Origin & Evolution of Life

Decoding Conversations Between Microbes Of Hypersaline Environments Reveals Insights Into Origins Of Complex Life

By Keith Cowing
Press Release
Max Planck Institute for Marine Microbiology
February 27, 2024
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Decoding Conversations Between Microbes Of Hypersaline Environments Reveals Insights Into Origins Of Complex Life
Extracellular vesicles of the haloarchaeon Haloferax volcanii, made visible with an electron microscope. The scale bar measures 500 nm. CREDIT S. Erdmann/Max Planck Institute for Marine Microbiology

Single-celled organisms, such as bacteria and archaea, have developed many ways to communicate with each other. For example, they might use tiny so-called extracellular vesicles (EVs) – membrane-enveloped packages smaller than 200nm in diameter (0.0002mm).

The organisms produce them by budding from their membrane into the surrounding space. These EVs can contain a variety of molecules such as enzymes, nutrients, RNA and even frag­ments of DNA. Though it is suspected that they play a key role in microbial communities, little is known about their function or how they are produced.

Speech balloons for RNA talk

In a study now published in the journal PNAS, Susanne Erdmann with her team at the Max Planck Institute for Marine Microbiology and collaborators from other institutions in Germany and Australia (see below), investigated EVs from microbes that thrive in extremely salty environments such as the Dead Sea, known as halophilic archaea or haloarchaea. They found that their EVs traffic RNA – nucleic acids that play a central role in protein synthesis and gene regulation – between cells. “Obviously, EVs can act as an RNA communication system between haloarchaea”, Erdmann explains. In particular, the EVs transported specific RNAs with the potential to regulate processes in a receiving cell. “We think that this represents a communication mechanism to regulate gene expression across a whole microbial population. One could say, RNA is their common language, and the EV is the speech balloon.”

A GTPase known from eukaryotic cells

The team around Erdmann also investigated how the haloarchaea produce these EVs. “We found a small GTPase – a class of enzymes serving as molecular switches or timers in many fundamental cellular processes – that was very similar to a GTPase in more complex cells”, reports first-author Joshua Mills, who conducted the study as party of his doctoral thesis. “That is quite astonishing, as GTPase-dependent vesicle formation was previously thought to be only carried out within eukaryotic cells, between the membrane-bound intracellular compartments. Our finding suggests that components of eukaryotic intracellular vesicle trafficking could have evolved much earlier in evolutionary history than previously assumed.”

“Few studies have investigated the role of EVs within the archaeal domain to date”, Erdmann adds. “Here we show that EVs in salt-loving archaea can transport an RNA cargo and thus help cells communicate with each other. Also, we reveal exciting new insights into the evolutionary development of this communication strategy. Our study provides the basis for further studies into the evolutionary relationships between prokaryotic and eukaryotic vesicle formation and might help solving the puzzle of the evolution of the eukaryotic cell.”

The new family of archaeal vesiculating GTPases, ArvA. (A) Unrooted phylogenetic tree of the identified ArvA homologs across the archaeal domain. The red arrow indicates position of H. volcanii ArvA. Blue dots represent branches with bootstrap value greater than 95. Structural prediction of tertiary structure of the ArvA dimer (monomer depicted in green) with (B) closed and (C) open conformations [AlphaFold v2 (41, 42)]. The modeled GDP ligand (displayed as balls), comes from the distant structural homolog EngA from Thermus thermophilus HB8 (rmsd of 3.29-Å out of 69 C-alphas, PDB 2DYK). Hydrophobic residues on N-terminal α-helix (displayed in yellow) are highlighted as balls.

Extracellular vesicle formation in Euryarchaeota is driven by a small GTPase, PNAS (open access)

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