SLC51B, as known as OSTB, is an organic solute transporter subunit. SLC51 is composed of two distinct proteins that must heterodimerize to generate transport activity, but the role of the individual subunits in mediating transport activity is unknown. The results demonstrate that SLC51B is required for both proper trafficking of SLC51A and formation of the functional transport unit, and identify specific residues of SLC51B critical for these processes. Recombinant human SLC51B protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography.