Abstract
Archaea are renowned for their ability to thrive in extreme environments, although they can be found in virtually all habitats. Their adaptive success is linked to their unique cell envelopes that are extremely resistant to chemical and thermal denaturation and that resist proteolysis by common proteases. Here we employ amyloid-specific conformation antibodies and biophysical techniques to show that the extracellular cell wall sheaths encasing the methanogenic archaea Methanosaeta thermophila PT are functional amyloids. Depolymerization of sheaths and subsequent MS/MS analyses revealed that the sheaths are composed of a single major sheath protein (MspA). The amyloidogenic nature of MspA was confirmed by in vitro amyloid formation of recombinant MspA under a wide range of environmental conditions. This is the first report of a functional amyloid from the archaeal domain of life. The amyloid nature explains the extreme resistance of the sheath, the elastic properties that allow diffusible substrates to penetrate through expandable hoop boundaries, and how the sheaths are able to split and elongate outside the cell. The archaeal sheath amyloids do not share homology with any of the currently known functional amyloids and clearly represent a new function of the amyloid protein fold.
Highlights
We show that the cell wall sheath of the methanogenic archaea Methanosaeta thermophila PT is a functional amyloid assembly composed of the major sheath protein (MspA)
The cell wall sheaths of filamentous methanogens share many traits with functional amyloids, but there is currently insufficient evidence to conclude that the sheaths are amyloid structures [16]
The width of these hoops is consistent with the common widths of mature amyloid fibrils, which are in the range of 6 –20 nm [38]
Summary
We employ amyloid-specific conformation antibodies and biophysical techniques to show that the extracellular cell wall sheaths encasing the methanogenic archaea Methanosaeta thermophila PT are functional amyloids. The sheaths display high stability against chemical (e.g. 6 M urea, 6 M GdmCl, 10 M LiSCN, or 1% SDS) and thermal (Ͼ100 °C) denaturation as well as resistance to proteolysis by common proteases [10, 14] Most of these features are shared with a group of proteins known as functional amyloids [15]. We show that the cell wall sheath of the methanogenic archaea Methanosaeta thermophila PT is a functional amyloid assembly composed of the major sheath protein (MspA). Boundaries, and the ability of the extracellular sheaths to split and elongate to accommodate growth of the archaeal filaments
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