Pal stabilises the bacterial outer membrane during constriction by a mobilisation-and-capture mechanism

Abstract

25Coordination of outer membrane constriction with septation is critical to faithful division 26 in Gram-negative bacteria and vital to the barrier function of the membrane. Recent 27 studies suggest this coordination is through the active accumulation of the 28 peptidoglycan-binding outer membrane lipoprotein Pal at division sites by the Tol 29 system, but the mechanism is unknown. Here, we show that Pal accumulation at 30Escherichia coli division sites is a consequence of three key functions of the Tol system. 31First, Tol mobilises Pal molecules in dividing cells, which otherwise diffuse very slowly 32 due to their binding of the cell wall. Second, Tol actively captures mobilised Pal 33 molecules and deposits them at the division septum. Third, the active capture 34 mechanism is analogous to that used by the inner membrane protein TonB to dislodge 35 the plug domains of outer membrane TonB-dependent nutrient transporters. We 36 conclude that outer membrane constriction is coordinated with cell division by active 37 mobilisation-and-capture of Pal at division septa by the Tol system. 38 39 Word count, 160 40 41 42 43 44 45 46 47 across the inner membrane 7-9 . Recently, Petiti et al have suggested that the 60multiprotein Tol system (also known as Tol-Pal) constricts the OM by populating the 61 division septum with Pal 10 . However, no mechanism has been proposed. In the present 62 work, through a combination of in vivo imaging, deletion analysis and mutagenesis, 63 structure determination, biophysical measurements, mathematical modelling and 64 molecular dynamics simulations, we demonstrate that the PMF is exploited by the Tol 65 system to both mobilise Pal in the OM of dividing cells and to then capture these 66 mobilised molecules at division sites. Mobilisation-and-capture circumvents Pal's 67 intrinsically low mobility in the OM and results in its accumulation at division sites, where 68 it invaginates the OM through non-covalent interactions with newly-formed septal 69 peptidoglycan. 70 4 tol genes, which are found in most Gram-negative bacteria, were originally 71 identified by Luria and co-workers in the 1960s through mutations that engendered 72Escherichia coli tolerance towards colicins and filamentous bacteriophages 11 . 73Concomittant with this tolerance is a pleiotropic OM instability phenotype that is manifest 74 through cell filamentation and division defects, hypersensitivity towards detergents and 75 bile salts and leakage of periplasmic contents. The Tol assembly is essential in bacteria 76expressing O-antigens, is a virulence factor in host-pathogen interactions 12-14 and is 77 implicated in biofilm formation 15 . The core components of Tol are three IM proteins, 78TolQ, TolR and TolA, periplasmic TolB and peptidoglycan associated lipoprotein Pal in 79 the inner leaflet of the OM (Figure 1a) 3 . TolA in the inner membrane spans the 80 periplasm and undergoes PMF-driven conformational changes by virtue of its interaction 81 partners TolQ and TolR, which are homologues of the MotA and MotB stato...