LipidII: Just Another Brick in the Wall?

Abstract

Nearly all bacteria contain a peptidoglycan cell wall. The peptidoglycan precursor molecule is LipidII, containing the basic peptidoglycan building block attached to a lipid. Although the suitability of LipidII as an antibacterial target has long been recognized, progress on elucidating the role(s) of LipidII in bacterial cell biology has been slow. The focus of this review is on exciting new developments, both with respect to antibacterials targeting LipidII as well as the emerging role of LipidII in organizing the membrane and cell wall synthesis. It appears that on both sides of the membrane, LipidII plays crucial roles in organizing cytoskeletal proteins and peptidoglycan synthesis machineries. Finally, the recent discovery of no less than three different categories of LipidII flippases will be discussed.

Fig 1. Organization of cell wall synthesis by LipidII.

Overview of recent work that highlights various new insights about the role of LipidII; for example, (1) in the identification of novel antibacterials that target LipidII (including teixobactin and bacteriocins), (2) how LipidII is translocated over the membrane by different families of flippases (such as FtsW or RodA, MurJ, and Amj), (3) how it is recruited to regions of increased fluidity (RIFs) and how it organizes attachment of MreB(-like) filaments, and (4) how cell wall synthesis enzymes (penicillin-binding proteins [PBPs]) are recruited to LipidII.

Fig 2. LipidII regulates membrane association of MreB.

Using Total Internal Reflection Fluorescence (TIRF) microscopy, association and dissociation of MreB-GFP with the membrane can be followed. Upon depletion of MurG and subsequent halt of conversion of LipidI to LipidII, MreB-GFP is released from the membrane (upper row); after induction of MurG expression, LipidII production is resumed and MreB-GFP is re-localized on the membrane (lower row). Shown are snapshots (A, B, D, E) of single TIRF images at the respective time points and an analysis of the variance in intensity over time (C, F), with red indicating regions of high protein mobility and blue denoting low mobility. (Adapted with permission from Macmillan Publishers Ltd.: Nature Chemical Biology; K. Schirner et al., Nat Chem Biol 11, 38–45 [2015], Macmillan Publishers Ltd. 2015.)

Fig 3. MreB is required for the generation of regions of increased fluidity (RIFs).

GFP-MreB (green; panel B, C) co-localizes with regions of increased fluidity (RIFs, stained with the lipid-dye DiI-C12, red; panel A, C) in a ΔMreBCD strain of B. subtilis (cells look round because of the resulting shape defect). (Adapted with permission from H. Strahl, F. Burmann, L. W. Hamoen, The actin homologue MreB organizes the bacterial cell membrane. Nat Commun 5, 3442 [2014].)

Fig 4. Expression of MreB in the non-MreB–containing bacterium S. aureus leads to accumulation of MreB and peptidoglycan.

Patches of MreB are formed (red, panel C), leading to aberrant production of peptidoglycan (asterisks, panel B). Wild type cells are shown for comparison (panel A). (Amended with permission from American Society for Microbiology from A. Yepes et al., 2014, Appl Environ Microbiol 80, 3868–3878, DOI: 10.1128/AEM.00759-14.)

Fig 5. Clustering of LipidII in nonphysiological domains leads to recruitment of elongation-specific PBPs.

Under normal circumstances, LipidII (stained with fluorescent vancomycin [Van-FL], green) and RFP-PBP2A (red) co-localize at the septum (yellow in control, panel A) and the lateral wall. When LipidII is clustered into nonphysiological domains with PP-nisin, RFP-PBP2A follows LipidII in 94% of the cases when cells exhibit both LipidII and PBP2A spots (panel B, arrows, strong co-localization; arrowheads, co-localization but weak Van-FL signal). (Adapted from The localization of key Bacillus subtilis penicillin binding proteins during cell growth is determined by substrate availability, Lages MC, Beilharz K, Morales Angeles D, Veening JW, Scheffers DJ, Environmental Microbiology 15, 3272–3281 [2013], John Wiley & Sons, Inc. http://onlinelibrary.wiley.com/doi/10.1111/1462-2920.12206/abstract.)