Pyrodictium abyssi is a hyperthermophilic archaeon, first isolated from shallow submarine hydrothermal systems, that grows optimally near 100°C. It is best known structurally for producing cannulae — hollow, tubular protein filaments that extend from the cell surface and, in dense cultures, interconnect large numbers of cells into an extracellular network. Cannulae networks have been documented by electron microscopy since the 1980s, but for decades the protein’s assembly mechanism and three-dimensional structure went unresolved, largely because the fibers are extremely stable and resistant to standard structural-biology preparation methods.
Two independent 2025 papers have now each solved a piece of that puzzle, using complementary structural methods on the same building-block protein, CanA.
A cryo-EM view of chaperone-free assembly
The more structurally complete of the two studies, led by Mike Sleutel with co-authors including Ravi R. Sonani, Fengbin Wang, Edward H. Egelman and Vincent P. Conticello, first appeared as a bioRxiv preprint in December 2024 and was published in Nature Communications on 13 October 2025 under the title “Donor Strand Complementation and Calcium Ion Coordination Drive the Chaperone-free Polymerization of Archaeal Cannulae” (DOI: 10.1038/s41467-025-64120-8).
Using cryo-electron microscopy on both native cannulae isolated from cells and cannulae reassembled in vitro, the team found that calcium ions alone — with no accessory chaperone proteins — are sufficient to drive CanA polymerization into the same hollow-fiber architecture seen in native filaments. The assembly mechanism is donor strand complementation: each CanA subunit completes the fold of its neighbor by contributing a beta-strand into the adjacent subunit’s core, a strategy structurally reminiscent of how bacterial chaperone-usher pili (such as type 1 pili) assemble — except that in cannulae this happens without any dedicated chaperone or usher protein to catalyze it. The paper frames this as evidence that structurally stable, self-templating polymers can form directly from calcium-ion coordination in an extreme thermal environment, without the folding machinery multicellular and many bacterial systems rely on. The authors note the paper’s own caveat directly: despite the structural clarity now available, cannulae’s native biological role — long postulated to be a primitive extracellular matrix for intercellular communication or material exchange — “remains obscure.”
The work was supported by multiple NIH grants, including K99 GM138756, R35 GM122510 and R01 GM120600, among others credited in the paper’s funding statement.
A complementary NMR view of the monomer
A second, independent paper — Claudia E. Munte, Raphael Kreitner, Reinhard Rachel, Karl O. Stetter, Werner Kremer and Hans Robert Kalbitzer, “Biophysical characterization and solution structure of the cannulae-forming protein CanA from the hyperthermophilic archaeon Pyrodictium abyssi,” Scientific Reports, 5 August 2025 (DOI: 10.1038/s41598-025-13242-6) — approached the same protein from a different angle. Rather than cryo-EM, the group used multidimensional NMR spectroscopy to solve the solution structure of a non-polymerizing CanA mutant (K1-CanA), avoiding the fiber-forming behavior that makes wild-type CanA hard to study by solution methods.
They report a jellyroll fold built mainly from beta-pleated sheets with two small alpha-helices, identify two distinct divalent-ion-binding sites on the monomer, and describe the polymerization process itself as biphasic, with two measurably different rate constants and a critical monomer concentration of 2.48 µM below which fiber formation does not proceed. The paper proposes that divalent-ion binding triggers a localized conformational change in the monomer — a conformational-selection mechanism — that then permits polymerization, a finding that sits comfortably alongside the cryo-EM group’s calcium-coordination result even though the two teams worked independently and used different structural methods.
Why a deep-sea archaeon’s filaments are relevant beyond microbiology
For research infrastructure and structural-biology audiences, the more durable story here is methodological as much as it is organismal. Cannulae are a rare confirmed case of a chaperone-independent, self-templating polymerization mechanism operating under extreme-thermophile conditions (near-boiling temperatures), which makes CanA a useful reference point for anyone studying calcium-triggered or donor-strand-complementation assembly logic in engineered biomaterials and synthetic self-assembling protein fibers. It is also a useful illustration of how cryo-EM and solution NMR can arrive at consistent, mutually reinforcing conclusions about the same protein from genuinely independent starting points — native fiber imaging in one case, an engineered non-polymerizing mutant in the other — published within a few months of each other in 2025. Raw cryo-EM image data of the kind used in structural studies like the Sleutel et al. paper is typically deposited to public repositories such as EMPIAR, the Electron Microscopy Public Image Archive, which is what makes independent re-analysis of density maps like these possible for other labs.
What is still unresolved
Both papers are explicit that the open question is biological function, not structure. Cannulae have been directly observed connecting Pyrodictium cells into extensive interconnected networks during growth, and researchers have long speculated that the filaments could serve in intercellular communication, material exchange, or structural support for the cell aggregate as a whole — but neither paper claims to have confirmed which, if any, of these roles cannulae actually play in the organism’s native hydrothermal-vent environment. That remains open for follow-up work.
