Structure of the archaellar motor and associated cytoplasmic cone inThermococcus kodakaraensis

Abstract

Archaeal swimming motility is driven by rotary motors called archaella. The structure of these motors, and particularly how they are anchored in the absence of a peptidoglycan cell wall, is unknown. Here, we use electron cryotomography to visualize the archaellar motor in vivo in Thermococcus kodakaraensis. Compared to the homologous bacterial type IV pilus (T4P), we observe structural similarities as well as several unique features. While the position of the cytoplasmic ATPase appears conserved, it is not braced by linkages that extend upward through the cell envelope as in the T4P, but rather by cytoplasmic components that attach it to a large conical frustum up to 500 nm in diameter at its base. In addition to anchoring the lophotrichous bundle of archaella, the conical frustum associates with chemosensory arrays and ribosome-excluding material and may function as a polar organizing center for the coccoid cells.

Structure and in situ organisation of the Pyrococcus furiosus archaellum machinery

“…This confirms earlier findings in H. salinarum (Kupper et al, 1994) and T. kodakarensis (Briegel et al, 2017), which also showed co-localisation of archaella and polar cap. This striking co-localisation indicates that archaella and polar cap are co-regulated and suggests a strong functional connection.…”

“…It remains to be investigated if the hexagonal protein arrays found in this study represent an unknown Pyrococcus -specific type of chemoreceptor or some other protein linked to motility. Our observations support the hypothesis that the polar cap does not only function as an anchor for the archaellum, but also as a platform that recruits protein complexes that control swimming motion (Briegel et al, 2017). …”

“…Each filament crosses the S-layer and spans the ~34 nm periplasmic gap (Figure 1B). As observed previously (Briegel et al, 2017), the archaella traverse the periplasm at variable angles of 60–90° between the filament axis and the membrane plane (Figure 1—figure supplement 3). In the tomograms, the filaments emerge from a basal density on the cytoplasmic surface of the plasma membrane, which most likely corresponds to the archaellar motor (Figure 1B).…”

“…The polar cap appears to be a hallmark of motile Euryarchaeota, as it has also been observed in related species (Briegel et al, 2017; Kupper et al, 1994). The polar cap has a thickness of ~3 nm, a variable diameter of 200–600 nm and typically straight edges.…”

“…This ring was not observed in averages of the archaellar motor complex of Thermococcus kodakarensis (Briegel et al, 2017), which may either be due to species-dependent structural differences of motor components or the smaller number of particles used for the T. kodakarensis average. In the periplasm, the filament is thought to be coordinated by FlaF (Banerjee et al, 2015).…”