3.A.1.25.8
An ECF ABC transporter with 4 subunits, EcfS/EcfT/EcfA/EcfA'.  EcfS is also called RibU; EcfT is also called CbiQ, EcfA is also called Cbi01, and EcfA' is also called Cbi02.  This system can take up riboflavin and possibly other vitamins (Karpowich et al. 2015). ATP binding to the EcfAA' ATPases drives a conformational change that dissociates the S subunit from the EcfAA'T ECF module. Upon release from the ECF module, the RibU S subunit then binds the riboflavin transport substrate.  S subunits for distinct substrates compete for the ATP-bound state of the ECF module (Karpowich et al. 2015). RibU appears to be capable of exporting riboflavin, FMN and FAD (Light et al. 2018). More recently, this system has been shown to functioin in three capacities; all use EcfA (Ecf1), EcfB (Ecf2), both ATPases. Two of them also use RibU (EcfS).  The first complex uses EcfT and RibU and imports FAD; the second uses FmnA and RibU and takes up vitamin B2 (riboflavin derivatives); the third uses FmnA and EetB and exports FAD (Rivera-Lugo et al. 2023). Thus, these three systems are partially redundant. FmnA is also called AuxA and can be found under TC# 9.B.414.1.1.