Mutational analysis of the Lem3p-Dnf1p putative phospholipid-translocating P-type ATPase reveals novel regulatory roles for Lem3p and a carboxyl-terminal region of Dnf1p independent of the phospholipid-translocating activity of Dnf1p in yeast

Crossing the membrane - what does it take to flip a phospholipid? Structural and biochemical advances on P4-ATPase flippases

Membrane asymmetry is critical for maintenance of several different processes such as cell signalling, apoptosis, and vesicular transport in various eukaryotic systems. Flippases of the P4-ATPase family are associated with flipping phospholipids from the luminal or exoplasmic leaflet to the cytosolic leaflet. P4-ATPases belong to the P-type ATPase family, which are activated by phosphorylation and couple ATPase activity to substrate translocation. These proteins possess a transmembrane domain responsible for substrate transport, while the cytosolic machinery perform the necessary ATP hydrolysis for this process. Several high-resolution structures of human or yeast P4-ATPases have recently been resolved, but a comprehensive overview of the changes for reaction cycle in different members was crucial for future research. In this review, we have compiled available data reflecting the reaction cycle-associated changes in conformation of P4-ATPases. Together, this will provide an improved understanding of the similarities and differences between these members, which will drive further structural, functional and computational studies to understand the mechanisms of these flippases.

Flipping the script: Advances in understanding how and why P4-ATPases flip lipid across membranes

Type IV P-type ATPases (P4-ATPases) are a family of transmembrane enzymes that translocate lipid substrates from the outer to the inner leaflet of biological membranes and thus create an asymmetrical distribution of lipids within membranes. On the cellular level, this asymmetry is essential for maintaining the integrity and functionality of biological membranes, creating platforms for signaling events and facilitating vesicular trafficking. On the organismal level, this asymmetry has been shown to be important in maintaining blood homeostasis, liver metabolism, neural development, and the immune response. Indeed, dysregulation of P4-ATPases has been linked to several diseases; including anemia, cholestasis, neurological disease, and several cancers. This review will discuss the evolutionary transition of P4-ATPases from cation pumps to lipid flippases, the new lipid substrates that have been discovered, the significant advances that have been achieved in recent years regarding the structural mechanisms underlying the recognition and flipping of specific lipids across biological membranes, and the consequences of P4-ATPase dysfunction on cellular and physiological functions. Additionally, we emphasize the requirement for additional research to comprehensively understand the involvement of flippases in cellular physiology and disease and to explore their potential as targets for therapeutics in treating a variety of illnesses. The discussion in this review will primarily focus on the budding yeast, C. elegans, and mammalian P4-ATPases.

The transport mechanism of P4 ATPase lipid flippases

P4 ATPase lipid flippases are ATP-driven transporters that translocate specific lipids from the exoplasmic to the cytosolic leaflet of biological membranes, thus establishing a lipid gradient between the two leaflets that is essential for many cellular processes. While substrate specificity, subcellular and tissue-specific expression, and physiological functions have been assigned to a number of these transporters in several organisms, the mechanism of lipid transport has been a topic of intense debate in the field. The recent publication of a series of structural models based on X-ray crystallography and cryo-EM studies has provided the first glimpse into how P4 ATPases have adapted the transport mechanism used by the cation-pumping family members to accommodate a substrate that is at least an order of magnitude larger than cations.

An optogenetic system to control membrane phospholipid asymmetry through flippase activation in budding yeast

Lipid asymmetry in biological membranes is essential for various cell functions, such as cell polarity, cytokinesis, and apoptosis. P4-ATPases (flippases) are involved in the generation of such asymmetry. In Saccharomyces cerevisiae, the protein kinases Fpk1p/Fpk2p activate the P4-ATPases Dnf1p/Dnf2p by phosphorylation. Previously, we have shown that a blue-light-dependent protein kinase, phototropin from Chlamydomonas reinhardtii (CrPHOT), complements defects in an fpk1Δ fpk2Δ mutant. Herein, we investigated whether CrPHOT optically regulates P4-ATPase activity. First, we demonstrated that the translocation of NBD-labelled phospholipids to the cytoplasmic leaflet via P4-ATPases was promoted by blue-light irradiation in fpk1Δ fpk2Δ cells with CrPHOT. In addition, blue light completely suppressed the defects in membrane functions (such as endocytic recycling, actin depolarization, and apical-isotropic growth switching) caused by fpk1Δ fpk2Δ mutations. All responses required the kinase activity of CrPHOT. Hence, these results indicate the utility of CrPHOT as a powerful and first tool for optogenetic manipulation of P4-ATPase activity.

Role of lipid transporters in fungal physiology and pathogenicity

The fungal cell wall and membrane are the most common targets of antifungal agents, but the potential of membrane lipid organization in regulating drug-target interactions has yet to be investigated. Energy-dependent lipid transporters have been recently associated with virulence and drug resistance in many pathogenic fungi. To illustrate this view, we discuss (i) the structural and biological aspects of ATP-driven lipid transporters, comprising P-type ATPases and ATP-binding cassette transporters, (ii) the role of these transporters in fungal physiology and virulence, and (iii) the potential of lipid transporters as targets for the development of novel antifungals. These recent observations indicate that the lipid-trafficking machinery in fungi is a promising target for studies on physiology, pathogenesis and drug development.

Target of rapamycin complex 2-dependent phosphorylation of the coat protein Pan1 by Akl1 controls endocytosis dynamics in Saccharomyces cerevisiae

Target of rapamycin complex 2 (TORC2) is a widely conserved serine/threonine protein kinase. In the yeast Saccharomyces cerevisiae, TORC2 is essential, playing a key role in plasma membrane homeostasis. In this role, TORC2 regulates diverse processes, including sphingolipid synthesis, glycerol production and efflux, polarization of the actin cytoskeleton, and endocytosis. The major direct substrate of TORC2 is the AGC-family kinase Ypk1. Ypk1 connects TORC2 signaling to actin polarization and to endocytosis via the flippase kinases Fpk1 and Fpk2. Here, we report that Fpk1 mediates TORC2 signaling to control actin polarization, but not endocytosis, via aminophospholipid flippases. To search for specific targets of these flippase kinases, we exploited the fact that Fpk1 prefers to phosphorylate Ser residues within the sequence RXS(L/Y)(D/E), which is present ∼90 times in the yeast proteome. We observed that 25 of these sequences are phosphorylated by Fpk1 in vitro We focused on one sequence hit, the Ark/Prk-family kinase Akl1, as this kinase previously has been implicated in endocytosis. Using a potent ATP-competitive small molecule, CMB4563, to preferentially inhibit TORC2, we found that Fpk1-mediated Akl1 phosphorylation inhibits Akl1 activity, which, in turn, reduces phosphorylation of Pan1 and of other endocytic coat proteins and ultimately contributes to a slowing of endocytosis kinetics. These results indicate that the regulation of actin polarization and endocytosis downstream of TORC2 is signaled through separate pathways that bifurcate at the level of the flippase kinases.

Cfs1p, a Novel Membrane Protein in the PQ-Loop Family, Is Involved in Phospholipid Flippase Functions in Yeast

Type 4 P-type ATPases (P4-ATPases) function as phospholipid flippases, which translocate phospholipids from the exoplasmic leaflet to the cytoplasmic leaflet of the lipid bilayer, to generate and maintain asymmetric distribution of phospholipids at the plasma membrane and endosomal/Golgi membranes. The budding yeast Saccharomyces cerevisiae has four heteromeric flippases (Drs2p, Dnf1p, Dnf2p, and Dnf3p), associated with the Cdc50p family noncatalytic subunit, and one monomeric flippase, Neo1p. They have been suggested to function in vesicle formation in membrane trafficking pathways, but details of their mechanisms remain to be clarified. Here, to search for novel factors that functionally interact with flippases, we screened transposon insertional mutants for strains that suppressed the cold-sensitive growth defect in the cdc50Δ mutant. We identified a mutation of YMR010W encoding a novel conserved membrane protein that belongs to the PQ-loop family including the cystine transporter cystinosin and the SWEET sugar transporters. We named this gene CFS1 (cdc fifty suppressor 1). GFP-tagged Cfs1p was partially colocalized with Drs2p and Neo1p to endosomal/late Golgi membranes. Interestingly, the cfs1Δ mutation suppressed growth defects in all flippase mutants. Accordingly, defects in membrane trafficking in the flippase mutants were also suppressed. These results suggest that Cfs1p and flippases function antagonistically in membrane trafficking pathways. A growth assay to assess sensitivity to duramycin, a phosphatidylethanolamine (PE)-binding peptide, suggested that the cfs1Δ mutation changed PE asymmetry in the plasma membrane. Cfs1p may thus be a novel regulator of phospholipid asymmetry.

