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. Forms a stable energy-coupling factor (ECF) transporter complex composed of 2 membrane-embedded substrate-binding proteins (S component), 2 ATP-binding proteins (A component) and 2 transmembrane proteins (T component) upon coexpression of the 4 components in E.coli. Belongs to the prokaryotic riboflavin transporter (P-RFT) (TC 2.A.87) family.