The bacteriophage [phi]29 tail possesses a pore-forming loop for cell membrane penetration

Structural and functional studies of the tail knob protein of bacteriophage [phi]29 shed light on how the phage breaches the membrane barrier and ejects its DNA genome into the host cell. How the [phi]29 phage crosses the cell membrane Bacteriophages have to overcome several physical barriers in order to introduce their genome into the cytoplasm of the host cell. How they get across the inner bacterial membrane has been unclear. Here Ye Xiang and colleagues describe structural and functional studies of the bacteriophage [phi]29 tail knob protein. The structure reveals that six gp9 molecules form a hexameric tube structure with six flexible hydrophobic loops that form a channel spanning the lipid bilayer of the membrane for the release of bacteriophage genomic DNA. This points to a pore-forming mechanism for membrane penetration similar to that of certain non-enveloped eukaryotic viruses. Most bacteriophages are tailed bacteriophages with an isometric or a prolate head attached to a long contractile, long non-contractile, or short non-contractile tail.sup.1. 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 [phi]29 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 viruses.sup.2,3,4. 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.
Author(s): Jingwei Xu [sup.1] , Miao Gui [sup.1] , Dianhong Wang [sup.1] , Ye Xiang [sup.1] Author Affiliations: (1) Department of Basic Medical Sciences, Centre for Infectious Diseases Research, Collaborative [...]