Membrane topology distinguishes a subfamily of the ATP‐binding cassette (ABC) transporters

Abstract

A group of ATP‐binding cassette (ABC) transporters, including the yeast cadmium transporter (YCF1), the mammalian multidrug resistance‐associated protein (MRP), the multispecific organic anion transporter and its congener (MOAT and EBCR), as well as the sulfonylurea receptor (SUR), group into a subfamily by sequence comparison. We suggest that these MRP‐related proteins are also characterized by a special, common membrane topology pattern. The most studied ABC transporters, the cystic fibrosis transmembrane conductance regulator (CFTR) and the multidrug resistance (MDR) proteins, were shown to contain a tandem repeat of six transmembrane helices, each set followed by an ATP‐binding domain. According to the present study, in contrast to various membrane topology predictions proposed for the different MRP‐related proteins, they all seem to have a CFTR/MDR‐like core structure, and an additional, large, N‐terminal hydrophobic region. This latter domain is predicted to contain 4–6 (most probably 5) transmembrane helices, and is occasionally glycosylated on the cell surface. Since all the MRP‐related transporters were shown to interact with anionic compounds, the N‐terminal membrane‐bound domain may have a key role in these interactions.