Yellowstone National Park is home to several stunning sights. The bright and vibrant Alkaline-chloride springs like Old Faithful and Grand Prismatic, the beautiful calcium-carbonate rock pools like those found at the Mammoth Hot Springs, showing off their gorgeous white waterfalls of limestone. These sights however, pale in comparison to the stunningly colourful Emerald Hot Spring. This gem of nature is incredibly acidic to the point that it dissolves the very rocks within it! Any unlucky wanderer who happens to fall into the Emerald Spring would find a gruesome end and completely dissolve. And yet, this pool of boiling acid is teeming with life!

The Archaeal domain of life is home to the most extreme of all extremophiles. One of these achaeal species is the acid and heat loving Sulfolobus acidocaldarius. This tough single celled organism optimally grows at 80°C and pH 2 and thrives in the acidic hot springs of Yellowstone. Through the oxidation of sulphur within the springs it calls home, S. acidocaldarius is responsible even for creating the toxic sulfuric acid it inhabits.

Our research group is interested in understanding how life can exist in such extreme places, and S. acidocaldarius has become the perfect model organism for this research. One of our interests is to understand how S. acidocaldarius cells interact with each other and form biofilms. To do this, they utilise hair-like surface extensions (filaments) made of protein. S. acidocaldarius produces four different protein filaments, each super stable and unique in its function. If we can determine the structure of these filaments, then not only do we gain insight into what biophysical parameters enable them to withstand hot and acidic environments, we will also understand how to design highly robust, yet biodegradable protein-based nanomaterials for a myriad of future applications.

We used a cutting-edge bioimaging technique called electron cryo-microscopy (cryoEM) to solve the structure of one of these filaments, called “thread”. We grew Sulfolobus acidocaldarius in special incubators and isolated the threads from the cells. We then froze the threads at very low temperatures and imaged them using a transmission electron microscope. Using sophisticated image analysis software, we generated a highly detailed 3-dimensional image of the thread, which allowed us to visualise it at atomic resolution.

To our surprise, the structure of the thread revealed a so-far unknown class of archaeal protein filaments. The threads are made of tadpole-shaped protein subunits, which are concatenated like beads on a string. The subunits are held together by extremely strong links; each tadpole-shaped subunit inserts its tail the head of the next subunit along the chain. The stability of these connections is further increased by so-called isopeptide bonds. These are covalent bonds between two proteins, which are highly stable, yet unusual in nature. To date, threads are the only type of archaeal filament seen to have these isopeptide bonds.

Another exciting feature is that the threads are coated with branched sugar molecules, called glycans. A vast range of eukaryotic and archaeal cell-surface proteins are have similar glycans. In humans, these glycans function in part to stop the immune system from targeting its own cells. Similarly, in Archaea, glycans are utilised in cell - cell interaction and communication. Our data show that many threads can line up in parallel. In these “cables”, the glycans mediate the interactions between the threads. We suggest that cable formation allows neighbouring cells to form connections with each and when these connections are formed between many cells, a biofilm is generated. The highly stable nature of the threads helps these biofilms to stay connected, even in the harsh conditions of a boiling hot acidic spring in Yellowstone National Park.