Substrate-binding (S) component of an energy-coupling factor (ECF) ABC-transporter complex. Mediates riboflavin uptake, may also transport FMN and roseoflavin. Probably a riboflavin-binding protein that interacts with the energy-coupling factor (ECF) ABC-transporter complex. Unlike classic ABC transporters this ECF transporter provides the energy necessary to transport a number of different substrates. The substrates themselves are bound by transmembrane, not extracytoplasmic soluble proteins (Probable). Expression of the complex plus RibU in E.coli allows riboflavin uptake; uptake does not occur in the absence of RibU or the EcfA1A2T complex. Forms a stable energy-coupling factor (ECF) transporter complex composed of 2 membrane-embedded substrate-binding proteins (S component, RibU, BioY), 2 ATP-binding proteins (A component) and 2 transmembrane proteins (T component) upon coexpression in E.coli. A stable subcomplex with both A and T components are also isolated. This complex interacts with at least 2 substrate-specific components, BioY and RibU. Belongs to the prokaryotic riboflavin transporter (P-RFT) (TC 2.A.87) family.