Dynamics and DNA substrate recognition by the catalytic domain of lambda integrase

Bacteriophage lambda integrase (lambda-Int) is the prototypical member of a large family of enzymes that catalyze site-specific DNA recombination via the formation of a Holliday junction intermediate. DNA strand cleavage by lambda-Int is mediated by nucleophilic attack on the scissile phosphate by a conserved tyrosine residue, forming an intermediate with the enzyme covalently attached to the 3'-end of the cleaved strand via a phosphotyrosine linkage. The crystal structure of the catalytic domain of lambda-Int (C170) obtained in the absence of DNA revealed the tyrosine nucleophile at the protein's C terminus to be located on a beta-hairpin far from the other conserved catalytic residues and adjacent to a disordered loop. This observation suggested that a conformational change in the C terminus of the protein was required to generate the active site in cis, or alternatively, that the active site could be completed in trans by donation of the tyrosine nucleophile from a neighboring molecule in the recombining synapse. We used NMR spectroscopy together with limited proteolysis to examine the dynamics of the lambda-Int catalytic domain in the presence and absence of DNA half-site substrates with the goal of characterizing the expected conformational change. Although the C terminus is indeed flexible in the absence of DNA, we find that conformational changes in the tyrosine-containing beta-hairpin are not coupled to DNA binding. To gain structural insights into C170/DNA complexes, we took advantage of mechanistic conservation with Cre and Flp recombinases to model C170 in half-site and tetrameric Holliday junction complexes. Although the models do not reveal the nature of the conformational change required for cis cleavage, they are consistent with much of the available experimental data and provide new insights into the how trans complementation could be accommodated.     

Replication of modified vaccinia virus Ankara in primary chicken embryo fibroblasts requires expression of the interferon resistance gene E3L

Highly attenuated modified vaccinia virus Ankara (MVA) serves as a candidate vaccine to immunize against infectious diseases and cancer. MVA was randomly obtained by serial growth in cultures of chicken embryo fibroblasts (CEF), resulting in the loss of substantial genomic information including many genes regulating virus-host interactions. The vaccinia virus interferon (IFN) resistance gene E3L is among the few conserved open reading frames encoding viral immune defense proteins. To investigate the relevance of E3L in the MVA life cycle, we generated the deletion mutant MVA-DeltaE3L. Surprisingly, we found that MVA-DeltaE3L had lost the ability to grow in CEF, which is the first finding of a vaccinia virus host range phenotype in this otherwise highly permissive cell culture. Reinsertion of E3L led to the generation of revertant virus MVA-E3rev and rescued productive replication in CEF. Nonproductive infection of CEF with MVA-DeltaE3L allowed viral DNA replication to occur but resulted in an abrupt inhibition of viral protein synthesis at late times. Under these nonpermissive conditions, CEF underwent apoptosis starting as early as 6 h after infection, as shown by DNA fragmentation, Hoechst staining, and caspase activation. Moreover, we detected high levels of active chicken alpha/beta IFN (IFN-alpha/beta) in supernatants of MVA-DeltaE3L-infected CEF, while moderate IFN quantities were found after MVA or MVA-E3rev infection and no IFN activity was present upon infection with wild-type vaccinia viruses. Interestingly, pretreatment of CEF with similar amounts of recombinant chicken IFN-alpha inhibited growth of vaccinia viruses, including MVA. We conclude that efficient propagation of MVA in CEF, the tissue culture system used for production of MVA-based vaccines, essentially requires conserved E3L gene function as an inhibitor of apoptosis and/or IFN induction.     

Unfolding and pH studies on manganese peroxidase: role of heme and calcium on secondary structure stability

The present study characterizes the unfolding and folding processes of recombinant manganese peroxidase. This enzyme contains five disulfide bonds, two calcium ions, and one heme prosthetic group. Circular dichroism in the far UV was used to monitor global changes of the protein secondary structure, whereas UV-visible spectroscopy of the Soret band provided information about local changes in the heme cavity. The effects of reducing agents, oxidizing agents, and denaturants on this process were investigated. In addition to affecting the secondary structure content, these factors also affect the binding of the heme and the calcium ions, both of which have a significant effect on the folding process. Our results also show that denaturants induce irreversible changes, which are most likely due to the inability of the denatured protein to rebind either calcium or the heme. Breaking of disulfide bonds by 30 mM dithiothreitol causes complete unfolding of recombinant manganese peroxidase. The unfolding process was also studied at low and high pH, where the protein reaches the final unfolded state through two different intermediate states. The data also indicate that only the acidic folding-unfolding process is reversible. Our results indicate a complex synergistic relationship between the secondary structure content, the tertiary structure arrangement, and the binding of the heme and the calcium ions and disulfide bridge formation.     