Role of post-translational modifications at the β-subunit ectodomain in complex association with a promiscuous plant P4-ATPase

P-type ATPases of subfamily IV (P4-ATPases) constitute a major group of phospholipid flippases that form heteromeric complexes with members of the Cdc50 (cell division control 50) protein family. Some P4-ATPases interact specifically with only one β-subunit isoform, whereas others are promiscuous and can interact with several isoforms. In the present study, we used a site-directed mutagenesis approach to assess the role of post-translational modifications at the plant ALIS5 β-subunit ectodomain in the functionality of the promiscuous plant P4-ATPase ALA2. We identified two N-glycosylated residues, Asn(181) and Asn(231) Whereas mutation of Asn(231) seems to have a small effect on P4-ATPase complex formation, mutation of evolutionarily conserved Asn(181) disrupts interaction between the two subunits. Of the four cysteine residues located in the ALIS5 ectodomain, mutation of Cys(86) and Cys(107) compromises complex association, but the mutant β-subunits still promote complex trafficking and activity to some extent. In contrast, disruption of a conserved disulfide bond between Cys(158) and Cys(172) has no effect on the P4-ATPase complex. Our results demonstrate that post-translational modifications in the β-subunit have different functional roles in different organisms, which may be related to the promiscuity of the P4-ATPase.

Protein kinase Gin4 negatively regulates flippase function and controls plasma membrane asymmetry

In yeast, the protein kinase Gin4 locally controls plasma membrane lipid asymmetry, which is necessary for optimal cytokinesis.

Plasma membrane function requires distinct leaflet lipid compositions. Two of the P-type ATPases (flippases) in yeast, Dnf1 and Dnf2, translocate aminoglycerophospholipids from the outer to the inner leaflet, stimulated via phosphorylation by cortically localized protein kinase Fpk1. By monitoring Fpk1 activity in vivo, we found that Fpk1 was hyperactive in cells lacking Gin4, a protein kinase previously implicated in septin collar assembly. Gin4 colocalized with Fpk1 at the cortical site of future bud emergence and phosphorylated Fpk1 at multiple sites, which we mapped. As judged by biochemical and phenotypic criteria, a mutant (Fpk1^11A), in which 11 sites were mutated to Ala, was hyperactive, causing increased inward transport of phosphatidylethanolamine. Thus, Gin4 is a negative regulator of Fpk1 and therefore an indirect negative regulator of flippase function. Moreover, we found that decreasing flippase function rescued the growth deficiency of four different cytokinesis mutants, which suggests that the primary function of Gin4 is highly localized control of membrane lipid asymmetry and is necessary for optimal cytokinesis.

Inositol depletion restores vesicle transport in yeast phospholipid flippase mutants

In eukaryotic cells, type 4 P-type ATPases function as phospholipid flippases, which translocate phospholipids from the exoplasmic leaflet to the cytoplasmic leaflet of the lipid bilayer. Flippases function in the formation of transport vesicles, but the mechanism remains unknown. Here, we isolate an arrestin-related trafficking adaptor, ART5, as a multicopy suppressor of the growth and endocytic recycling defects of flippase mutants in budding yeast. Consistent with a previous report that Art5p downregulates the inositol transporter Itr1p by endocytosis, we found that flippase mutations were also suppressed by the disruption of ITR1, as well as by depletion of inositol from the culture medium. Interestingly, inositol depletion suppressed the defects in all five flippase mutants. Inositol depletion also partially restored the formation of secretory vesicles in a flippase mutant. Inositol depletion caused changes in lipid composition, including a decrease in phosphatidylinositol and an increase in phosphatidylserine. A reduction in phosphatidylinositol levels caused by partially depleting the phosphatidylinositol synthase Pis1p also suppressed a flippase mutation. These results suggest that inositol depletion changes the lipid composition of the endosomal/TGN membranes, which results in vesicle formation from these membranes in the absence of flippases.

