2.A.66.4.3
Peptidoglycan biosynthesis protein MurJ (Ruiz 2008). A 3-d structural model showed a solvent-exposed cavity within the plane of the membrane (Butler et al. 2013). MurJ has 14 TMSs, and specific charged residues localized in the central cavity are essential for function. This structural homology model suggests that MurJ functions as an essential transporter in PG biosynthesis (Butler et al. 2013). Based on an in vivo assay, MurJ is a flippase for the lipid-linked cell wall precursors, polyisoprenoid-linked disaccharide-peptapeptides (Sham et al. 2014).  There is controversy about the role of this porter and FtsW/RodA which on the basis of an in vitro assay, were thought to be flippases for the same intermediate (Young 2014). MurJ, the bacterial lipid II flippase, functions by an alternating-access mechanism (Kumar et al. 2019). The crystal structure of MurJ in a "squeezed" form, distinct from its inward- and outward-facing forms, has been published (Kohga et al. 2022). These authors reported two crystal structures of inward-facing forms from Arsenophonus endosymbiont MurJ and a crystal structure of E. coli MurJ in a "squeezed" form, which lacks a cavity to accommodate the substrate, mainly because of the increased proximity of transmembrane helices 2 and 8. Molecular dynamics simulations support the hypothesis that the squeezed form is an intermediate conformation (Kohga et al. 2022).