Biochemical and structural analyses of the extracellular matrix fibrils of Myxococcus xanthus

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Abstract
It is characteristic of myxobacteria to produce large amounts of extracellular material. This report demonstrates that this material in Myxococcus xanthus is fibrillar and describes the structure and chemical composition of the fibrils. The extracellular matrix fibrils are the mediators of cell-cell cohesion in M. xanthus. As such, the fibrils play an important role in the cell-cell interactions that form the basis for the social and developmental lifestyle of this organism. The fibrils are composed of protein and carbohydrate in a 1.0:1.2 ratio. Combined, the two fractions accounted for greater than 85% of the mass of isolated fibrils, and the fibrils were found to compose up to 10% of the dry weight of cells grown at high density on a solid surface. The polysaccharide portion of the fibrils was shown to be composed of five different monosaccharides: galactose, glucosamine, glucose, rhamnose, and xylose. Glucosamine, one of the component monosaccharides of the fibrils and a known morphogen for M. xanthus, inhibited cohesion to a level near that of Congo red (the positive control for cohesion inhibition). Glucose and xylose also inhibited cohesion but less than did glucosamine. Analysis of the morphology of the fibrils, the periodicities within the distribution of fibril diameters observed by field emission scanning electron microscopy, and the observation of fibrils on hydrated cells strongly suggested that the extracellular matrix of M. xanthus was indeed arranged as fibrils. Furthermore, results suggested that the fibrils were constructed as carbohydrate structures with associated proteins.