Comparative analyses of the sheath structures of Methanothrix concilii GP6 and Methanospirillum hungatei strains GP1 and JF1

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Abstract
The proteinaceous sheath of the aceticlastic methanogen Methanothrix concilii strain GP6 was isolated by exposure of the cells to 5 N NaOH at room temperature, followed by incubation with 1% (w/v) sodium dodecyl sulphate (100 °C, 10 min). The sheaths of Methanospirillum hungatei strains JF1 and GP1 could be isolated by milder alkali treatment (0.1 N NaOH at room temperature) and subsequent sodium dodecyl sulphate treatment (90 °C, 15 min). The isolated sheaths from each bacterium resembled hollow tubes which were composed of hoops piled on top of one another; all sheaths were several micrometers long but those of both Msp. hungatei strains were 0.5 μm wide and those of Mtx. concilii were 0.8μm wide. All three types of sheath possessed an identical subunit arrangement with p2 symmetry (a = 5.6, b = 2.8, and γ = 86°) which was confirmed by electron diffraction. On isolation, the Msp. hungatei sheaths contained complex, multilayered "cell spacer" plugs which possessed hexagonal symmetry, whereas those of Mtx. concilii were not as complex and their subunit arrangements resembled concentric rings. The plugs of Msp. hungatei could be removed by the mild alkali used in sheath isolation, but alkali did not affect the Mtx. concilii plugs. Whereas the GP1 sheath could be extensively disintegrated by a combination of sodium dodecyl sulphate and mercaptoethanol (90 °C, pH 9.0), the sheaths of strain JF1 and Mtx. concilii were unaffected. Treatment with 0.1 N NaOH at 80 °C resulted in complete disintegration of the sheaths isolated from the two Msp. hungatei strains, whereas the Mtx. concilii sheaths were resistant. Amino acid analysis of hydrolyzed sheaths indicated that unlike Mtx. concilii, those of the Msp. hungatei strains had higher proportions (on a mol% basis) of alanine and aspartate but considerably lower proportions of glutamate, histidine, and arginine. The predominant sugar was mannose, arabinose and glucose for the sheaths of Mtx. concilii, Msp. hungatei strains JF1 and GP1, respectively. All sheath preparations contained metals, notably zinc in the case of Mtx. concilii, and calcium in the Msp. hungatei strains. We conclude that although the subunit arrangements of the three sheath types are similar, there are marked differences in their chemical compositions. In addition, the subunit arrangements of the spacer plugs differ from those of the respective sheaths, and the plugs of Mtx. concilii are physically different from those of Msp. hungatei.