Plasma membrane transporter mediating the uptake by cells of the water soluble vitamin B2/riboflavin that plays a key role in biochemical oxidation-reduction reactions of the carbohydrate, lipid, and amino acid metabolism. May also act as a receptor for 4-hydroxybutyrate. (Microbial infection) In case of infection by retroviruses, acts as a cell receptor to retroviral envelopes similar to the porcine endogenous retrovirus (PERV-A). riboflavin(in) = riboflavin(out) Riboflavin transport is Na(+)-independent but moderately pH-sensitive (By similarity). Activity is strongly inhibited by riboflavin analogs, such as lumiflavin (By similarity). Weakly inhibited by flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) (By similarity). Belongs to the riboflavin transporter family.