CHARACTERIZATION OF MOAT-C AND MOAT-D, NEW MEMBERS OF THE MRP CMOAT SUBFAMILY OF TRANSPORTER PROTEINS/

Background: Multidrug resistance-associated protein (MRP) and canalicu lar multispecific organic anion transporter (cMOAT) are transporter pr oteins that pump organic anions across cellular membranes and have bee n linked to resistance to cytotoxic drugs. We previously identified MO AT-B, an MRP/cMOAT-related transporter, by use of a polymerase chain r eaction approach. However, analysis of expressed sequence tag (EST) da tabases indicated that there might be additional MRP/cMOAT-related tra nsporters. To further define the MRP/cMOAT subfamily of transporters, we used EST probes to isolate complementary DNAs for two related trans porter proteins, MOAT-C and MOAT-D, Methods: MOAT-C and MOAT-D express ion patterns in human tissues were determined by RNA blot analysis, an d chromosomal localization of the genes was determined by fluorescence in situ hybridization. Results: MOAT-C is predicted to encode a 1437- amino-acid protein that, among eukaryotic transporters, is most closel y related to MRP, cMOAT, and MOAT-B (about 36% identity). However, MOA T-C is less related to MRP and cMOAT than MRP and cMOAT are to each ot her (about 48% identity). Like MOAT-B, MOAT-C lacks an N-terminal memb rane-spanning domain, indicating that the topology of this protein is similarly distinct from that of MRP and cMOAT, MOAT-D is predicted to encode a 1527-amino-acid protein that is the closest known relative of MRP (about 58% identity). MOAT-D is also highly related to cMOAT (abo ut 47% identity). The presence of an N-terminal membrane-spanning doma in indicates that the topology of MOAT-D is quite similar to that of M RP and cMOAT, MOAT-C transcripts are widely expressed in human tissues ; however, MOAT-D transcript expression is more restricted, The MOAT-C and MOAT-D genes are located at chromosomes 3q27 and 17q21.3, respect ively. Conclusions: On the basis of amino acid identity and protein to pology, the MRP/cMOAT transporter subfamily falls into two groups; the first group consists of MRP, cMOAT, and MOAT-D, and the second group consists of MOAT-B and MOAT-C.