The recently cloned apical renal transport system for organic cations (OCT2) exists in dopamine-rich tissues such as kidney and some brain areas (Grü ndemann, D., Babin-Ebell, J., Martel, F., Ö rding, N., Schmidt, A., and Schö mig, E.

The recent cloning of renal transport systems for organic anions and cations (OAT1, OCT1, and OCT2) opened the possibility to search, via polymerase chain reaction (PCR) homology screening, for novel transport proteins. Two integral membrane proteins, UST1 and UST2, were cloned from rat kidney. RT-PCR revealed that UST1 is confined to the kidney whereas UST2 mRNA was detected in all tested tissues. Sequence analyses suggest that UST1 and UST2, together with four related transporters, comprise, within the major facilitator superfamily, a so far unrecognized transporter family, termed amphiphilic solute facilitator (ASF) family. Characteristic signatures for the ASF family were identified.z 1998 Federation of European Biochemical Societies.