BFR1(+), A NOVEL GENE OF SCHIZOSACCHAROMYCES-POMBE WHICH CONFERS BREFELDIN-A RESISTANCE, IS STRUCTURALLY RELATED TO THE ATP-BINDING CASSETTE SUPERFAMILY

We have isolated a Schizosaccharomyces pombe gene, bfr1(+), which on a multicopy plasmid vector, pDB248', confers resistance to brefeldin A (BFA), an inhibitor of intracellular protein transport. This gene encodes a novel protein of 1,531 amino acids with an intramolecular duplicated structure, each half containing a single ATP-binding consensus sequence and a set of six transmembrane sequences. This structural characteristic of bfr1(+) protein resembles that of mammalian P-glycoprotein, which, by exporting a variety of anticancer drugs, has been shown to be responsible for multidrug resistance in tumor cells, Consistent with this is that S. pombe cells harboring bfr1(+) on pDB248' are resistant to actinomycin D, cerulenin, and cytochalasin B, as well as to BFA. The relative positions of the ATP-binding sequences and the clusters of transmembrane sequences within the bfr1(+) protein are, however, transposed in comparison with those in P-glycoprotein; the bfr1(+) protein has N-terminal ATP-binding sequence followed by transmembrane segments in each half of the molecule, The bfr1(+) protein exhibited significant homology in primary and secondary structures with two recently identified multidrug resistance gene products of Saccharomyces cerevisiae, Snq2 and Sts1/Pdr5/Ydr1. The bfr1(+) gene is not essential for cell growth or mating, but a Delta bfr1 mutant exhibited hypersensitivity to BFA, We propose that the bfr1(+) protein is another member of the ATP-binding cassette superfamily and serves as an efflux pump of various antibiotics.