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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 of the peptidoglycan. Although its association with the peptidoglycan later 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 freeze-drying and shadowing shows that the molecules are arranged according to three fold 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 3nm. They are readily penetrated by stain and may represent channels which span the protein monolayer.
Insights
The major matrix protein of Escherichia coli forms a hexagonal lattice on the cell surface, maintaining its structure independently of peptidoglycan. This protein array features indentations, potentially serving as channels through the monolayer.
Area of Science:
- Microbiology
- Structural Biology
- Biophysics
Background:
- The Escherichia coli cell envelope contains a major matrix protein, a polypeptide of 36,500 daltons.
- This protein forms a periodic monolayer on the outer surface of the peptidoglycan layer.
- Its association with peptidoglycan is strong, yet the protein's periodic structure persists even without peptidoglycan, indicating robust protein-protein interactions.
Purpose of the Study:
- To analyze the ultrastructure of the matrix protein array in Escherichia coli.
- To elucidate the arrangement and symmetry of matrix protein molecules.
- To investigate potential functional features like channels within the protein monolayer.
Main Methods:
- Electron microscopy was employed for ultrastructural analysis.
- Image processing techniques were used to analyze high-resolution data.
- Specimens were prepared using negative staining and freeze-drying with shadowing.
Main Results:
- The matrix protein molecules are arranged in a hexagonal lattice with a 7.7 nm repeat.
- The arrangement exhibits three-fold symmetry.
- Each unit cell likely contains three matrix protein molecules, featuring a prominent triplet of indentations (approx. 2 nm diameter each) with 3 nm center-to-center spacing.
Conclusions:
- The matrix protein of Escherichia coli self-assembles into a stable, periodic hexagonal lattice.
- Strong protein-protein interactions stabilize this structure, independent of the peptidoglycan layer.
- The observed indentations may function as channels spanning the protein monolayer, suggesting a role in transport or interaction with the external environment.