The Saccharomyces cerevisiae genome encodes 15 fullsize ATP binding cassette transporters (ABC), of which PDR5, SNQ2, and YOR1 are known to be regulated by the transcription factors Pdr1p and Pdr3p (pleiotropic drug resistance). We have identified two new ABC transporter-encoding genes, PDR10 and PDR15, which were upregulated by the PDR1-3 mutation. These genes, as well as four other ABC transporter-encoding genes, were deleted in order to study the properties of Yor1p. The PDR1-3 gain-of-function mutant was then used to overproduce Yor1p up to 10% of the total plasma membrane proteins. Despite their different topologies, both Yor1p and Pdr5p mediated the ATP-dependent translocation of similar drugs and phospholipids across the yeast cell membrane. Both ABC transporters exhibit ATP hydrolysis in vitro, but Pdr5p ATPase activity is about 15 times higher than that of Yor1p, which may indicate mechanistic or regulatory differences between the two enzymes. The yeast YOR11 gene confers oligomycin resistance on overexpression in a 2-m plasmid (1). Its nucleotide sequence reveals an ORF of 1477 amino acids encoding an ABC protein highly homologous to mammalian transporters such as the multidrug resistance-conferring enzyme MRP (BLAST (see Ref. 2) sequence homology score: p ϭ e Ϫ228 ), the organic anion transporter cMOAT (p ϭ e Ϫ216 ), the sulfonylurea receptor (p ϭ e Ϫ164 ), and the cystic fibrosis transmembrane conductance regulator CFTR (p ϭ e Ϫ132 ). Yor1p is a "full-size" ABC transporter with the topology (TM-NBF) 2 (3, 4). It consists of two homologous halves, with each containing a putative ATP-binding domain (NBF) and a transmembrane domain of six membrane spans (TM). Cui et al. (5) showed that Yor1p confers resistance to a series of drugs including reveromycin A and suggested that Yor1p may be involved in the cellular efflux of organic anions including the fluorescent dye rhodamine B. They also showed that incubation with reveromycin A increases the YOR1 mRNA level. The transcription of YOR1 is controlled by the homologous pair of transcription factors Pdr1p/Pdr3p. The level of YOR1 transcription is decreased by the deletion of either PDR1 or PDR3 and increased in the presence of the gain-of-function PDR1 alleles (1).In this paper, we have investigated the transport activity of Yor1p. Building on previous studies, which indicated that the (TM-NBF) 2 -type Yor1p, together with the (NBF-TM) 2 -type Pdr5p and Snq2p ABC transporters, are overexpressed in the PDR1-3 mutant plasma membrane (6 -8), the PDR1-3 mutant has been used as a tool that enhances the Yor1p protein level. As another investigative tool, we constructed a set of isogenic strains, in the PDR1-3 mutant, with multiple deletions of homologous ABC genes since, in situations where two or more proteins located in the same subcellular compartment share a common substrate, a clear phenotype is only seen when all the corresponding genes are deleted, as illustrated by the work of Mahé et al. (9), who showed that Pdr5p and Snq2p have an overlapping transport ...

The fluorescent phospholipid 1-acyl-2-[12-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]dodecanoyl]phosphatidylcholine (C12-NBD-PC) was used to study the kinetics of lipid transfer between phospholipid vesicles. A model based on lipid transfer resulting from the diffusion of soluble monomers was found to accurately predict the kinetics of this transfer process. From these studies, we conclude that (i) C12-NBD-PC transfer between vesicles results from the diffusion of soluble monomers and not from vesicle collision, (ii) the rate at which a lipid molecule enters or leaves a bilayer is dependent upon both its molecular structure and the characteristics of the donor and acceptor bilayers, and (iii) under the appropriate conditions, either the rate of lipid association or dissociation from the bilayer or a combination of both may determine the rate of transfer.