Plasma membrane aminoglycerolipid flippase function is required for signaling competence in the yeast mating pheromone response pathway

The class 4 P-type ATPases ("flippases") maintain membrane asymmetry by translocating phosphatidylethanolamine and phosphatidylserine from the outer leaflet to the cytosolic leaflet of the plasma membrane. In Saccharomyces cerevisiae, five related gene products (Dnf1, Dnf2, Dnf3, Drs2, and Neo1) are implicated in flipping of phosphatidylethanolamine, phosphatidylserine, and phosphatidylcholine. In MAT A: cells responding to α-factor, we found that Dnf1, Dnf2, and Dnf3, as well as the flippase-activating protein kinase Fpk1, localize at the projection ("shmoo") tip where polarized growth is occurring and where Ste5 (the central scaffold protein of the pheromone-initiated MAPK cascade) is recruited. Although viable, a MAT A: dnf1∆ dnf2∆ dnf3∆ triple mutant exhibited a marked decrease in its ability to respond to α-factor, which we could attribute to pronounced reduction in Ste5 stability resulting from an elevated rate of its Cln2⋅Cdc28-initiated degradation. Similarly, a MAT A: dnf1∆ dnf3∆ drs2∆ triple mutant also displayed marked reduction in its ability to respond to α-factor, which we could attribute to inefficient recruitment of Ste5 to the plasma membrane due to severe mislocalization of the cellular phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate pools. Thus proper remodeling of plasma membrane aminoglycerolipids and phosphoinositides is necessary for efficient recruitment, stability, and function of the pheromone signaling apparatus.

Opt2 mediates the exposure of phospholipids during cellular adaptation to altered lipid asymmetry

Plasma membrane lipid asymmetry is important for various membrane-associated functions and is regulated by membrane proteins termed flippases and floppases. The Rim101 pathway senses altered lipid asymmetry in the yeast plasma membrane. The mutant lem3Δ cells, in which lipid asymmetry is disturbed owing to the inactivation of the plasma membrane flippases, showed a severe growth defect when the Rim101 pathway was impaired. To identify factors involved in the Rim101-pathway-dependent adaptation to altered lipid asymmetry, we performed DNA microarray analysis and found that Opt2 induced by the Rim101 pathway plays an important role in the adaptation to altered lipid asymmetry. Biochemical investigation of Opt2 revealed its localization to the plasma membrane and the Golgi, and provided several lines of evidence for the Opt2-mediated exposure of phospholipids. In addition, Opt2 was found to be required for the maintenance of vacuolar morphology and polarized cell growth. These results suggest that Opt2 is a novel factor involved in cell homeostasis by regulating lipid asymmetry.

Role of phosphatidylserine in phospholipid flippase-mediated vesicle transport in Saccharomyces cerevisiae

Phospholipid flippases translocate phospholipids from the exoplasmic to the cytoplasmic leaflet of cell membranes to generate and maintain phospholipid asymmetry. The genome of budding yeast encodes four heteromeric flippases (Drs2p, Dnf1p, Dnf2p, and Dnf3p), which associate with the Cdc50 family noncatalytic subunit, and one monomeric flippase Neo1p. Flippases have been implicated in the formation of transport vesicles, but the underlying mechanisms are largely unknown. We show here that overexpression of the phosphatidylserine synthase gene CHO1 suppresses defects in the endocytic recycling pathway in flippase mutants. This suppression seems to be mediated by increased cellular phosphatidylserine. Two models can be envisioned for the suppression mechanism: (i) phosphatidylserine in the cytoplasmic leaflet recruits proteins for vesicle formation with its negative charge, and (ii) phosphatidylserine flipping to the cytoplasmic leaflet induces membrane curvature that supports vesicle formation. In a mutant depleted for flippases, a phosphatidylserine probe GFP-Lact-C2 was still localized to endosomal membranes, suggesting that the mere presence of phosphatidylserine in the cytoplasmic leaflet is not enough for vesicle formation. The CHO1 overexpression did not suppress the growth defect in a mutant depleted or mutated for all flippases, suggesting that the suppression was dependent on flippase-mediated phospholipid flipping. Endocytic recycling was not blocked in a mutant lacking phosphatidylserine or depleted in phosphatidylethanolamine, suggesting that a specific phospholipid is not required for vesicle formation. These results suggest that flippase-dependent vesicle formation is mediated by phospholipid flipping, not by flipped phospholipids.

Circulating biologically active oxidized phospholipids show on-going and increased oxidative stress in older male mice

Background

The biologically active phospholipids Platelet-activating Factor (PAF) and oxidatively truncated phospholipids from chemical oxidation are increased in the circulation of rats subject to the oxidant stress of chronic ethanol ingestion. Potentially, circulating inflammatory and apoptotic phospholipids correlate to physiologic oxidative stress.

Results

PAF and the common oxidatively truncated and biologically active phospholipid azelaoyl phosphatidylcholine (Az-PC) were significantly increased in the plasma of older mice, and in male mice. PAF and Az-PC are very rapidly cleared from the circulation, which was unaffected by age or sex. Platelets exposed to Az-PC display phosphatidylserine on their surface, and occlusive platelet carotid arterial thrombosis is enhanced by aging.

Conclusion

Biologically active phospholipids vary in the circulation, with the highest levels being found in older, male mice. Turnover of PAF and the biologically active Az-PC are rapid and are invariant with age and sex, so increased production accounts for the increased concentration and flux of both lipids. Platelets are exposed to plasma Az-PC that depolarizes their mitochondria to increase pro-thrombotic phosphatidylserine expression, and occlusive platelet thrombosis is enhanced in aged mice.

Significance

Oxidatively modified phospholipids are increased in the circulation during common, mild oxidant stresses of aging, or in male compared to female animals. Turnover of these biologically active phospholipids by rapid transport into liver and kidney is unchanged, so circulating levels reflect continuously increased production.

Mapping functional interactions in a heterodimeric phospholipid pump

Type 4 P-type ATPases (P(4)-ATPases) catalyze phospholipid transport to generate phospholipid asymmetry across membranes of late secretory and endocytic compartments, but their kinship to cation-transporting P-type transporters raised doubts about whether P(4)-ATPases alone are sufficient to mediate flippase activity. P(4)-ATPases form heteromeric complexes with Cdc50 proteins. Studies of the enzymatic properties of purified P(4)-ATPase·Cdc50 complexes showed that catalytic activity depends on direct and specific interactions between Cdc50 subunit and transporter, whereas in vivo interaction assays suggested that the binding affinity for each other fluctuates during the transport reaction cycle. The structural determinants that govern this dynamic association remain to be established. Using domain swapping, site-directed, and random mutagenesis approaches, we here show that residues throughout the subunit contribute to forming the heterodimer. Moreover, we find that a precise conformation of the large ectodomain of Cdc50 proteins is crucial for the specificity and functionality to transporter/subunit interactions. We also identified two highly conserved disulfide bridges in the Cdc50 ectodomain. Functional analysis of cysteine mutants that disrupt these disulfide bridges revealed an inverse relationship between subunit binding and P(4)-ATPase-catalyzed phospholipid transport. Collectively, our data indicate that a dynamic association between subunit and transporter is crucial for the transport reaction cycle of the heterodimer.

Functions of phospholipid flippases

Asymmetrical distribution of phospholipids is generally observed in the eukaryotic plasma membrane. Maintenance and changes of this phospholipid asymmetry are regulated by ATP-driven phospholipid translocases. Accumulating evidence indicates that type 4 P-type ATPases (P4-ATPases, also called flippases) translocate phospholipids from the exoplasmic leaflet to the cytoplasmic leaflet of the plasma membrane and internal membranes. Among P-type ATPases, P4-ATPases are unique in that they are associated with a conserved membrane protein of the Cdc50 family as a non-catalytic subunit. Recent studies indicate that flippases are involved in various cellular functions, including transport vesicle formation and cell polarity. In this review, we will focus on the functional aspect of phospholipid flippases.

Biochemical and cellular functions of P4 ATPases

P4 ATPases (subfamily IV P-type ATPases) form a specialized subfamily of P-type ATPases and have been implicated in phospholipid translocation from the exoplasmic to the cytoplasmic leaflet of biological membranes. Pivotal roles of P4 ATPases have been demonstrated in eukaryotes, ranging from yeast, fungi and plants to mice and humans. P4 ATPases might exert their cellular functions by combining enzymatic phospholipid translocation activity with an enzyme-independent action. The latter could be involved in the timely recruitment of proteins involved in cellular signalling, vesicle coat assembly and cytoskeleton regulation. In the present review, we outline the current knowledge of the biochemical and cellular functions of P4 ATPases in the eukaryotic membrane.

