Abstract
Lipid membrane fusion is an essential function in many biological processes. Detailed mechanisms of membrane fusion and the protein structures involved have been mainly studied in eukaryotic systems, whereas very little is known about membrane fusion in prokaryotes. Haloarchaeal pleomorphic viruses (HRPVs) have a membrane envelope decorated with spikes that are presumed to be responsible for host attachment and membrane fusion. Here we determine atomic structures of the ectodomains of the 57-kDa spike protein VP5 from two related HRPVs revealing a previously unreported V-shaped fold. By Volta phase plate cryo-electron tomography we show that VP5 is monomeric on the viral surface, and we establish the orientation of the molecules with respect to the viral membrane. We also show that the viral membrane fuses with the host cytoplasmic membrane in a process mediated by VP5. This sheds light on protein structures involved in prokaryotic membrane fusion.
Highlights
Lipid membrane fusion is an essential function in many biological processes
In order to characterize the VP4-like fusion proteins from Haloarchaeal pleomorphic viruses (HRPVs)-2 and HRPV-6, we purified them from infection-competent virions using either detergent (HRPV-2) or protease digestion (HRPV-6)
A wealth of information describing the mechanisms and the protein structures involved in eukaryotic membrane fusion exists for both cellular processes and infection by enveloped viruses[1]
Summary
Lipid membrane fusion is an essential function in many biological processes. Detailed mechanisms of membrane fusion and the protein structures involved have been mainly studied in eukaryotic systems, whereas very little is known about membrane fusion in prokaryotes. Haloarchaeal pleomorphic viruses (HRPVs) have a membrane envelope decorated with spikes that are presumed to be responsible for host attachment and membrane fusion. We show that the viral membrane fuses with the host cytoplasmic membrane in a process mediated by VP5 This sheds light on protein structures involved in prokaryotic membrane fusion. In the absence of atomic structures for the spike proteins from enveloped viruses infecting prokaryotes, it is unknown whether they contain fusion proteins that are similar to those characterized in eukaryotic viruses. Pleolipovirus VP4-like surface spike proteins are good candidates to be involved in membrane fusion[5], their structure and function remains to be established. Our structural and functional analysis of the VP4-like spike proteins and virions allows us to suggest a model of prokaryotic virus-cell membrane fusion
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