For successful infection, bacteriophages must overcome multiple barriers to transport their genome and proteins across the bacterial cell envelope. We use cryo-electron tomography to study the infection initiation of phage P22 in Salmonella enterica serovar Typhimurium, revealing how a channel forms to allow genome translocation into the cytoplasm. Our results show free phages that initially attach obliquely to the cell through interactions between the O antigen and two of the six tailspikes; the tail needle also abuts the cell surface. The virion then orients perpendicularly and the needle penetrates the outer membrane. The needle is released and the internal head protein gp7* is ejected and assembles into an extracellular channel that extends from the gp10 baseplate to the cell surface. A second protein, gp20, is ejected and assembles into a structure that extends the extracellular channel across the outer membrane into the periplasm. Insertion of the third ejected protein, gp16, into the cytoplasmic membrane probably completes the overall trans-envelope channel into the cytoplasm. Construction of a trans-envelope channel is an essential step during infection of Gram-negative bacteria by all short-tailed phages, because such virions cannot directly deliver their genome into the cell cytoplasm.
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We thank S. Casjens for providing phage strains and amber mutant sequence data. We also thank M. M. Susskind and A. R. Poteete for phage and bacterial strains. This work was supported by grant nos. GM124378 and GM110243 to I.J.M. and J.L. C.W., J.T. and J.L. were also supported in part by grant no. AI087946 from the NIAID and grant no. AU-1714 from the Welch Foundation.
The authors declare no competing interests.
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Wang, C., Tu, J., Liu, J. et al. Structural dynamics of bacteriophage P22 infection initiation revealed by cryo-electron tomography. Nat Microbiol 4, 1049–1056 (2019) doi:10.1038/s41564-019-0403-z
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