown of periphagosomal F-actin and phagolysosome fusion are calcium dependent processes in neutrophils interacting with serum-opsonized prey, but appears to be calcium independent in macrophages interacting with serum- or IgG-opsonized prey. In the present study, we found that calcium was necessary for phagocytosis, breakdown of periphagosomal F-actin, and phagosomal maturation in J774 macrophages interacting with unopsonized prey. We also observed that lipophosphoglycan (LPG) from Leishmania donovani promastigotes required calcium to exert its inhibitory effect on macrophage phagocytosis and periphagosomal F-actin breakdown. We conclude that calcium is essential for phagocytosis, depolymerization of periphagosomal F-actin, and phagosomal maturation in J774 macrophages interacting with unopsonized prey, as well as for proper functioning of LPG.     

The spectrin repeat: a structural platform for cytoskeletal protein assemblies

Spectrin repeats are three-helix bundle structures which occur in a large number of diverse proteins, either as single copies or in tandem arrangements of multiple repeats. They can serve structural purposes, by coordination of cytoskeletal interactions with high spatial precision, as well as a 'switchboard' for interactions with multiple proteins with a more regulatory role. We describe the structure of the alpha-actinin spectrin repeats as a prototypical example, their assembly in a defined antiparallel dimer, and the interactions of spectrin repeats with multiple other proteins. The alpha-actinin rod domain shares several features common to other spectrin repeats. (1) The rod domain forms a rigid connection between two actin-binding domains positioned at the two ends of the alpha-actinin dimer. The exact distance and rigidity are important, for example, for organizing the muscle Z-line and maintaining its architecture during muscle contraction. (2) The spectrin repeats of alpha-actinin have evolved to make tight antiparallel homodimer contacts. (3) The spectrin repeats are important interaction sites for multiple structural and signalling proteins. The interactions of spectrin repeats are, however, diverse and defy any simple classification of their preferred interaction sites, which is possible for other domains (e.g. src-homology domains 3 or 2). Nevertheless, the binding properties of the repeats perform important roles in the biology of the proteins where they are found, and lead to the assembly of complex, multiprotein structures involved both in cytoskeletal architecture as well as in forming large signal transduction complexes.     

Monthly patterns of testosterone and behavior in prospective fathers

The individual time patterns of salivary testosterone of adult healthy men, self-reported sexual behavior and their co-occurrence with regular weekly or monthly intervals were studied. Twenty-seven volunteer males (mean age 33 +/- 1 years) collected daily morning saliva over a period of 90 days. Evening questionnaires provided daily information on sexual activity. From the saliva, testosterone immunoreactive substances were determined using enzyme immunoassay. To detect events in which increases of testosterone were associated with sexual activity and at the same time controlling for regular internal patterns in men, data were analyzed using Theme software. First results indicated a varying number of complex nonrandom interaction patterns of testosterone with sexual activity, but also with weekly (i.e., Saturdays) and monthly intervals (i.e., 28-day full-moon intervals). The social context of the occurrence of specific pattern combinations was elaborated using parameters from the men's self-reported general life history profiles. Peak hormone levels occurred around weekends in the majority of the males. The 28-day monthly interval coincided with testosterone peaks only in those of the paired men who reported a current wish for children ("prospective fathers"), but not in unpaired men or in those who did not wish to have children with their current partner. Rather than representing a direct regular pattern of the male testosterone per se, the observed patterns suggest that men have the facultative potential to adjust their testosterone responses to their female partner's cycle. In line with the interactions between behavior and androgens observed in vertebrates in general, this study adds an example of the mutual character of hormone-behavior interactions and, thus, for the social context of testosterone patterns in human males.     

The cysteine desulfurase IscS is required for synthesis of all five thiolated nucleosides present in tRNA from Salmonella enterica serovar typhimurium

Deficiency of a modified nucleoside in tRNA often mediates suppression of +1 frameshift mutations. In Salmonella enterica serovar Typhimurium strain TR970 (hisC3737), which requires histidine for growth, a potential +1 frameshifting site, CCC-CAA-UAA, exists within the frameshifting window created by insertion of a C in the hisC gene. This site may be suppressed by peptidyl-tRNAProcmo5UGG (cmo(5)U is uridine-5-oxyacetic acid), making a frameshift when decoding the near-cognate codon CCC, provided that a pause occurs by, e.g., a slow entry of the tRNAGlnmnm5s2UUG (mnm(5)s(2)U is 5-methylaminomethyl-2-thiouridine) to the CAA codon located in the A site. We selected mutants of strain TR970 that were able to grow without histidine, and one such mutant (iscS51) was shown to have an amino acid substitution in the L-cysteine desulfurase IscS. Moreover, the levels of all five thiolated nucleosides 2-thiocytidine, mnm(5)s(2)U, 5-carboxymethylaminomethyl-2-thiouridine, 4-thiouridine, and N-6-(4-hydroxyisopentenyl)-2-methylthioadenosine present in the tRNA of S. enterica were reduced in the iscS51 mutant. In logarithmically growing cells of Escherichia coli, a deletion of the iscS gene resulted in nondetectable levels of all thiolated nucleosides in tRNA except N-6-(4-hydroxyisopentenyl)-2-methylthioadenosine, which was present at only 1.6% of the wild-type level. After prolonged incubation of cells in stationary phase, a 20% level of 2-thiocytidine and a 2% level of N-6-(4-hydroxyisopentenyl)-2-methylthioadenosine was observed, whereas no 4-thiouridine, 5-carboxymethylaminomethyl-2-thiouridine, or mnm(5)s(2)U was found. We attribute the frameshifting ability mediated by the iscS51 mutation to a slow decoding of CAA by the tRNAGlnmnm5s2UUG due to mnm(5)s(2)U deficiency. Since the growth rate of the iscS deletion mutant in rich medium was similar to that of a mutant (mnmA) lacking only mnm(5)s(2)U, we suggest that the major cause for the reduced growth rate of the iscS deletion mutant is the lack of mnm(5)s(2)U and 5-carboxymethylaminomethyl-2-thiouridine and not the lack of any of the other three thiolated nucleosides that are also absent in the iscS deletion mutant.