Sheath disassembly in Methanospirillum hungatei strain GP1

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Abstract
Sheaths of Methanospirillum hungatei are very resilient structures and consist of circumferential rings, which have been likened to the hoops of a barrel. The isolated sheaths are dramatically disassembled to the individual hoops upon treatment with β-mercaptoethanol at 90 °C. Sheath disassembly resulted in a large decrease in suspension turbidity and a release of approximately 10% of sheath protein in the form of 4.6 to 7.0 kiloDalton (kDa) peptides. These are referred to as "glue peptides" to suggest a function in linking the hoops together to form the intact sheath. The liberated hoops consisted of a protein surface array crystallized on a matrix material. Intact sheaths, or hoops largely freed of the glue peptides, could be solubilized at 90 °C by (i) a combination of β-mercaptoethanol and sodium dodecyl sulfate, (ii) by alkaline conditions of pH 12 or more, or partially (iii) by carbonate anion at pH 11. Fractionation by liquid chromatography of hoops solubilized by alkaline conditions of pH 12.6 revealed major polypeptides of about 24 and 45 kDa; a smaller peak occurred at 12 kDa. Based on the dimensions of the unit cell of the crystalline array, we suggest that two copies of the 24-kDa polypeptide form the 5.6 × 2.8 nm unit cell as a dimer, which then tends to form larger aggregates. The 2.8-nm subunit may be a dimer of the 12-kDa species. Sheaths of M. hungatei strain JF1 exhibited a similar polypeptide profile, but in this case, the 45-kDa species predominated.