Most bacteriophages are tailed bacteriophages with an isometric or a prolate head attached to a long contractile, long non-contractile, or short non-contractile tail1. The tail is a complex machine that plays a central role in host cell recognition and attachment, cell wall and membrane penetration, and viral genome ejection. The mechanisms involved in the penetration of the inner host cell membrane by bacteriophage tails are not well understood. Here we describe structural and functional studies of the bacteriophage ϕ29 tail knob protein gene product 9 (gp9). The 2.0 Å crystal structure of gp9 shows that six gp9 molecules form a hexameric tube structure with six flexible hydrophobic loops blocking one end of the tube before DNA ejection. Sequence and structural analyses suggest that the loops in the tube could be membrane active. Further biochemical assays and electron microscopy structural analyses show that the six hydrophobic loops in the tube exit upon DNA ejection and form a channel that spans the lipid bilayer of the membrane and allows the release of the bacteriophage genomic DNA, suggesting that cell membrane penetration involves a pore-forming mechanism similar to that of certain non-enveloped eukaryotic viruses2,3,4. A search of other phage tail proteins identified similar hydrophobic loops, which indicates that a common mechanism might be used for membrane penetration by prokaryotic viruses. These findings suggest that although prokaryotic and eukaryotic viruses use apparently very different mechanisms for infection, they have evolved similar mechanisms for breaching the cell membrane.
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Electron Microscopy Data Bank
Protein Data Bank
The atomic coordinates and structure factor files have been deposited into the Protein Data Bank (PDB) under accession numbers 5FB4, 5FB5 and 5FEI. The electron microscopy maps have been deposited into the Electron Microscopy Data Bank (EMDB) under accession numbers EMD-6556, EMD-6557 and EMD-6558.
We thank L. Q. Zhang, N. Yan, H. T. Li, S. L. Fan, N. Gao, C. Z. Zhou, D. L. Anderson and M. G. Rossmann for support; the Tsinghua University Branch of the China National Center for Protein Sciences for the facility support; and the staff at the Shanghai Synchrotron Research Facility beam line BL17U for assistance with data collection. This work was supported by funds from the 973 program (2015CB910102), the National Natural Science Foundation of China (31470721 and 81550001), the Junior Thousand Talents Program of China (20131770418) and the Beijing Advanced Innovation Center for Structural Biology to Y.X.
About this article
Nature Communications (2017)