Both neural crest and placode contribute to the ciliary ganglion and oculomotor nerve

The chick ciliary ganglion is a neural crest-derived parasympathetic ganglion that innervates the eye. Here, we examine its axial level of origin and developmental relationship to other ganglia and nerves of the head. Using small, focal injections of DiI, we show that neural crest cells arising from both the caudal half of the midbrain and the rostral hindbrain contribute to the ciliary as well as the trigeminal ganglion. Precursors to both ganglia have overlapping migration patterns, moving first ventrolaterally and then rostrally toward the optic vesicle. At the level of the midbrain/forebrain junction, precursors to the ciliary ganglion separate from the main migratory stream, turn ventromedially, and condense in the vicinity of the rostral aorta and Rathke's pouch. Ciliary neuroblasts first exit the cell cycle at early E2, prior to and during ganglionic condensation, and neurogenesis continues through E5.5. By E3, markers of neuronal differentiation begin to appear in this population. By labeling the ectoderm with DiI, we discovered a new placode, caudal to the eye and possibly contiguous to the trigeminal placode, that contributes a few early differentiating neurons to the ciliary ganglion, oculomotor nerve, and connecting branches to the ophthalmic nerve. These results suggest for the first time a dual neural crest and placodal contribution to the ciliary ganglion and associated nerves.     

The C-terminal peptide of thrombospondin-1 stimulates distinct signaling pathways but induces an activation-independent agglutination of platelets and other cells

A peptide from the C-terminal domain of thrombospondin-1 (4N1-1) has been proposed to stimulate platelet aggregation by a novel mechanism involving both an activation-independent agglutination and an activation-dependent, glycoprotein (GP) IIb/IIIa-mediated aggregation which involves GPVI signaling but does not involve CD47. The present study demonstrates that 4N1-1 stimulated a different pattern of signal transduction pathways than the GPVI agonist convulxin. Furthermore, 4N1-1-induced platelet aggregation was activation-independent and not dependent on GPVI or GPIIb/IIIa. Interestingly, 4N1-1 also stimulated activation-independent agglutination of different megakaryocytic and non-megakaryocytic cells. 4N1-1-induced cell agglutination but not platelet signaling was inhibited by anti-CD47 antibodies.     

Complex effects of different green tea catechins on human platelets

Epigallocatechin gallate (EGCG), a major component of green tea, has been previously shown to inhibit platelet aggregation. The effects of other green tea catechins on platelet function are not known. Pre-incubation with EGCG concentration-dependently inhibited thrombin-induced aggregation and phosphorylation of p38 mitogen-activated protein kinase and extracellular signal-regulated kinases-1/2. In contrast EGCG stimulated tyrosine phosphorylation of platelet proteins, including Syk and SLP-76 but inhibited phosphorylation of focal adhesion kinase. Other catechins did not inhibit platelet aggregation. Interestingly, when EGCG was added to stirred platelets, a tyrosine kinase-dependent stimulation of platelet aggregation was observed. The two other catechins containing a galloyl group in the 3' position (catechin gallate, epicatechin gallate) also stimulated platelet aggregation, while catechins without a galloyl group (catechin, epicatechin) or the catechin with a galloyl group in the 2' position (epigallocatechin) did not.     

Omega-3 fatty acids suppress monocyte adhesion to human endothelial cells: role of endothelial PAF generation

Monocyte-endothelium interaction is a fundamental process in many acute and chronic inflammatory diseases. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are fish oil-derived alternative (omega-3) precursor fatty acids implicated in the suppression of inflammatory events. We investigated their influence on rolling and adhesion of monocytes to human umbilical vein endothelial cells (HUVEC) under laminar flow conditions in vitro. Exposure of HUVEC to tumor necrosis factor (TNF-alpha) strongly increased 1) surface expression of intercellular adhesion molecule (ICAM-1), vascular cell adhesion molecule (VCAM-1), and E-selectin, 2) platelet-activating factor (PAF) synthesis as assessed by thrombin challenge, and 3) rate of rolling and adhesion of monocytes. Preincubation of HUVEC with EPA or DHA markedly suppressed PAF synthesis, monocyte rolling, and adherence, whereas expression of endothelial adhesion molecules was unchanged. Also, PAF receptor antagonists markedly suppressed the adhesion rate of monocytes, and EPA or DHA revealed no additional inhibitory capacity. In contrast, arachidonic acid partially reversed the effect of the antagonist. We conclude that omega-3 fatty acids suppress rolling and adherence of monocytes on activated endothelial cells in vitro by affecting endothelial PAF generation.     

The proteasome regulatory particle subunit Rpn6 is required for Drosophila development and interacts physically with signalosome subunit Alien/CSN2

The eukaryotic 26S proteasome plays a central role in ubiquitin-dependent intracellular protein metabolism. The multimeric holoenzyme is composed of two major subcomplexes, known as the 20S proteolytic core particle and the 19S regulatory particle (RP). The RP can be further dissected into two multisubunit complexes, the lid and the base complex. The lid complex shares striking similarities with another multiprotein complex, the COP9 signalosome. Several subunits of both complexes contain the characteristic PCI domain, a structural motif important for complex assembly. The COP9 signalosome was shown to act as a versatile regulator in numerous pathways. To help define the molecular interactions of the signalosome during Drosophila development, we performed a yeast two-hybrid screen to identify proteins that physically interact with subunit 2 of the complex, namely Alien/CSN2. Here, we report that Drosophila Rpn6, a non-ATPase subunit of the RP lid complex, interacts with Alien/CSN2 via its PCI domain. The temporal and spatial expression patterns of Rpn6 and alien/CSN2 overlap on a large scale during development providing additional evidence for their interaction in vivo. Analyses of an Rpn6 P element insertion mutant and newly generated Rpn6 alleles reveal that Rpn6 is essential for Drosophila development.