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In many bacterial viruses and in certain animal viruses, the double-stranded DNA genome enters and exits the capsid through a portal gatekeeper. We report a pseudoatomic structure of a complete portal system. The bacteriophage SPP1 gatekeeper is composed of dodecamers of the portal protein gp6, the adaptor gp15, and the stopper gp16. The solution structures of gp15 and gp16 were determined by NMR. They were then docked together with the X-ray structure of gp6 into the electron density of the ≈1-MDa SPP1 portal complex purified from DNA-filled capsids. The resulting structure reveals that gatekeeper assembly is accompanied by a large rearrangement of the gp15 structure and by folding of a flexible loop of gp16 to form an intersubunit parallel 13-sheet that closes the portal channel. This stopper system prevents release of packaged DNA. Disulfide cross-linking between 13-strands of the stopper blocks the key conformational changes that control genome ejection from the virus at the beginning of host infection. [ABSTRACT FROM AUTHOR]

NMR spectra of ubiquitin in the presence of bicelles at a concentration of 32% w/v have been recorded at 700 MHz under sample spinning conditions at the magic angle (54.7°) and at an angle of 45.5°. At the magic angle, the 1H–15N HSQC spectrum of ubiquitin in bicelles is virtually indistinguishable from the one recorded on the protein in solution. Spinning the sample at the magic angle creates an isotropic environment with no preferred bicelle orientations, thus allowing the determination of scalar coupling constants. For an angle of rotation of 45.5°, the bicelles orient with their normal perpendicular to the spinning axis leading to the observation of strong residual dipolar couplings and chemical shift variations of the 15N resonances. [ABSTRACT FROM AUTHOR]