Structural analysis of a symbiotic system involving a Nanobdellati archaeon by cryo-electron tomography.

Nanobdellati (formerly DPANN) archaea are considered as primitive archaeal organisms that often live in symbiosis with archaeal hosts. In this study, we investigated the symbiotic mechanism between a Nanobdellati archaeon, Nanobdella aerobiophila strain MJ1, and its host archaeon Metallosphaera sedula strain MJ1HA, using cryo-electron tomography. In our tomographic observations, we identified a conical attachment organelle at the interface between MJ1 and MJ1HA during symbiosis. This structure consists of a concentric array of short cylindrical shells, consistent with a previous report. Subtomogram averaging, combined with AlphaFold 3 structural predictions, allowed us to identify a potential component of attachment organelles. Additionally, we inferred potential components of the S-layers in MJ1 and MJ1HA based on tomographic data and subtomogram averages. Based on these analyses, we hypothesize that a MJ1 S-layer component may undergo conformational changes to also serve as a component of attachment organelles, warranting further investigation.