An ancestral dual function of OmpM as outer membrane tether and nutrient uptake channel in diderm Firmicutes
The outer membrane (OM) in diderm, or Gram-negative, bacteria must be tethered to peptidoglycan (PG) for mechanical stability and to maintain cell morphology. Most diderm phyla from the Terrabacteria group lack well-characterised OM attachment systems such as Brauns lipoprotein, Pal and OmpA, but instead have the OmpM protein. OmpM has a periplasmic region containing an S-layer homology domain and a {beta}-barrel region that is embedded in the OM. Here, we have determined the structure of the most abundant OmpM protein from the diderm firmicute Veillonella parvula by single particle cryogenic electron microscopy (cryo-EM). We reveal an OM portion similar to well-characterised trimeric general porins and a mobile periplasmic PG-binding region. Combining the cryo-EM data with crystal structures, structure predictions and molecular dynamics simulations, we show that the periplasmic region of OmpM likely adopts multiple conformations. Single-channel electrophysiology demonstrates ion conductance properties similar to E. coli OmpF. Finally, we demonstrate via functional assays that the four OmpM paralogues are likely to be the only general porins in V. parvula. Together, our results show that OM tethering and nutrient acquisition are genetically linked in V. parvula (and other diderm Firmicutes), suggesting that concurrent loss of two key OM-related functions, tethering and nutrient uptake, promoted the multiple OM loss events that have been inferred in the Terrabacteria.