Composition and in situ structure of the Methanospirillum hungatei cell envelope and surface layer.

Archaea share genomic similarities with Eukarya and cellular architectural similarities with Bacteria, though archaeal and bacterial surface layers (S-layers) differ. Using cellular cryo-electron tomography, we visualized the S-layer lattice surrounding Methanospirillum hungatei, a methanogenic archaeon. Though more compact than known structures, M. hungatei's S-layer is a flexible hexagonal lattice of dome-shaped tiles, uniformly spaced from both the overlying cell sheath and the underlying cell membrane. Subtomogram averaging resolved the S-layer hexamer tile at 6.4-angstrom resolution. By fitting an AlphaFold model into hexamer tiles in flat and curved conformations, we uncover intra- and intertile interactions that contribute to the S-layer's cylindrical and flexible architecture, along with a spacer extension for cell membrane attachment. M. hungatei cell's end plug structure, likely composed of S-layer isoforms, further highlights the uniqueness of this archaeal cell. These structural features offer advantages for methane release and reflect divergent evolutionary adaptations to environmental pressures during early microbial emergence.