Structure of a dodecameric double-ferritin-fold protein from an Asgard archaeon
Ferritins are ubiquitous iron homeostasis proteins found across the tree of life that form conserved 24-subunit cages with octahedral (4-3-2) symmetry. New types of ferritins and ferritin-like proteins are being continuously discovered, such as mini-bacterioferritins, which form smaller shells of 12 subunits, and double-ferritin-fold proteins, which act as ferroxidases but do not form shells.…
Ferritins, essential proteins for iron regulation, are found throughout all life forms and exhibit a consistent 24-subunit octahedral structure. Recently, various new types of ferritins and ferritin-like proteins have been discovered, including mini-bacterioferritins forming 12-subunit shells and double-ferritin-fold proteins (dFTNs), which act as ferroxidases without forming shells.
In this study, the researchers investigate dFTNs from Asgard archaea and determine their structure using Cryo-EM, revealing a representative from Candidatus Heimdallarchaeum endolithica. This protein assembles into a dodecameric shell with tetrahedral symmetry. The N-terminal and C-terminal domains are linked by an ordered linker and share two-fold rotational pseudosymmetry.
In dFTNs, the C-terminal beta-helix (helix E) that forms the four-fold channel in traditional ferritins shifts to become helix 2 out of five ferritin domain -helices, resulting in pseudo-four-fold symmetry. Four three-fold channels are generated by N-terminal domains, while four more are formed by C-terminal domains. The overall dFTN ferritin domain arrangement is similar to that of classic ferritin protomers.
These findings broaden our understanding of ferritin family proteins and shed light on the iron metabolism of Asgard archaea.
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