Structure of the Ulster Strain Newcastle Disease Virus Hemagglutinin-Neuraminidase Reveals Auto-Inhibitory Interactions Associated with Low Virulence

Paramyxovirus hemagglutinin-neuraminidase (HN) plays roles in viral entry and maturation, including binding to sialic acid receptors, activation of the F protein to drive membrane fusion, and enabling virion release during virus budding. HN can thereby directly influence virulence and in a subset of avirulent Newcastle disease virus (NDV) strains, such as NDV Ulster, HN must be proteolytically activated to remove a C-terminal extension not found in other NDV HN proteins. Ulster HN is 616 amino acids long and the 45 amino acid C-terminal extension present in its precursor (HN₀) form has to be cleaved to render HN biologically active. Here we show that Ulster HN contains an inter-subunit disulfide bond within the C-terminal extension at residue 596, which regulates HN activities and neuraminidase (NA) domain dimerization. We determined the crystal structure of the dimerized NA domain containing the C-terminal extension, which extends along the outside of the sialidase β-propeller domain and inserts C-terminal residues into the NA domain active site. The C-terminal extension also engages a secondary sialic acid binding site present in NDV HN proteins, which is located at the NA domain dimer interface, that most likely blocks its attachment function. These results clarify how the Ulster HN C-terminal residues lead to an auto-inhibited state of HN, the requirement for proteolytic activation of HN₀ and associated reduced virulence.     

Structural basis of specificity and cross-reactivity in T cell receptors specific for cytochrome c-I-E(k)

T cells specific for the cytochrome c Ag are widely used to investigate many aspects of TCR specificity and interactions with peptide-MHC, but structural information has long been elusive. In this study, we present structures for the well-studied 2B4 TCR, as well as a naturally occurring variant of the 5c.c7 TCR, 226, which is cross-reactive with more than half of possible substitutions at all three TCR-sensitive residues on the peptide Ag. These structures alone and in complex with peptide-MHC ligands allow us to reassess many prior mutagenesis results. In addition, the structure of 226 bound to one peptide variant, p5E, shows major changes in the CDR3 contacts compared with wild-type, yet the TCR V-region contacts with MHC are conserved. These and other data illustrate the ability of TCRs to accommodate large variations in CDR3 structure and peptide contacts within the constraints of highly conserved TCR-MHC interactions.     

The fish community of an East African mangrove creek

The results of a beach seine survey of an East African mangrove creek are presented. The fish community of the creek is described and is compared with that of a nearby lagoonal site and with those described for other mangrove and estuarine systems. The species composition was found to differ substantially between the creek and the lagoon site, though diversity indices for the two areas were similar. Eighty-three species of teleost fish werecollected from the mangrove area of the creek. This number is considered high in relation to comparable studies and is attributed to the constant high salinity (approximately 35%o) measured throughout the study period. Approximately 90% of the fish caught were juveniles. Plankton sampling was also carried out and representatives of 21 fish families were collected as larvae within the creek. Catches from both beach seining and plankton sampling in the mangrove areas were dominated numerically by resident clupeid and gobiid species. The majority of species, however, were considered to have a widespread distribution as adults. A discrepancy between the catch composition of larvae and juveniles suggests that species that use the creek as a nursery area enter the system principally at a post-larval/ juvenile stage of development. No systematic spatial or temporal variation in the community structure was identified over the study period.     

Molecular evolution of mitochondrial 12S RNA and cytochrome b sequences in the pantherine lineage of Felidae

DNA sequence comparisons of two mitochondrial DNA genes were used to infer phylogenetic relationships among 17 Felidae species, notably 15 in the previously described pantherine lineage. The polymerase chain reaction (PCR) was used to generate sequences of 358 base pairs of the mitochondrial 12S RNA gene and 289 base pairs of the cytochrome b protein coding gene. DNA sequences were compared within and between 17 felid and five nonfelid carnivore species. Evolutionary trees were constructed using phenetic, cladistic, and maximum likelihood algorithms. The combined results suggested several phylogenetic relationships including (1) the recognition of a recently evolved monophyletic genus Panthera consisting of Panthera leo, P. pardus, P. onca, P. uncia, P. tigris, and Neofelis nebulosa; (2) the recent common ancestry of Acinonyx jubatus, the African cheetah, and Puma concolor, the American puma; and (3) two golden cat species, Profelis temmincki and Profelis aurata, are not sister species, and the latter is strongly associated with Caracal caracal. These data add to the growing database of vertebrate mtDNA sequences and, given the relatively recent divergence among the felids represented here (1-10 Myr), allow 12S and cytochrome b sequence evolution to be addressed over a time scale different from those addressed in most work on vertebrate mtDNA.      

Regulation of cellular adhesion molecule expression in murine oocytes, peri-implant ation and post-implantation embryos

Expression of the adhesion molecules, ICAM-1, VCAM-1, NCAM, CD44, CD49d (VLA-4, a chain), and CDlla (LFA-1, a chain) on mouse oocytes, and pre- and peri-implantation stage embryos was examined by quantitative indirect immunofluorescence microscopy. ICAM-1 was most strongly expressed at the oocyte stage, gradually declining almost to undetectable levels by the expanded blastocyst stage. NCAM, also expressed maximally on the oocyte, declined to undetectable levels beyond the morula stage. On the other hand, CD44 declined from highest expression at the oocyte stage to show a second maximum at the compacted 8-cell/morula. This molecule exhibited high expression around contact areas between trophecto-derm and zona pellucida during blastocyst hatching. CD49d was highly expressed in the oocyte, remained significantly expressed throughout and after blastocyst hatching was expressed on the polar trophecto-derm. Like CD44, CD49d declined to undetectable levels at the blastocyst outgrowth stage. Expression of both