Ultrastructural characterization of goblet-shaped particles from the cell wall of Flexibacter polymorphus

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Abstract
The ultrastructure of submicroscopic goblet-shaped particles (goblets) from the cell wall of the marine-gliding microbe F. polymorphus was investigated. The goblets, which were partially purified by CsCl density-gradient centrifugation, were rich in protein, exhibiting a single absorption maximum in UV at about 276 nm; they also contained a small amount of carbohydrate. As determined by EM, goblets negatively contrasted with ammonium molybdate were about 30 nm in diameter by 36 nm in length. When viewed in profile, each apparently consisted of 5 morphologically distinct kinds of components: the C-1, C-2 and C-3 subunits which formed the cup-shaped moiety of the goblet; a globular base unit; and a tubular stem-like structure connecting the cup with the base unit. A long fiber emerged from the interior of some goblets. The fine structural evidence suggested that goblets may be constructed from 3 stacked subunit rings (each composed of repeating C-1, C-2 or C-3 protomers) arranged concentrically. X-ray images of a clay model closely resembled electron micrographs of negatively stained goblets, thereby lending support to the proposed structure. Goblets probably function in vivo as macromolecular pores through the outer membrane which mediate extrusion of extracellular fibers, possibly of importance in gliding motility or in attachment of cells to solid surfaces.