Heteromeric interactions required for abundance and subcellular localization of human CDC50 proteins and class 1 P4-ATPases

Members of the P(4) family of P-type ATPases (P(4)-ATPases) are believed to function as phospholipid flippases in complex with CDC50 proteins. Mutations in the human class 1 P(4)-ATPase gene ATP8B1 cause a severe syndrome characterized by impaired bile flow (intrahepatic cholestasis), often leading to end-stage liver failure in childhood. In this study, we determined the specificity of human class 1 P(4)-ATPase interactions with CDC50 proteins and the functional consequences of these interactions on protein abundance and localization of both protein classes. ATP8B1 and ATP8B2 co-immunoprecipitated with CDC50A and CDC50B, whereas ATP8B4, ATP8A1, and ATP8A2 associated only with CDC50A. ATP8B1 shifted from the endoplasmic reticulum (ER) to the plasma membrane upon coexpression of CDC50A or CDC50B. ATP8A1 and ATP8A2 translocated from the ER to the Golgi complex and plasma membrane upon coexpression of CDC50A, but not CDC50B. ATP8B2 and ATP8B4 already displayed partial plasma membrane localization in the absence of CDC50 coexpression but displayed a large increase in plasma membrane abundance upon coexpression of CDC50A. ATP8B3 did not bind CDC50A and CDC50B and was invariably present in the ER. Our data show that interactions between CDC50 proteins and class 1 P(4)-ATPases are essential for ER exit and stability of both subunits. Furthermore, the subcellular localization of the complex is determined by the P(4)-ATPase, not the CDC50 protein. The interactions of CDC50A and CDC50B with multiple members of the human P(4)-ATPase family suggest that these proteins perform broader functions in human physiology than thus far assumed.

The E646D-ATP13A4 mutation associated with autism reveals a defect in calcium regulation

ATP13A4 is a member of the subfamily of P5-type ATPases. P5-type ATPases are the least studied of the P-type ATPase subfamilies with no ion specificities assigned to them. In order to elucidate ATP13A4 function, we studied the protein's subcellular localization and tested whether it is involved in calcium regulation. The intracellular calcium concentration was measured in COS-7 cells over-expressing mouse ATP13A4 using ratiometric calcium imaging with fura-2 AM as a calcium indicator. The results of this study show that ATP13A4 is localized to the endoplasmic reticulum (ER). Furthermore, we demonstrate that over-expression of ATP13A4 in COS-7 cells caused a significant increase in the intracellular calcium level. Interestingly, over-expression of the sequence variant containing a substitution of aspartic acid for a glutamic acid (E646D), previously found in patients with autism spectrum disorder (ASD), did not increase the free cellular calcium likely due to the mutation. In this study, we also describe the expression of ATP13A4 during mouse embryonic development. Quantitative real-time PCR revealed that ATP13A4 was highly expressed at embryonic days 15-17, when neurogenesis takes place. The present study is the first to provide further insights into the biological role of a P5-type ATPase. Our results demonstrate that ATP13A4 may be involved in calcium regulation and that its expression is developmentally regulated. Overall, this study provides support for the hypothesis that ATP13A4 may play a vital role in the developing nervous system and its impairment can contribute to the symptoms seen in ASD.

Incorporation and remodeling of phosphatidylethanolamine containing short acyl residues in yeast

Phosphatidylethanolamine (PE) is one of the essential phospholipids in the yeast Saccharomyces cerevisiae. We have previously shown that a yeast strain, the endogenous PE synthesis of which was controllable, grew in the presence of PE containing decanoyl residues (diC10PE) when PE synthesis was repressed. In this study, we investigated the fate of diC10PE, its uptake and remodeling in yeast. Deletion of the genes encoding Lem3p/Ros3p or P-type ATPases, Dnf1p and Dnf2p, impaired the growth of the mutants in the medium containing diC10PE, suggesting the involvement of these proteins in the uptake of diC10PE. Analysis of the metabolism of deuterium-labeled diC10PE by electrospray ionization tandem mass spectrometry revealed that it was rapidly converted to deuterium-labeled PEs containing C16 or C18 acyl residues. The probable intermediate PEs that contained decanoic acid and C16 or C18 fatty acids as acyl residues were also detected. In addition, a substantial amount of decanoic acid was released into the culture medium during growth in the presence of diC10PE. These results imply that diC10PE was remodeled to PEs with longer acyl residues and used as membrane components. Defects in the remodeling of diC10PE in the deletion mutants of ALE1 and SLC1, products of which were capable of acyl-transfer to the sn-2 position of lyso-phospholipids, suggested their involvement in the introduction of acyl residues to the sn-2 position of lyso-phosphatidylethanolamine in the remodeling reaction of diC10PE. Our results also suggest the presence of a mechanism to maintain the physiological length of PE acyl residues in yeast.

