Extracellular membrane vesicles and nanotubes in Archaea

Abstract

Membrane-bound extracellular vesicles (EVs) are secreted by cells from all three domains of life and their implication in various biological processes is increasingly recognized. In this review, we summarize the current knowledge on archaeal EVs and nanotubes, and emphasize their biological significance. In archaea, the EVs and nanotubes have been largely studied in representative species from the phyla Crenarchaeota and Euryarchaeota. The archaeal EVs have been linked to several physiological processes such as detoxification, biomineralization and transport of biological molecules, including chromosomal, viral or plasmid DNA, thereby taking part in genome evolution and adaptation through horizontal gene transfer. The biological significance of archaeal nanotubes is yet to be demonstrated, although they could participate in EV biogenesis or exchange of cellular contents. We also discuss the biological mechanisms leading to EV/nanotube biogenesis in Archaea. It has been recently demonstrated that, similar to eukaryotes, EV budding in crenarchaea depends on the ESCRT machinery, whereas the mechanism of EV budding in euryarchaeal lineages, which lack the ESCRT-III homologues, remains unknown.

Keywords: DNA transfer; archaea; extracellular vesicles; extremophiles; nanotubes; vesiduction.

Figure 1.

Extracellular vesicles produced by Sulfolobus islandicus. (A) EV purification by ultracentrifugation in the 25–50% sucrose gradient. EVs form an opalescent band in the region corresponding to 30–40% sucrose. The figure is modified from Liu et al. (2021b). (B)Transmission electron micrograph of negatively stained EVs. (C) EVs budding from a dividing Sulfolobus cell. Scale bars: 400 nm.

Figure 2.

Extracellular vesicles produced by Thermococcales. Transmission electron microscopy images of purified EVs produced by (A)T. nautili showing the size diversity (scale bar, 200 nm) and (B) by T. gammatolerans (scale bar, 100 nm). (C) Scanning electron micrograph of T. kodakarensis cells producing EVs (scale bar, 500 nm). Cryo-electron micrographs of (D)Thermococcus sp. 15-2 and (E)T. prieurii cells associated with dark vesicles named sulfur vesicles (SVs). The arrows indicate SVs. (F) Transmission electron microscopy image of T. kodakarensis cells (a) and EVs (b) entirely mineralized within FeS₂ pyrite. The EVs are dispersed among Fe₃S₄ greigite nanocrystals (indicated by arrow). Scale bar, 500 nm. (G) Cryo-electron micrographs of a T. prieurii cell producing very small EVs (indicated by an arrow) grouped into a spherical structure at the cell surface.

Figure 3.

Nanotubes produced by Thermococcales.(A–C)Transmission electron microscopy images: (A)T. prieurii cell producing both EVs and nanotubes containing EVs (scale bar, 200 nm); discrete EVs are surrounded by the cellular S-layer forming the nanotube structure. (B) Long nanotubes containing EVs produced by T. prieurii (scale bar, 200 nm). (C) A cell of Thermococcus sp. 15-2 producing a long nanotube. (D) Scanning electron microscopy image of long nanotubes connecting clusters of Thermococcus sp. 15-2 cells. EVs can be also observed at the surface of most cells (scale bar, 1 µm) (Marguet and Forterre, unpublished observations).

Figure 4.

Nanotubes produced by Haloferax volcanii. Cells of H. volcanii are linked by nanotubes as observed by (A) phase-contrast and (B) fluorescence microscopy. In panel (B), H. volcanii cells are labeled with Alexa Fluor 488. The cell–cell bridge is indicated by a red arrow. Scale bar, 4 µm. (C, D) Electron cryo-tomography images of H. volcanii nanotubes. In panel (D), ribosomes are indicated by small red arrows. Scale bar, 100 nm. All images are reproduced from Sivabalasarma et al. (2020).