The gram-negative bacterial periplasm: Size matters

Abstract

Gram-negative bacteria are surrounded by two membrane bilayers separated by a space termed the periplasm. The periplasm is a multipurpose compartment separate from the cytoplasm whose distinct reducing environment allows more efficient and diverse mechanisms of protein oxidation, folding, and quality control. The periplasm also contains structural elements and important environmental sensing modules, and it allows complex nanomachines to span the cell envelope. Recent work indicates that the size or intermembrane distance of the periplasm is controlled by periplasmic lipoproteins that anchor the outer membrane to the periplasmic peptidoglycan polymer. This periplasm intermembrane distance is critical for sensing outer membrane damage and dictates length of the flagellar periplasmic rotor, which controls motility. These exciting results resolve longstanding debates about whether the periplasmic distance has a biological function and raise the possibility that the mechanisms for maintenance of periplasmic size could be exploited for antibiotic development.

Fig 1. Architecture of the gram-negative bacterial cell envelope.

Shown is the asymmetric bilayer of lipopolysaccharide and glycerolphospholipids that comprise the outer membrane. The inner membrane is a symmetric bilayer of glycerolphospholipids. The periplasmic space is the region between these membranes that includes a variety of enzymes and functions, including the oxidation and quality control of proteins. Also within the periplasmic space is a layer of crosslinked sugars and amino acids termed peptidoglycan, which surrounds the cell. The peptidoglycan is linked to the outer membrane in enteric bacteria through covalent transpeptidase linkages between an abundant outer membrane lipoprotein Lpp. A variety of sensors sit in the inner membrane with periplasmic domains sensing environmental change and, in the case of the Rcs system, a change in location of the RcsF outer membrane lipoprotein. Multicomponent protein complexes such as the flagellar machine span the two membranes. IM, inner membrane; Lpp, Braun’s lipoprotein; LPS, lipopolysaccharide; RcsF, Regulator of capsule synthesis F.

Fig 2. RcsF signaling is altered by a change in size of the periplasmic space.

The RcsF outer membrane lipoprotein sensor must contact its inner membrane signaling partners to activate sensing. This sensing requires a specific periplasmic distance because lengthening of the Lpp linkages to peptidoglycan increases the distance of the periplasm, and unless RcsF is lengthened, signaling can no longer occur. In panel A: the state in which RcsF is not activating signaling because no envelope disorder is ongoing. In panel B: envelope disorder leads to RcsF physical interactions with the inner membrane-sensing system, and the Rcs regulon is activated. In panel C, in which Lpp has been lengthened and the periplasmic intermembrane distance lengthened, the Rcs regulon cannot be activated despite envelope disorder. In panel D: the defect of the long Lpp is corrected by lengthening RcsF. IM, inner membrane; Lpp, Braun’s lipoprotein; OM, outer membrane; PG, peptidoglycan; RcsF, Regulator of capsule synthesis F.