FIC1-mediated stimulation of FXR activity is decreased with PFIC1 mutations in HepG2 cells

PURPOSE

Progressive familial intrahepatic cholestasis type 1 (PFIC1) is a specific form of genetic cholestasis caused by functional defects in FIC1/ATP8B1. Although the way FIC1 deficiency leads to PFIC1 remains unclear, some reports suggest that the loss of FIC1 function results in decreased activity of the farnesoid X receptor (FXR) in PFIC1 patients. In this study, in order to elucidate the molecular mechanism of the pathogenesis of PFIC1, we constructed an experimental system for the evaluation of FIC1-mediated stimulatory effects on FXR activity.

METHODS AND RESULTS

Luciferase assays revealed that FIC1 expression increased FXR-dependent transcription and that the effects of three PFIC1 mutants (G308V, T456M and D554N) were smaller than that of wild-type FIC1. In addition, the PFIC1 mutants could not locate to the plasma membrane even in the presence of CDC50A, which brings wild-type FIC1 to the plasma membrane. The results of coprecipitation assays suggested a defect in the ability of the PFIC1 mutants to interact with CDC50A. Furthermore, it was revealed that the expression of CDC50A elevated the FIC1-mediated transcriptional stimulation when coexpressed with wild-type FIC1, but not with mutated FIC1.

CONCLUSIONS

These results suggest that the PFIC1 mutants have a lower stimulatory effect on FXR activity and cannot interact with CDC50A, which may lead to the development of the features of PFIC1.

ABC transporter Pdr10 regulates the membrane microenvironment of Pdr12 in Saccharomyces cerevisiae

The eukaryotic plasma membrane exhibits both asymmetric distribution of lipids between the inner and the outer leaflet and lateral segregation of membrane components within the plane of the bilayer. In budding yeast (Saccharomyces cerevisiae), maintenance of leaflet asymmetry requires P-type ATPases, which are proposed to act as inward-directed lipid translocases (Dnf1, Dnf2, and the associated protein Lem3), and ATP-binding cassette (ABC) transporters, which are proposed to act as outward-directed lipid translocases (Pdr5 and Yor1). The S. cerevisiae genome encodes two other Pdr5-related ABC transporters: Pdr10 (67% identity) and Pdr15 (75% identity). We report the first analysis of Pdr10 localization and function. A Pdr10-GFP chimera was located in discrete puncta in the plasma membrane and was found in the detergent-resistant membrane fraction. Compared to control cells, a pdr10 mutant was resistant to sorbate but hypersensitive to the chitin-binding agent Calcofluor White. Calcofluor sensitivity was attributable to a partial defect in endocytosis of the chitin synthase Chs3, while sorbate resistance was attributable to accumulation of a higher than normal level of the sorbate exporter Pdr12. Epistasis analysis indicated that Pdr10 function requires Pdr5, Pdr12, Lem3, and mature sphingolipids. Strikingly, Pdr12 was shifted to the detergent-resistant membrane fraction in pdr10 cells. Pdr10 therefore acts as a negative regulator for incorporation of Pdr12 into detergent-resistant membranes, a novel role for members of the ABC transporter superfamily.

P4 ATPases - lipid flippases and their role in disease

P4 ATPases (type 4 P-type ATPases) are multispan transmembrane proteins that have been implicated in phospholipid translocation from the exoplasmic to the cytoplasmic leaflet of biological membranes. Studies in Saccharomyces cerevisiae have indicated that P4 ATPases are important in vesicle biogenesis and are required for vesicular trafficking along several intracellular vesicular transport routes. Although little is known about mammalian P4 ATPases, some members of this subfamily appear to be associated with human disease or mouse pathophysiology. ATP8B1, a phosphatidylserine translocase, is the most extensively studied mammalian P4 ATPase. This protein is important for maintaining the detergent resistant properties of the apical membrane of the hepatocyte. Mutations in ATP8B1 give rise to severe liver disease. Furthermore, a role for Atp8b3 in mouse sperm cell capacitation has been suggested, whereas deficiency of Atp10a and Atp10d leads to insulin resistance and obesity in mice. Here we review the present status on the pathophysiological consequences of P4 ATPase deficiency.

