The archaeal cell envelope exhibits unique adaptations distinct from those of bacteria and eukaryotes, often comprising proteinaceous layers such as S-layers, sheaths, or pseudomurein that contribute to structural integrity, protection, and cellular processes. The Methanospirillum species, with their elongated cylindrical morphology, offer a valuable model for investigating archaeal envelope architectures and their roles in motility and cellular organization. Using cryogenic electron tomography (cryo-ET) combined with focused ion beam (FIB) milling and subtomogram averaging, we resolved the ultrastructure of the cell end of Methanospirillum hungatei at sub-nanometer resolution. This complex architecture is composed of multiple concentric protein layers and associated components. Structure-guided analysis, supported by AI-based protein structure prediction, identified these layers as distinct S-layer paralogs encoded within a single operon. Notably, we visualized the archaeal flagellum (archaellum) embedded within the end plug, revealing a localized remodeling of the otherwise continuous, pore-less S-layer to permit flagellar extrusion—providing insight into mechanisms of directional motility. Additionally, cell end structures captured at different life stages suggest an ordered assembly process, potentially coordinated with archaellum biogenesis. These findings advance our understanding of archaeal envelope organization and highlight the structural versatility of the cell end in supporting motility and morphological regulation.
Agnew et al. (Sun,) studied this question.