No abstract

The alkylphosphocholine class of drugs, including edelfosine and miltefosine, has recently shown promise in the treatment of protozoal and fungal diseases, most notably, leishmaniasis. One of the major barriers to successful treatment of these infections is the development of drug resistance. To understand better the mechanisms underlying the development of drug resistance, we performed a combined mutant selection and screen in Saccharomyces cerevisiae, designed to identify genes that confer resistance to the alkylphosphocholine drugs by inhibiting their transport across the plasma membrane. Mutagenized cells were first selected for resistance to edelfosine, and the initial collection of mutants was screened a second time for defects in internalization of a short chain, fluorescent (7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD))-labeled phosphatidylcholine reporter. This approach identified mutations in a single gene, YNL323W/LEM3, that conferred resistance to alkylphosphocholine drugs and inhibited internalization of NBD-labeled phosphatidylcholine. Loss of YNL323W/ LEM3 does not confer resistance to N-nitroquinilone Noxide or ketoconazole and actually increases sensitivity to cycloheximide. The defect in internalization is specific to NBD-labeled phosphatidylcholine and phosphatidylethanolamine. Labeled phosphatidylserine is internalized at normal levels in lem3 strains. LEM3 is a member of an evolutionarily conserved family and has two homologues in S. cerevisiae. Single point mutations that produce resistance to alkylphosphocholine drugs and inhibition of NBD-labeled phosphatidylcholine internalization were identified in several highly conserved domains. These data demonstrate a requirement for Lem3p expression for normal phosphatidylcholine and alkylphosphocholine drug transport across the plasma membrane of yeast.

The transcription regulators, PDR1 and PDR3, have been shown to activate the transcription of numerous genes involved in a wide range of functions, including resistance to physical and chemical stress, membrane transport, and organelle function in Saccharomyces cerevisiae. We report here that PDR1 and PDR3 also regulate the transcription of one or more undetermined genes that translocate endogenous and fluorescent-labeled (M-C6-NBD-PE) phosphatidylethanolamine across the plasma membrane. A combination of fluorescence microscopy, fluorometry, and quantitative analysis demonstrated that M-C6-NBD-PE can be translocated both inward and outward across the plasma membrane of yeast cells. Mutants, defective in the accumulation of M-C6-NBD-PE, were isolated by selectively photokilling normal cells that accumulated the fluorescent phospholipid. This led to the isolation of numerous trafficking in phosphatidylethanolamine (tpe) mutants that were defective in intracellular accumulation of M-C6-NBD-PE. Complementation cloning and linkage analysis led to the identification of the dominant mutation TPE1-1 as a new allele of PDR1 and the semidominant mutation tpe2-1 as a new allele of PDR3. The amount of endogenous phosphatidylethanolamine exposed to the outer leaflet of the plasma membrane was measured by covalent labeling with the impermeant amino reagent, trinitrobenzenesulfonic acid. The amount of outer leaflet phosphatidylethanolamine in both mutant strains increased four- to fivefold relative to the parent Tpe+ strain, indicating that the net inward flux of endogenous phosphatidylethanolamine as well as M-C6-NBD-PE was decreased. Targeted deletions of PDR1 in the new allele, PDR1-11, and PDR3 in the new allele, pdr3-11, resulted in normal M-C6-NBD-PE accumulation, confirming that PDR1-11 and pdr3-11 were gain-of-function mutations in PDR1 and PDR3, respectively. Both mutant alleles resulted in resistance to the drugs cycloheximide, oligomycin, and 4-nitroquinoline N-oxide (4-NQO). However, a previously identified drug-resistant allele, pdr3-2, accumulated normal amounts of M-C6-NBD-PE, indicating allele specificity for the loss of M-C6-NBD-PE accumulation. These data demonstrated that PDR1 and PDR3 regulate the net rate of M-C6-NBD-PE translocation (flip-flop) and the steady-state distribution of endogenous phosphatidylethanolamine across the plasma membrane.