AUTHOR=Christensen Line Friis Bakmann , Hansen Lonnie Maria , Finster Kai , Christiansen Gunna , Nielsen Per Halkjær , Otzen Daniel Erik , Dueholm Morten Simonsen 

TITLE=The Sheaths of Methanospirillum Are Made of a New Type of Amyloid Protein

JOURNAL=Frontiers in Microbiology

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2018.02729

DOI=10.3389/fmicb.2018.02729

ISSN=1664-302X

ABSTRACT=<p>The genera <italic>Methanospirillum</italic> and <italic>Methanosaeta</italic> contain species of anaerobic archaea that grow and divide within proteinaceous tubular sheaths that protect them from environmental stressors. The sheaths of <italic>Methanosaeta thermophila</italic> PT are composed of the 60.9 kDa major sheath protein MspA. In this study we show that sheaths purified from <italic>Methanospirillum hungatei</italic> JF-1 are regularly striated tubular structures with amyloid-like properties similar to those of <italic>M. thermophila</italic> PT. Depolymerizing the sheaths from <italic>M. hungatei</italic> JF-1 allowed us to identify a 40.6 kDa protein (WP_011449234.1) that shares 23% sequence similarity to MspA from <italic>M. thermophila</italic> PT (ABK14853.1), indicating that they might be distant homologs. The genome of <italic>M. hungatei</italic> JF-1 encodes six homologs of the identified MspA protein. Several homologs also exist in the related strains <italic>Methanospirillum stamsii</italic> Pt1 (7 homologs, 28–66% sequence identity), <italic>M. lacunae</italic> Ki8-1 C (15 homologs, 29–60% sequence identity) and <italic>Methanolinea tarda</italic> NOBI-1 (2 homologs, 31% sequence identity). The MspA protein discovered here could accordingly represent a more widely found sheath protein than the MspA from <italic>M. thermophila</italic> PT, which currently has no homologs in the NCBI Reference Sequence database (RefSeq).</p>