Structural Insights into Protein-Protein Interactions Involved in Bacterial Cell Wall Biogenesis

Abstract

The bacterial cell wall is essential for survival, and proteins that participate in its biosynthesis have been the targets of antibiotic development efforts for decades. The biosynthesis of its main component, the peptidoglycan, involves the coordinated action of proteins that are involved in multi-member complexes which are essential for cell division (the "divisome") and/or cell wall elongation (the "elongasome"), in the case of rod-shaped cells. Our knowledge regarding these interactions has greatly benefitted from the visualization of different aspects of the bacterial cell wall and its cytoskeleton by cryoelectron microscopy and tomography, as well as genetic and biochemical screens that have complemented information from high resolution crystal structures of protein complexes involved in divisome or elongasome formation. This review summarizes structural and functional aspects of protein complexes involved in the cytoplasmic and membrane-related steps of peptidoglycan biosynthesis, with a particular focus on protein-protein interactions whereby disruption could lead to the development of novel antibacterial strategies.

Keywords: MraY; Mur enzymes; bacterial cytoskeleton; cell division; elongation; peptidoglycan; protein complexes.

Figure 1

A simplified view of cytoplasmic and membrane-related steps of PG biosynthesis. The concerted action of MurA and MurB generates the initial precursor, UDP-N-acetyl muramic acid (UDP-NAM). Mur ligases (C–F) catalyze the stepwise addition of a pentapeptide to UDP-N-acetyl muramic acid (UDP-NAM). MraY anchors the UDP-NAM-pentapeptide unit to the inner membrane through an undecaprenyl phosphate carrier lipid, forming lipid I. MurG participates in the formation of the final peptidoglycan building block (Lipid II), which is then flipped to the periplasm by flippases. C: cytoplasm; IM: inner membrane; PG: peptidoglycan layer. Cytoskeletal elements are not shown for simplicity. PDB codes of molecules depicted here: MurA (1NAW); MurB (1MBT); MurC (1J6U); MurD (4BUC); MurE (4BUB); MurF (3ZL8); MurG (1F0K); MraY (4J72).

Figure 2

The crystal structure of MraY from Aquifex aeolicus (PDB 4J72) reveals (A) a dimer displaying 10 TM helices per monomer, whose N- and C-termini face the periplasm. (B) A view from the cytoplasmic side indicates a tunnel formed in the monomer-monomer interaction region, buttressed by cytoplasmic loops (in red) that could interact with substrate, partner proteins, or both.

Figure 3

Schematic representation of a section of the dcw (division and cell wall) cluster in different bacterial genomes. Adjacent arrows represent contiguous genes involved in cell wall synthesis and division. In a number of species, adjacent genes are fused, such as in numerous strains of B. pertussis (murE/murF), and in the actinomycetes K. flavida (ftsW/murG) and C. gilvus (murG/murC).

Figure 4

Protein interactions involving cytoskeletal proteins that play key roles in cytoplasmic and membrane-embedded PG biosynthesis steps. (A) MreB:RodZ from T. maritima (2WUS), where MreB’s subdomains IA and IIA are shown in blue and green, respectively; (B) FtsA:FtsZ (res 338–351) from T. maritima (4A2A); (C) ZipA:FtsZ (res 367–383) from E. coli (1F47); (D) SulA:FtsZ from P. aeruginosa (1OFU); (E) MciZ:FtsZ from B. subtilis (4U39), where MciZ‘s β-hairpin completes FtsZ’s 4-stranded sheet ; (F) MinC:MinD from A. aeolicus (4V02).