Riboflavin transporter. Riboflavin transport is Na+-independent but moderately pH-sensitive. Activity is strongly inhibited by riboflavin analogs, such as lumiflavin. Weakly inhibited by flavin adenine dinucleotide (FAD). In case of infection by retroviruses, acts as a cell receptor to retroviral envelopes similar to the porcine endogenous retrovirus (PERV-A).