RESEARCH HIGHLIGHTS
MACMILLAN
Nature Reviews Microbiology | AOP, published online 21 July 2014; doi:10.1038/nrmicro3328
B AC T E R I A L P H Y S I O LO GY
Flipping out over MurJ
MurJ is essential for the translocation of lipid II
Assembly of the peptidoglycan sacculus requires the precursor lipid II to be translocated or ‘flipped’ across the cytoplasmic membrane into the periplasm for subsequent insertion into the growing sacculus. The identity of the flippase has long been debated, but Sham et al. now show that the inner-membrane protein MurJ mediates this event in Escherichia coli. To investigate flippase activity in vivo, the authors developed a sensitive assay in which the toxin colicin M (ColM) was used to specifically cleave flipped lipid II in the periplasm, which generates a soluble product (PP-Mpep5-G) that is detectable by high-performance liquid chromo tography. To determine whether MurJ functions as the flippase, the authors examined a MurJ-inactivated mutant and found that, compared with wild-type bacteria, the mutant produced only negligible amounts of PP-Mpep5-G, which would be insufficient to support bacterial survival. This result suggests that inactivation of MurJ protects lipid II from ColM cleavage either by blocking lipid II translocation or by interfering with ColM import and/or activity. The
authors showed that removal of the outer membrane, which provides ColM with direct access to flipped lipid II in the periplasm, failed to restore PP-Mpep5-G production in the MurJ mutant, which confirms that MurJ is essential for the translocation of lipid II. An in vitro study previously proposed that another innermembrane protein, FtsW, functions as the flippase in E. coli. However, Sham et al. found that lipid II translocation was relatively unperturbed in a mutant lacking FtsW, which suggests that FtsW is not essential for flippase activity in vivo or has only a minor contribution. This study provides compelling in vivo evidence that MurJ functions as the lipid II flippase, so it will be interesting to see whether these findings will put an end to the flippase controversy. Christina Tobin Kåhrström ORIGINAL RESEARCH PAPER Sham, L.-T. et al. MurJ is the flippase of lipid-linked precursors for peptidoglycan biogenesis. Science 345, 220–222 (2014) FURTHER READING Typas, A. et al. From the regulation of peptidoglycan synthesis to bacterial growth and morphology. Nature Rev. Microbiol. 10, 123–136 (2012)
NATURE REVIEWS | MICROBIOLOGY
VOLUME 12 | SEPTEMBER 2014 © 2014 Macmillan Publishers Limited. All rights reserved
Copyright of Nature Reviews Microbiology is the property of Nature Publishing Group and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.
MACMILLAN
Nature Reviews Microbiology | AOP, published online 21 July 2014; doi:10.1038/nrmicro3328
B AC T E R I A L P H Y S I O LO GY
Flipping out over MurJ
MurJ is essential for the translocation of lipid II
Assembly of the peptidoglycan sacculus requires the precursor lipid II to be translocated or ‘flipped’ across the cytoplasmic membrane into the periplasm for subsequent insertion into the growing sacculus. The identity of the flippase has long been debated, but Sham et al. now show that the inner-membrane protein MurJ mediates this event in Escherichia coli. To investigate flippase activity in vivo, the authors developed a sensitive assay in which the toxin colicin M (ColM) was used to specifically cleave flipped lipid II in the periplasm, which generates a soluble product (PP-Mpep5-G) that is detectable by high-performance liquid chromo tography. To determine whether MurJ functions as the flippase, the authors examined a MurJ-inactivated mutant and found that, compared with wild-type bacteria, the mutant produced only negligible amounts of PP-Mpep5-G, which would be insufficient to support bacterial survival. This result suggests that inactivation of MurJ protects lipid II from ColM cleavage either by blocking lipid II translocation or by interfering with ColM import and/or activity. The
authors showed that removal of the outer membrane, which provides ColM with direct access to flipped lipid II in the periplasm, failed to restore PP-Mpep5-G production in the MurJ mutant, which confirms that MurJ is essential for the translocation of lipid II. An in vitro study previously proposed that another innermembrane protein, FtsW, functions as the flippase in E. coli. However, Sham et al. found that lipid II translocation was relatively unperturbed in a mutant lacking FtsW, which suggests that FtsW is not essential for flippase activity in vivo or has only a minor contribution. This study provides compelling in vivo evidence that MurJ functions as the lipid II flippase, so it will be interesting to see whether these findings will put an end to the flippase controversy. Christina Tobin Kåhrström ORIGINAL RESEARCH PAPER Sham, L.-T. et al. MurJ is the flippase of lipid-linked precursors for peptidoglycan biogenesis. Science 345, 220–222 (2014) FURTHER READING Typas, A. et al. From the regulation of peptidoglycan synthesis to bacterial growth and morphology. Nature Rev. Microbiol. 10, 123–136 (2012)
NATURE REVIEWS | MICROBIOLOGY
VOLUME 12 | SEPTEMBER 2014 © 2014 Macmillan Publishers Limited. All rights reserved
Copyright of Nature Reviews Microbiology is the property of Nature Publishing Group and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.
