Hierarchical organization and assembly of the archaeal cell sheath from an amyloid-like protein

Certain archaeal cells possess external proteinaceous sheath, whose structure and organization are both unknown. By cellular cryogenic electron tomography (cryoET), here we have determined sheath organization of the prototypical archaeon, Methanospirillum hungatei. Fitting of Alphafold-predicted model of the sheath protein (SH) monomer into the 7.9 angstrom-resolution structure reveals that the sheath cylinder consists of axially stacked beta-hoops, each of which is comprised of two to six 400 nm-diameter rings of beta-strand arches (beta-rings). With both similarities to and differences from amyloid cross-beta fibril architecture, each beta-ring contains two giant beta-sheets contributed by similar to 450 SH monomers that entirely encircle the outer circumference of the cell. Tomograms of immature cells suggest models of sheath biogenesis: oligomerization of SH monomers into beta-ring precursors after their membrane-proximal cytoplasmic synthesis, followed by translocation through the unplugged end of a dividing cell, and insertion of nascent beta-hoops into the immature sheath cylinder at the junction of two daughter cells.