Deep-sea rhodopsin assembles into trimers as pressure climbs to 120 megapascals

The design is a two-protein comparison. PoXeR comes from Parvularcula oceani, a deep-sea bacterium adapted to high pressure; GR comes from Gloeobacter, a terrestrial bacterium that lives at atmospheric pressure ^[4]. Both went through the same custom-built apparatus and the same ramp, 0.1 to 120 MPa, which the team equates with sea level down to the deepest point in the oceans ^[6]. That is a 1,200-fold span in pressure ^[17]. Samples were prepared at the University of Tokyo's Institute for Solid State Physics, and the experiments were run in the Department of Chemistry at the Institute of Science Tokyo ^[3].

The readout throughout is light absorption ^[7]. The criterion that does the work is not what the spectrum looks like at 120 MPa but whether it comes back afterwards. "When GR was pressurized, its structure became unstable, essentially causing the protein to denature," Fukuhara said ^[8]. "Then after the pressure was released, GR's absorption spectrum, which is its light-absorption ability, did not return to its original state. The protein had undergone irreversible structural changes," he said ^[9]. PoXeR's spectrum returned almost entirely to its original state, with no visible signs of denaturation ^[10]. The account does not say how far short of entirely it landed.

Then the mechanism. "As pressure increases, the molecules adapt to the high-pressure environment by forming groups of three, known as a trimeric structure. This structure is highly suited to the extreme conditions of the deep sea. This is how PoXeR remains stable under pressure," Fukuhara said ^[13]. The team's name for it is oligomerization-mediated structural stabilization ^[12].

The trimer appeared as pressure rose, and so did the evidence of stability ^[12]. Nothing in the reported work blocks trimer formation in PoXeR or pushes GR into a trimer ^[19]. Without one of those two experiments, assembly state and pressure tolerance are two things observed together in a single protein. The comparison also carries one protein per environment ^[18], and PoXeR and GR are different proteins from different organisms ^[4], so every difference between them is a candidate explanation and the trimer is the candidate this method can see.

What an absorption spectrum does not tell you is whether the protein still works. Microbial rhodopsins use light energy to do biological work ^[5], and the reported measurement tracks how they interact with light, not what they accomplish with it ^[20]. Fukuhara's group says the finding could feed into light-responsive protein materials for harsh conditions ^[15], and points somewhere further out. "Now, our next step is to move into the medical field. We want to understand how the human body responds to increasing hydrostatic pressure and explore how this might affect the development and treatment of cancer and other diseases," Fukuhara said ^[16].