Protein kinases Fpk1p and Fpk2p are novel regulators of phospholipid asymmetry

Type 4 P-type ATPases (flippases) are implicated in the generation of phospholipid asymmetry in membranes by the inward translocation of phospholipids. In budding yeast, the DRS2/DNF family members Lem3p-Dnf1p/Dnf2p and Cdc50p-Drs2p are putative flippases that are localized, respectively, to the plasma membrane and endosomal/trans-Golgi network (TGN) compartments. Herein, we identified a protein kinase gene, FPK1, as a mutation that exhibited synthetic lethality with the cdc50Delta mutation. The kinase domain of Fpk1p exhibits high homology to plant phototropins and the fungus Neurospora crassa NRC-2, both of which have membrane-associated functions. Simultaneous disruption of FPK1 and its homolog FPK2 phenocopied the lem3Delta/dnf1Delta dnf2Delta mutants, exhibiting the impaired NBD-labeled phospholipid uptake, defects in the early endosome-to-TGN pathway in the absence of CDC50, and hyperpolarized bud growth after exposure of phosphatidylethanolamine at the bud tip. The fpk1Delta fpk2Delta mutation did not affect the subcellular localization of Lem3p-Dnf1p or Lem3p-Dnf2p. Further, the purified glutathione S-transferase (GST)-fused kinase domain of Fpk1p phosphorylated immunoprecipitated Dnf1p and Dnf2p to a greater extent than Drs2p. We propose that Fpk1p/Fpk2p are upstream activating protein kinases for Lem3p-Dnf1p/Dnf2p.

ATP8B1 requires an accessory protein for endoplasmic reticulum exit and plasma membrane lipid flippase activity

Mutations in ATP8B1 cause progressive familial intrahepatic cholestasis type 1 and benign recurrent intrahepatic cholestasis type 1. Previously, we have shown in mice that Atp8b1 deficiency leads to enhanced biliary excretion of phosphatidylserine, and we hypothesized that ATP8B1 is a flippase for phosphatidylserine. However, direct evidence for this function is still lacking. In Saccharomyces cerevisiae, members of the Cdc50p/Lem3p family are essential for proper function of the ATP8B1 homologs. We have studied the role of two human members of this family, CDC50A and CDC50B, in the routing and activity of ATP8B1. When only ATP8B1 was expressed in Chinese hamster ovary cells, the protein localized to the endoplasmic reticulum. Coexpression with CDC50 proteins resulted in relocalization of ATP8B1 from the endoplasmic reticulum to the plasma membrane. Only when ATP8B1 was coexpressed with CDC50 proteins was a 250%-500% increase in the translocation of fluorescently labeled phosphatidylserine observed. Importantly, natural phosphatidylserine exposure in the outer leaflet of the plasma membrane was reduced by 17%-25% in cells coexpressing ATP8B1 and CDC50 proteins in comparison with cells expressing ATP8B1 alone. The coexpression of ATP8B1 and CDC50A in WIF-B9 cells resulted in colocalization of both proteins in the canalicular membrane.

CONCLUSION

Our data indicate that CDC50 proteins are pivotal factors in the trafficking of ATP8B1 to the plasma membrane and thus may be essential determinants of ATP8B1-related disease. In the plasma membrane, ATP8B1 functions as a flippase for phosphatidylserine. Finally, CDC50A may be the potential beta-subunit or chaperone for ATP8B1 in hepatocytes.

Characterization and expression of mouse Cdc50c during spermatogenesis

Cdc50p is a transmembrane protein required for polarized growth in Saccharomyces cerevisiae. The knowledge on physiological functions of its mammalian homologs, however, is limited. Using database analysis, we identified one mouse testis expressed sequence tag, named Cdc50c, encoding a previously uncharacterized homolog of Cdc50p. Similar to yeast Cdc50p, the putative Cdc50c protein contains three transmembrane spanning regions. Its orthologs are present in many species such as fish, avian and human, suggesting its evolutionary conservation. In multitissue reverse transcription-polymerase chain reaction analyses the mRNA for Cdc50c was predominately detected in testis. The onset of the gene expression coincides with the first appearance of spermatocytes during testicular development. In situ hybridization analyses revealed that Cdc50c mRNA localized in pachytene spermatocytes and round and elongated spermatids. Our data suggest that Cdc50c might play important roles during spermatogenesis.