Membrane structure and interactions with protein and DNA in bacteriophage PRD1

Membranes are essential for selectively controlling the passage of molecules in and out of cells and mediating the response of cells to their environment. Biological membranes and their associated proteins present considerable difficulties for structural analysis. Although enveloped viruses have been imaged at about 9 A resolution by cryo-electron microscopy and image reconstruction, no detailed crystallographic structure of a membrane system has been described. The structure of the bacteriophage PRD1 particle, determined by X-ray crystallography at about 4 A resolution, allows the first detailed analysis of a membrane-containing virus. The architecture of the viral capsid and its implications for virus assembly are presented in the accompanying paper. Here we show that the electron density also reveals the icosahedral lipid bilayer, beneath the protein capsid, enveloping the viral DNA. The viral membrane contains about 26,000 lipid molecules asymmetrically distributed between the membrane leaflets. The inner leaflet is composed predominantly of zwitterionic phosphatidylethanolamine molecules, facilitating a very close interaction with the viral DNA, which we estimate to be packaged to a pressure of about 45 atm, factors that are likely to be important during membrane-mediated DNA translocation into the host cell. In contrast, the outer leaflet is enriched in phosphatidylglycerol and cardiolipin, which show a marked lateral segregation within the icosahedral asymmetric unit. In addition, the lipid headgroups show a surprising degree of order.     

Active genes are tri-methylated at K4 of histone H3

Lysine methylation of histones in vivo occurs in three states: mono-, di- and tri-methyl. Histone H3 has been found to be di-methylated at lysine 4 (K4) in active euchromatic regions but not in silent heterochromatic sites. Here we show that the Saccharomyces cerevisiae Set1 protein can catalyse di- and tri-methylation of K4 and stimulate the activity of many genes. Using antibodies that discriminate between the di- and tri-methylated state of K4 we show that di-methylation occurs at both inactive and active euchromatic genes, whereas tri-methylation is present exclusively at active genes. It is therefore the presence of a tri-methylated K4 that defines an active state of gene expression. These findings establish the concept of methyl status as a determinant for gene activity and thus extend considerably the complexity of histone modifications.     

Magnification of light from many distant quasars by gravitational lenses

Exceptionally bright quasars with redshifts up to z = 6.28 have recently been discovered. Quasars are thought to be powered by the accretion of gas onto supermassive black holes at the centres of galaxies. Their maximum (Eddington) luminosity depends on the mass of the black hole, and the brighter quasars are inferred to have black holes with masses of more than a few billion solar masses. The existence of such massive black holes poses a challenge to models for the formation of structures in the early Universe, as it requires their formation within one billion years of the Big Bang. Here we show that up to one-third of known quasars with z approximately equal to 6 will have had their observed flux magnified by a factor of ten or more, as a consequence of gravitational lensing by galaxies along the line of sight. The inferred abundance of quasar host galaxies, as well as the luminosity density provided by the quasars, has therefore been substantially overestimated.     

Excitatory glycine receptors containing the NR3 family of NMDA receptor subunits

The N-methyl-D-aspartate subtype of glutamate receptor (NMDAR) serves critical functions in physiological and pathological processes in the central nervous system, including neuronal development, plasticity and neurodegeneration. Conventional heteromeric NMDARs composed of NR1 and NR2A-D subunits require dual agonists, glutamate and glycine, for activation. They are also highly permeable to Ca2+, and exhibit voltage-dependent inhibition by Mg2+. Coexpression of NR3A with NR1 and NR2 subunits modulates NMDAR activity. Here we report the cloning and characterization of the final member of the NMDAR family, NR3B, which shares high sequence homology with NR3A. From in situ and immunocytochemical analyses, NR3B is expressed predominantly in motor neurons, whereas NR3A is more widely distributed. Remarkably, when co-expressed in Xenopus oocytes, NR3A or NR3B co-assembles with NR1 to form excitatory glycine receptors that are unaffected by glutamate or NMDA, and inhibited by D-serine, a co-activator of conventional NMDARs. Moreover, NR1/NR3A or -3B receptors form relatively Ca2+-impermeable cation channels that are resistant to Mg2+, MK-801, memantine and competitive antagonists. In cerebrocortical neurons containing NR3 family members, glycine triggers a burst of firing, and membrane patches manifest glycine-responsive single channels that are suppressible by D-serine. By itself, glycine is normally thought of as an inhibitory neurotransmitter. In contrast, these NR1/NR3A or -3B 'NMDARs' constitute a type of excitatory glycine receptor.