Ultrastructure of a periodic protein layer in the outer membrane of Escherichia coli.

Abstract

Matrix protein (36,500 daltons), one of the major polypeptides of the Escherichia coli cell envelope, is arranged in a periodic monolayer which covers the outer surface of the peptidoglycan. Although its association with the peptidoglycan layer is probably tight, the periodic structure is maintained in the absence of peptidoglycan, and is therefore based on strong protein-protein interactions. A detailed analysis of the ultrastructure of the matrix protein array by electron microscopy and image processing of specimens prepared by negative staining or by freezedrying and shadowing shows that the molecules are arranged according to threefold symmetry on a hexagonal lattice whose repeat is 7.7 nm. The most pronounced feature of the unit cell, which probably contains three molecules of matrix protein, is a triplet of indentations, each approx. 2 nm in diameter, with a center-to-center spacing of 3 nm. They are readily penetrated by stain and may represent channels which span the protein monolayer.The envelopes of Gram-negative bacteria have a multilaminar organization consisting of a cytoplasmic membrane, a rigid peptidoglycan layer, and an outer membrane. The latter is composed of phospholipids, lipopolysaccharides, and proteins. These include the matrix protein 1 (16) which, in Escherichia coli, has a mol wt of 36,500. The cellular complement of approx. 105 molecules forms a periodic array in close association with the underlying peptidoglycan layer. Numerically, the most abundant outer membrane protein is a lipoprotein (7,000 daltons) which has been extensively characterized (4). Each cell has approx. 6 • 105 copies, of which a third (11) are covalently linked to the peptide cross-bridges of the peptidoglycan layer (4). In addition, the outer membrane contains several other major (9) as well as a number of minor protein species, which have not yet 1 This protein has been designated by various names (cf. reference 7). been so well characterized and will not be considered further here.The structural organization of the outer membrane and the interactions between its components are not yet well understood. Likewise, the functions of its major polypeptides are still unclear. Motivated by the idea that information on the architecture of the periodic arrangement of matrix protein which effectively covers the peptidoglycan cell wall might afford insight into these questions, we have undertaken a study of it by electron microscopy combined with image processing. Our findings are reported in this communication.

Direct visualization of receptor arrays in frozen‐hydrated sections and plunge‐frozen specimens of E. coli engineered to overproduce the chemotaxis receptor Tsr

“…3e,f). Interestingly, this value matches the 7.7‐nm spacing previously reported for the outer membrane porin protein layer using rotary shadowing studies (Steven et al ., 1977). We cannot determine at present whether this is merely a coincidence, or whether it implies some type of co‐registration between the peptidoglycan layer and the outer membrane layer.…”

Functional reassembly of membrane proteins in planar lipid bilayers

“…It exhibits a 3-fold symmetry and triplet indentations penetrated by stain. These indentations were tentatively interpreted as membrane channels (Steven et al 1977). Evidence that matrix protein forms membranespanning pores was inferred from the higher permeability of wild type Salmonella typhimurium to the antibiotic cephaloridine compared to a mutant deficient in porin (Nikaido et al 1977).…”

Inhibition of bacteria by photocatalytic nano-TiO2 particles in the absence of light

“…Nikaido et al (1980) explained that diffusion channels in the outer membrane of intact cells, namely E. coli, had a diameter between 0.6 and 1.2 nm. In an early study, Steven et al (1977) showed that an E. coli porin revealed indentations with a diameter of 1.5-2 nm. When the NPs and bacteria are in contact, the NPs may plug the pores of the diffusion channels and prevent nutrition uptake.…”