Post-translational flavinylation is associated with diverse extracytosolic redox functionalities throughout bacterial life

Abstract

Disparate redox activities that take place beyond the bounds of the prokaryotic cell cytosol must connect to membrane or cytosolic electron pools. Proteins post-translationally flavinylated by the enzyme ApbE mediate electron transfer in several characterized extracytosolic redox systems but the breadth of functions of this modification remains unknown. Here we present a comprehensive bioinformatic analysis of 31,910 prokaryotic genomes that provides evidence of extracytosolic ApbEs within ~50% of bacteria and the involvement of flavinylation in numerous uncharacterized biochemical processes. By mining flavinylation-associated gene clusters, we identify five protein classes responsible for transmembrane electron transfer and two domains of unknown function (DUF2271 and DUF3570) that are flavinylated by ApbE. We observe flavinylation/iron transporter gene colocalization patterns that implicate functions in iron reduction and assimilation. We find associations with characterized and uncharacterized respiratory oxidoreductases that highlight roles of flavinylation in respiratory electron transport chains. Finally, we identify interspecies gene cluster variability consistent with flavinylation/cytochrome functional redundancies and discover a class of 'multi-flavinylated proteins' that may resemble multiheme cytochromes in facilitating longer distance electron transfer. These findings provide key mechanistic insight into an important facet of bacterial physiology and establish flavinylation as a functionally diverse mediator of extracytosolic electron transfer.

Listeria monocytogenes requires cellular respiration for NAD+ regeneration and pathogenesis

“…We recently identified a second flavin-based electron transport chain that transfers electrons to extracytosolic acceptors (including ferric iron and fumarate) via a putative demethylmenaquinone intermediate and can promote growth in anaerobic conditions ( Figure 1A ; Light et al, 2018 ; Light et al, 2019 ; Zeng et al, 2021 ). Final electron transfer steps in this flavin-based electron transport mechanism are catalyzed by PplA and FrdA, which are post-translationally linked to an essential cofactor by the flavin mononucleotide transferase (FmnB) ( Light et al, 2018 ; Méheust et al, 2021 ).…”

Distinct energy-coupling factor transporter subunits enable flavin acquisition and extracytosolic trafficking for extracellular electron transfer inListeria monocytogenes

“…While the role of flavin-binding proteins (flavoproteins) in cytosolic redox activities is well established, the importance of flavins for extracytosolic activities in prokaryotic biology has become increasingly apparent. [2][3][4] In prokaryotes, many extracytosolic flavoproteins are post-translationally linked to their flavin cofactors (flavinylated) through the action of the enzyme ApbE. 5 ApbE specifically uses FAD as a substrate, catalyzing a reaction that links the FMN portion of the molecule to a serine/threonine residue in substrate proteins via a phosphodiester bond (Figure 1A).…”

RibU is an essential determinant of Listeria pathogenesis that mediates acquisition of FMN and FAD during intracellular growth

“…Recently, there has been renewed interest in flavin metabolism in the context of pathogenesis stemming from the discovery that intermediates of riboflavin biosynthesis activate innate-like mucosal-associated invariant T (MAIT) cells ( 23 ). In addition, we recently discovered that flavins are implicated in distinct extracytosolic redox activities in thousands of bacterial species ( 24 ). These bacterial redox systems allow bacteria to transfer electrons from the cytosol to various electron acceptors.…”