Sequential conformational transition of ArnB, an archaeal ortholog with Sec23/Sec24 core motif

ArnA and ArnB serve as regulators within the archaellum regulatory network by affecting the levels of archaellum components ArlB and ArlX in response to nutrient cues. Together, they form either a loose or a tight complex, whose transition is directed by phosphorylation via the kinase ArnC. For a structure-based analysis of this transition we solved a cocrystal structure of the ArnA/ArnB complex revealing that the zinc finger domain of ArnA interacts with the β-sandwich and C-terminal domain of ArnB. HDX data corroborate the phosphorylation-dependent transition from loose to tight ArnAB complexes. This transition depends on a structural transformation of ArnB by sequential phosphorylation, exposing the interaction surface of the C-terminal domain of ArnB for the forkhead-associated domain of ArnA. Furthermore, we found a striking structural similarity between ArnB and the membrane-curving proteins of the COPII vesicle system, Sec23/Sec24. The common Sec23/Sec24 core motif can be found in all domains of life, where it can apparently adopt a multitude of different functions. Overall, this implies that Sec23/Sec24 orthologs with a function in vesicle formation arose in Lokiarchaeota from related, but not necessarily functionally linked relatives as found in TACK Archaea.