P-glycoprotein genes in the winter flounder, Pleuronectes americanus: isolation of two types of genomic clones carrying 3' terminal exons

Abstract

In mammals, P-glycoprotein (P-gp) is encoded by two or more highly conserved genes that differ in their abilities to transport drugs. One isoform class (class I) is consistently associated with the multidrug resistance phenotype, while the other (class III) is not. This study was designed to enumerate the P-gp genes in fish and determine how they are related to the two functional classes already defined in mammals. Southern blot analysis using a conserved single exon from the 3' terminal region of hamster P-gp cDNA (pEX1-172) as a probe indicated that there were two P-gp genes in right-eye flounders. Subsequently, two sets of clones were isolated from a winter flounder genomic library that correspond to the 3' ends of the two flounder P-gp genes. Sequence analysis was done on two key areas: the 3' ATP binding site and the 3' terminal exon, both of which were found to be homologous with their mammalian counterparts. Despite high levels of sequence identity in the predicted coding regions of the gene fragments it has not been possible to use these sequences to relate the homologs to particular mammalian classes of P-gp genes, perhaps because of gene conversion between mammalian P-gp genes. These cloned sequences are the first set of P-gp genes reported in lower vertebrates and will be useful for delineating the expression of P-gp genes in fish and understanding the role of P-gp in fish physiology.