To maintain cellular structure and integrity during division, Gram-negative bacteria must carefully coordinate constriction of a tripartite cell envelope of inner membrane, peptidoglycan (PG), and outer membrane (OM). It has remained enigmatic how this is accomplished. Here, we show that envelope machines facilitating septal PG synthesis (PBP1B-LpoB complex) and OM constriction (Tol system) are physically and functionally coordinated via YbgF, renamed CpoB (Coordinator of PG synthesis and OM constriction, associated with PBP1B). CpoB localizes to the septum concurrent with PBP1B-LpoB and Tol at the onset of constriction, interacts with both complexes, and regulates PBP1B activity in response to Tol energy state. This coordination links PG synthesis with OM invagination and imparts a unique mode of bifunctional PG synthase regulation by selectively modulating PBP1B cross-linking activity. Coordination of the PBP1B and Tol machines by CpoB contributes to effective PBP1B function in vivo and maintenance of cell envelope integrity during division.DOI: http://dx.doi.org/10.7554/eLife.07118.001

Surface plasmon resonance (SPR) is one of the most powerful label‐free methods to determine the kinetic parameters of molecular interactions in real time and in a highly sensitive way. Penicillin‐binding proteins (PBPs) are peptidoglycan synthesis enzymes present in most bacteria. Established protocols to analyze interactions of PBPs by SPR involve immobilization to an ampicillin‐coated chip surface (a β‐lactam antibiotic mimicking its substrate), thereby forming a covalent complex with the PBPs transpeptidase (TP) active site. However, PBP interactions measured with a substrate‐bound TP domain potentially affect interactions near the TPase active site. Furthermore, in vivo PBPs are anchored in the inner membrane by an N‐terminal transmembrane helix, and hence immobilization at the C‐terminal TPase domain gives an orientation contrary to the in vivo situation. We designed a new procedure: immobilization of PBP by copper‐free click chemistry at an azide incorporated in the N terminus. In a proof‐of‐principle study, we immobilized Escherichia coli PBP1B on an SPR chip surface and used this for the analysis of the well‐characterized interaction of PBP1B with LpoB. The site‐specific incorporation of the azide affords control over protein orientation, thereby resulting in a homogeneous immobilization on the chip surface. This method can be used to study topology‐dependent interactions of any (membrane) protein.

To maintain cellular structure and integrity during division, Gram-negative bacteria must carefully coordinate constriction of a tripartite cell envelope of inner membrane, peptidoglycan (PG), and outer membrane (OM). It has remained enigmatic how this is accomplished. Here, we show that envelope machines facilitating septal PG synthesis (PBP1B-LpoB complex) and OM constriction (Tol system) are physically and functionally coordinated via YbgF, renamed CpoB (Coordinator of PG synthesis and OM constriction, associated with PBP1B). CpoB localizes to the septum concurrent with PBP1B-LpoB and Tol at the onset of constriction, interacts with both complexes, and regulates PBP1B activity in response to Tol energy state. This coordination links PG synthesis with OM invagination and imparts a unique mode of bifunctional PG synthase regulation by selectively modulating PBP1B crosslinking activity. Coordination of the PBP1B and Tol machines by CpoB contributes to effective PBP1B function in vivo and maintenance of cell envelope integrity during division.

The inside cover picture shows a schematic representation of the creation of a protein with a site specifically incorporated unnatural amino acid containing an azide functionality by using nonsense suppression mutagenesis (upper left). This creates the possibility to immobilize the protein on a cyclooctyne‐functionalized surface plasmon resonance (SPR) chip surface in a homogeneous way with respect to its orientation (lower). This is used to measure protein–protein interactions by SPR (upper right). More information can be found in the full paper by E. Breukink et al. on page 2250 in Issue 23, 2016 (DOI: 10.1002/cbic.201600461).