Substitutions in the capsids of poliovirus mutants selected in human neuroblastoma cells confer on the Mahoney type 1 strain a phenotype neurovirulent in mice.

Poliovirus (PV) type 1 mutants selected in human neuroblastoma cells persistently infected (PVpi) with the wild-type Mahoney strain exhibited a mouse-neurovirulent phenotype. Four of the five substitutions present in the capsid proteins of a PVpi were demonstrated to extend the host range of the Mahoney strain to mice. These new mouse-neurovirulent determinants were located in the three-dimensional structure of the viral capsid; two of them (residues 142 of VP2 and 60 of VP3) were located in loops exposed at the surface of the protein shell, whereas the other two (residues 43 of VP1 and 62 of VP4) were located on the inside of the capsid. VP1 residue 43 and VP2 residue 142 substitutions were also selected in a PVpi derived from the attenuated Sabin strain. We suggest that the selective pressure of human neuroblastoma cell factor(s) involved in early steps of PV multiplication could be responsible for the arising of amino acid substitutions which confer adaptation to the mouse central nervous system to PV.     

The F-box protein Skp2 is a ubiquitylation target of a Cul1-based core ubiquitin ligase complex: evidence for a role of Cul1 in the suppression of Skp2 expression in quiescent fibroblasts

The ubiquitin protein ligase SCF(Skp2) is composed of Skp1, Cul1, Roc1/Rbx1 and the F-box protein Skp2, the substrate-recognition subunit. Levels of Skp2 decrease as cells exit the cell cycle and increase as cells re-enter the cycle. Ectopic expression of Skp2 in quiescent fibroblasts causes mitogen-independent S-phase entry. Hence, mechanisms must exist for limiting Skp2 protein expression during the G(0)/G(1) phases. Here we show that Skp2 is degraded by the proteasome in G(0)/G(1) and is stabilized when cells re-enter the cell cycle. Rapid degradation of Skp2 in quiescent cells depends on Skp2 sequences that contribute to Cul1 binding and interference with endogenous Cul1 function in serum-deprived cells induces Skp2 expression. Furthermore, recombinant Cul1-Roc1/Rbx1-Skp1 complexes can catalyse Skp2 ubiquitylation in vitro. These results suggest that degradation of Skp2 in G(0)/G(1) is mediated, at least in part, by an autocatalytic mechanism involving a Skp2-bound Cul1-based core ubiquitin ligase and imply a role for this mechanism in the suppression of SCF(Skp2) ubiquitin protein ligase function during the G(0)/G(1) phases of the cell cycle.     

Degradation of Hof1 by SCF(Grr1) is important for actomyosin contraction during cytokinesis in yeast

SCF-type (SCF: Skp1-Cullin-F-box protein complex) E3 ligases regulate ubiquitin-dependent degradation of many cell cycle regulators, mainly at the G1/S transition. Here, we show that SCF(Grr1) functions during cytokinesis by degrading the PCH protein Hof1. While Hof1 is required early in mitosis to assemble a functional actomyosin ring, it is specifically degraded late in mitosis and remains unstable during the entire G1 phase of the cell cycle. Degradation of Hof1 depends on its PEST motif and a functional 26S proteasome. Interestingly, degradation of Hof1 is independent of APC(Cdh1), but instead requires the SCF(Grr1) E3 ligase. Grr1 is recruited to the mother-bud neck region after activation of the mitotic-exit network, and interacts with Hof1 in a PEST motif-dependent manner. Our results also show that downregulation of Hof1 at the end of mitosis is necessary to allow efficient contraction of the actomyosin ring and cell separation during cytokinesis. SCF(Grr1)-mediated degradation of Hof1 may thus represent a novel mechanism to couple exit from mitosis with initiation of cytokinesis.     

Blue Tit song repertoire size, male quality and interspecific competition

A recent hypothesis suggested that bird song repertoire size is a reliable indication of male quality because the number of songs emitted by an individual and the expression of many traits important for fitness are highly dependent on developmental conditions. To test this hypothesis, we studied the relationship between song repertoire size and a trait known to be strongly affected by developmental conditions: tarsus length of Blue Tits Parus caeruleus . We studied this relationship in three populations presenting differences in density, quantity of food available and relative and absolute densities of Great Tits Parus major . Great Tit density may be an important factor because this species is supposed to be in competition for acoustic space with the Blue Tit. We found a positive correlation between Blue Tit dawn repertoire size and tarsus length in the three study populations which is consistent with the tested hypothesis. However, this correlation was only significant in the population with the lower Great Tit density. We discuss the significance of this variation and propose that interspecific competition may limit the action of sexual selection on song repertoire size.     

Bacterial intracellularly active toxins: Membrane localisation of the active domain

Numerous bacterial toxins exert their activity by inactivating or modulating a specific intracellular host target. For this purpose, these toxins have developed efficient strategies to overcome the different host cell defences including specific binding to cell surface, internalisation, passage through the endosome or plasma membrane, exploiting intracellular trafficking and addressing to intracellular targets. Several intracellularly active toxins deliver an active domain into the cytosol that interacts with a target localised to the inner face of the plasma membrane. Thus, the large clostridial glucosylating toxins (LCGTs) target Rho/Ras‐GTPases, certain virulence factors of Gram negative bacteria, Rho‐GTPases, while Pasteurella multocida toxin (PMT) targets trimeric G‐proteins. Others such as botulinum neurotoxins and tetanus neurotoxin have their substrate on synaptic vesicle membrane. LCGTs, PMT, and certain virulence factors from Vibrio sp. show a particular structure constituted of a four‐helix bundle membrane (4HBM) protruding from the catalytic site that specifically binds to the membrane phospholipids and then trap the catalytic domain at the proximity of the membrane anchored substrate. Structural and functional analysis indicate that the 4HBM tip of the Clostridium sordellii lethal toxin (TcsL) from the LCGT family contain two loops forming a cavity that mediates the binding to phospholipids and more specifically to phosphatidylserine.     

Bax is activated during rotavirus-induced apoptosis through the mitochondrial pathway

Rotaviruses are the leading cause of infantile viral gastroenteritis worldwide. Mature enterocytes of the small intestine infected by rotavirus undergo apoptosis, and their replacement by less differentiated dividing cells probably leads to defective absorptive function of the intestinal epithelium, which, in turn, contributes to osmotic diarrhea and rotavirus pathogenesis. Here we show that infection of MA104 cells by the simian rhesus rotavirus strain RRV induced caspase-3 activation, DNA fragmentation, and cleavage of poly(ADP-ribose) polymerase; all three phenomena are features of apoptosis. RRV induced the release of cytochrome c from mitochondria to the cytosol, indicating that the mitochondrial apoptotic pathway was activated. RRV infection of MA104 cells activated Bax, a proapoptotic member of the Bcl-2 family, as revealed by its conformational change. Most importantly, Bax-specific small interfering RNAs partially inhibited cytochrome c release in RRV-infected cells. Thus, mitochondrial dysfunction induced by rotavirus is Bax dependent. Apoptosis presumably leads to impaired intestinal functions, so our findings contribute to improving our understanding of rotavirus pathogenesis at the cellular level.