SUMMARY The restriction factor Bst2/tetherin contains two membrane anchors which are employed to retain some enveloped viruses including HIV-1 tethered to the plasma membrane in the absence of virus encoded antagonists. The 2.77 Å crystal structure of the extracellular core presented here reveals a parallel 90 Å long disulfide linked coiled-coil domain while the complete extracellular domain forms an extended 170 Å long rod-like structure based on small angle X-ray scattering data. Mutagenesis analyses indicate that both the coiled-coil and the N-terminal region are required for retention of HIV-1, suggesting that the elongated structure can function as a molecular ruler to bridge long distances. The structure reveals substantial irregularities and instabilities throughout the coiled-coil, which contribute to its low stability in the absence of disulfide bonds. We propose that the irregular coiled-coil provides conformational flexibility and ensures that Bst2/tetherin anchoring in the plasma and the newly formed virus membrane do not interfere with budding.

ALIX recruits ESCRT-III CHMP4 and is involved in membrane remodeling during endosomal receptor sorting, budding of some enveloped viruses and cytokinesis. We show that ALIX dimerizes via the middle domain (ALIX-V) in solution. Structural modeling based on small angle X-ray scattering (SAXS) data reveal an elongated crescent shaped conformation for dimeric ALIX lacking the proline rich domain (ALIXBRO1-V). Mutations at the dimerization interface prevent dimerization and induce an open elongated monomeric conformation of ALIX-V as determined by SAXS modeling. ALIX dimerizes in vivo and dimeric ALIX co-localizes with CHMP4B upon co-expression. We show further that ALIX dimerization affects HIV-1 budding. C-terminally truncated activated CHMP4B retaining the ALIX binding site forms linear, circular and helical filaments in vitro, which can be bridged by ALIX. Our data suggest that dimeric ALIX represents the active form that interacts with ESCRT-III CHMP4 polymers and functions as a scaffolding protein during membrane remodeling processes.

Background: Ebola virus VP30 is an essential transcription factor dispensable for viral replication whose activity is regulated via phosphorylation. Results: Phosphorylation of VP30 impacts viral transcription and replication by modulating interaction with the nucleocapsid proteins VP35 and NP. Conclusion: VP30 phosphorylation influences the composition of the viral polymerase complex via phosphorylation-dependent interaction with VP35. Significance: VP30 phosphorylation status modulates both viral transcription and replication.