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

Abstract

Coordination of outer membrane constriction with septation is critical to faithful division in Gram-negative bacteria and vital to the barrier function of the membrane. This coordination requires the recruitment of the peptidoglycan-binding outer-membrane lipoprotein Pal at division sites by the Tol system. Here, we show that Pal accumulation at Escherichia coli division sites is a consequence of three key functions of the Tol system. First, Tol mobilises Pal molecules in dividing cells, which otherwise diffuse very slowly due to their binding of the cell wall. Second, Tol actively captures mobilised Pal molecules and deposits them at the division septum. Third, the active capture mechanism is analogous to that used by the inner membrane protein TonB to dislodge the plug domains of outer membrane TonB-dependent nutrient transporters. We conclude that outer membrane constriction is coordinated with cell division by active mobilisation-and-capture of Pal at division septa by the Tol system.

The multifarious roles of Tol-Pal in Gram-negative bacteria

“…The recent solution state structure for Pseudomonas aeruginosa TolA III bound to a TolB peptide shows a β-strand augmentation binding mechanism; the C-terminal helix of TolA III is displaced by the N-terminal residues of TolB, which form a parallel β-strand (Szczepaniak et al . 2020 ) (Fig. 1D ).…”

“…TolB binds through a β-strand augmentation mechanism in which the C-terminal α-helix (α4) of TolA is displaced by the N-terminus of TolB (Glu22-Ser33, in orange) (Szczepaniak et al . 2020 ). E , Crystal structure of E. coli TolB (PDB code: 1CRZ).…”

“… 2010 ) and TolQ-TolR energises TolA to dissociate TolB-Pal complexes at the outer membrane (Szczepaniak et al . 2020 ). These simple comparisons emphasise three important points.…”

“… 2009 ) and abolishes accumulation of Pal at division sites (Szczepaniak et al . 2020 ). Biophysical studies have shown that the TolA III -TolB complex has a low affinity (K d ∼40–200 μM, depending on the species), a consequence of the structural rearrangements in TolA III .…”

“…We suggest that TolA through its coupling with the PMF-linked TolQ-TolR stator complex adopts a similar role, but one in which TolB is dislodged from its complex with Pal at the outer membrane via a TolA-TolB-Pal ternary complex (Szczepaniak et al . 2020 ). The biological rationale for energised dissociation of the TolB-Pal complex is explored below.…”