The ultrastructure and polypeptide composition of a novel membrane junction in magnesium-starved Escherichia coli are described in this report. Freeze-fracture replicas reveal the junction as a site-specific membrane particle array with four fracture faces. Each junction consists of a cell membrane, a midline zone and a coupled membrane. Membrane particles associated with the junction extend from the hydrophobic region of the cell membrane across the hydrophilic midline zone and into the hydrophobic region of the coupled membrane. After negative staining or after rotary shadowing of freeze-fractured specimens, these particles were seen to consist of two similar but slightly offset bracket-shaped subunits separated by a small space. Optical analysis confirms this structure. Since the apposing membranes are bracketed or linked by their component particles, the name "bracket junction" is proposed for the complex. Methods are described for isolating a membrane fraction enriched in these junctional complexes; the fraction contains a prominent glycoprotein (mol wt 90,000) as well as a number of other components. The bracket junction is compared with the vertebrate gap junction in terms of both structure and possible roles in facilitating the permeation of the cell by small molecules.

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