Short MRP may not be short

Detailed structural analysis on both human MRP5 and mouse mrp5 transcripts

The multidrug-resistant phenotype in tumor cells is attributed in part to anti-cancer drug efflux transporters such as the MRP family. The amino-terminal structure of MRP5 has not been refined. To determine the amino-terminal structure of a major transcript of the MRP5 gene, we performed primer extension analysis to determine a major transcriptional start site of this gene and compared the structure of human MRP5 and that of mouse mrp5. We successfully determined the structures of human MRP5 and mouse mrp5. Estimated amino acid sequences are 1437 and 1436 amino acids for human MRP5 and mouse mrp5 respectively, and were highly conserved (94.1%). We further showed that our previously identified SMRP mRNA was a splicing variant of the MRP5 gene, which was expressed in various human tissues, suggesting that a short form of MRP5 protein encoded by the SMRP mRNA may have a physiological role.

An inventory of the human ABC proteins

Currently 30 human ABC proteins are represented by full sequences in various databases, and this paper provides a brief overview of these proteins. ABC proteins are composed of transmembrane domains (TMDs), and nucleotide binding domains (NBDs, or ATP-binding cassettes, ABSs). The arrangement of these domains, together with available membrane topology models of the family members, are presented. Based on their sequence similarity scores, the members of the human ABC protein family can be grouped into eight subfamilies. At present the MDR/TAP, the ALD, the MRP/CFTR, the ABC1, the White, the RNAseL inhibitor, the ANSA, and the GCN20 subfamilies are identified. Mutations of many human ABC proteins are known to be causative in inherited diseases, and a short description of the molecular pathology of these ABC gene-related genetic diseases is also provided.

cDNA cloning and inducible expression of human multidrug resistance associated protein 3 (MRP3)

Previously, we cloned rat MRP3 as a candidate for an inducible transporter for the biliary excretion of organic anions [Hirohashi et al. (1998) Mol. Pharmacol. 53, 1068-10751. In the present study, we cloned human MRP3 (1527 amino acids) from Caco-2 cells. Human MRP3 is predominantly expressed in liver, small intestine and colon; hepatic expression of MRP3 was observed in humans but not in normal rats. In HepG2 cells, the expression of MRP3 was induced by phenobarbital. These results suggest that MRP3 may act as an inducible transporter in the biliary and intestinal excretion of organic anions.