Multi-disciplinary methods reveal a novel type of ion binding in the rotor ring of the F1Fo-ATP synthase from the opportunistic pathogen Fusobacterium nucleatum.

Serratia sp. strain ATCC 39006 produces the red-pigmented antibiotic prodigiosin. Prodigiosin biosynthesis is regulated by a complex hierarchy that includes the uncharacterized protein YgfX (DUF1434). The ygfX gene is co-transcribed with sdhE, an FAD assembly factor essential for the flavinylation and activation of the SdhA subunit of succinate dehydrogenase (SDH), a central enzyme in the tricarboxylic acid cycle and electron transport chain. The sdhEygfX operon is highly conserved within the Enterobacteriaceae, suggesting that SdhE and YgfX function together. We performed an extensive mutagenesis to gain molecular insights into the uncharacterized protein YgfX, and have investigated the relationship between YgfX and SdhE. YgfX localized to the membrane, interacted with itself, forming dimers or larger multimers, and interacted with SdhE. The transmembrane helices of YgfX were critical for protein function and the formation of YgfX multimers. Site-directed mutagenesis of residues conserved in DUF1434 proteins revealed a periplasmic tryptophan and a cytoplasmic aspartate that were crucial for YgfX activity. Both of these amino acids were required for the formation of YgfX multimers and interactions with SdhE but not membrane localization. Multiple cell division proteins were identified as putative interaction partners of YgfX and overexpression of YgfX had effects on cell morphology. These findings represent an important step in understanding the function of DUF1434 proteins. In contrast to a recent report, we found no evidence that YgfX and SdhE form a toxin-antitoxin system. In summary, YgfX functions as a multimeric membrane-bound protein that interacts with SdhE, an important FAD assembly factor that controls SDH activity.