An unexpectedly high degree of specialization and a widespread involvement in sterol metabolism among the C. elegans putative aminophospholipid translocases

Towards a Unitary Hypothesis of Alzheimer's Disease Pathogenesis

The "amyloid cascade" hypothesis of Alzheimer's disease (AD) pathogenesis invokes the accumulation in the brain of plaques (containing the amyloid-β protein precursor [AβPP] cleavage product amyloid-β [Aβ]) and tangles (containing hyperphosphorylated tau) as drivers of pathogenesis. However, the poor track record of clinical trials based on this hypothesis suggests that the accumulation of these peptides is not the only cause of AD. Here, an alternative hypothesis is proposed in which the AβPP cleavage product C99, not Aβ, is the main culprit, via its role as a regulator of cholesterol metabolism. C99, which is a cholesterol sensor, promotes the formation of mitochondria-associated endoplasmic reticulum (ER) membranes (MAM), a cholesterol-rich lipid raft-like subdomain of the ER that communicates, both physically and biochemically, with mitochondria. We propose that in early-onset AD (EOAD), MAM-localized C99 is elevated above normal levels, resulting in increased transport of cholesterol from the plasma membrane to membranes of intracellular organelles, such as ER/endosomes, thereby upregulating MAM function and driving pathology. By the same token, late-onset AD (LOAD) is triggered by any genetic variant that increases the accumulation of intracellular cholesterol that, in turn, boosts the levels of C99 and again upregulates MAM function. Thus, the functional cause of AD is upregulated MAM function that, in turn, causes the hallmark disease phenotypes, including the plaques and tangles. Accordingly, the MAM hypothesis invokes two key interrelated elements, C99 and cholesterol, that converge at the MAM to drive AD pathogenesis. From this perspective, AD is, at bottom, a lipid disorder.

PGP-14 establishes a polar lipid permeability barrier within the C. elegans pharyngeal cuticle

The cuticles of ecdysozoan animals are barriers to material loss and xenobiotic insult. Key to this barrier is lipid content, the establishment of which is poorly understood. Here, we show that the p-glycoprotein PGP-14 functions coincidently with the sphingomyelin synthase SMS-5 to establish a polar lipid barrier within the pharyngeal cuticle of the nematode C. elegans. We show that PGP-14 and SMS-5 are coincidentally expressed in the epithelium that surrounds the anterior pharyngeal cuticle where PGP-14 localizes to the apical membrane. pgp-14 and sms-5 also peak in expression at the time of new cuticle synthesis. Loss of PGP-14 and SMS-5 dramatically reduces pharyngeal cuticle staining by Nile Red, a key marker of polar lipids, and coincidently alters the nematode's response to a wide-range of xenobiotics. We infer that PGP-14 exports polar lipids into the developing pharyngeal cuticle in an SMS-5-dependent manner to safeguard the nematode from environmental insult.

LINKIN-associated proteins necessary for tissue integrity during collective cell migration

Cell adhesion plays essential roles in almost every aspect of metazoan biology. LINKIN (Human: ITFG1, Caenorhabditis elegans: lnkn-1) is a conserved transmembrane protein that has been identified to be necessary for tissue integrity during migration. In C. elegans, loss of lnkn-1 results in the detachment of the lead migratory cell from the rest of the developing male gonad. Previously, three interactors of ITFG1/lnkn-1 - RUVBL1/ruvb-1, RUVBL2/ruvb-2, and alpha-tubulin - were identified by immunoprecipitation-mass spectrometry (IP-MS) analysis using human HEK293T cells and then validated in the nematode male gonad. The ITFG1-RUVBL1 interaction has since been independently validated in a breast cancer cell line model that also implicates the involvement of the pair in metastasis. Here, we showed that epitope-tagged ITFG1 localized to the cell surface of MDA-MB-231 breast cancer cells. Using IP-MS analysis, we identified a new list of potential interactors of ITFG1. Loss-of-function analysis of their C. elegans orthologs found that three of the interactors - ATP9A/tat-5, NME1/ndk-1, and ANAPC2/apc-2 - displayed migratory detachment phenotypes similar to that of lnkn-1. Taken together with the other genes whose reduction-of-function phenotype is similar to that of lnkn-1 (notably cohesion and condensin), suggests the involvement of membrane remodeling and chromosome biology in LINKIN-dependent cell adhesion and supports the hypothesis for a structural role of chromosomes in post-mitotic cells.

ATP9A deficiency causes ADHD and aberrant endosomal recycling via modulating RAB5 and RAB11 activity

ATP9A, a lipid flippase of the class II P4-ATPases, is involved in cellular vesicle trafficking. Its homozygous variants are linked to neurodevelopmental disorders in humans. However, its physiological function, the underlying mechanism as well as its pathophysiological relevance in humans and animals are still largely unknown. Here, we report two independent families in which the nonsense mutations c.433C>T/c.658C>T/c.983G>A (p. Arg145*/p. Arg220*/p. Trp328*) in ATP9A (NM_006045.3) cause autosomal recessive hypotonia, intellectual disability (ID) and attention deficit hyperactivity disorder (ADHD). Atp9a null mice show decreased muscle strength, memory deficits and hyperkinetic movement disorder, recapitulating the symptoms observed in patients. Abnormal neurite morphology and impaired synaptic transmission are found in the primary motor cortex and hippocampus of the Atp9a null mice. ATP9A is also required for maintaining neuronal neurite morphology and the viability of neural cells in vitro. It mainly localizes to endosomes and plays a pivotal role in endosomal recycling pathway by modulating small GTPase RAB5 and RAB11 activation. However, ATP9A pathogenic mutants have aberrant subcellular localization and cause abnormal endosomal recycling. These findings provide strong evidence that ATP9A deficiency leads to neurodevelopmental disorders and synaptic dysfunctions in both humans and mice, and establishes novel regulatory roles for ATP9A in RAB5 and RAB11 activity-dependent endosomal recycling pathway and neurological diseases.

An antisteatosis response regulated by oleic acid through lipid droplet-mediated ERAD enhancement

Although excessive lipid accumulation is a hallmark of obesity-related pathologies, some lipids are beneficial. Oleic acid (OA), the most abundant monounsaturated fatty acid (FA), promotes health and longevity. Here, we show that OA benefits Caenorhabditis elegans by activating the endoplasmic reticulum (ER)-resident transcription factor SKN-1A (Nrf1/NFE2L1) in a lipid homeostasis response. SKN-1A/Nrf1 is cleared from the ER by the ER-associated degradation (ERAD) machinery and stabilized when proteasome activity is low and canonically maintains proteasome homeostasis. Unexpectedly, OA increases nuclear SKN-1A levels independently of proteasome activity, through lipid droplet-dependent enhancement of ERAD. In turn, SKN-1A reduces steatosis by reshaping the lipid metabolism transcriptome and mediates longevity from OA provided through endogenous accumulation, reduced H3K4 trimethylation, or dietary supplementation. Our findings reveal an unexpected mechanism of FA signal transduction, as well as a lipid homeostasis pathway that provides strategies for opposing steatosis and aging, and may mediate some benefits of the OA-rich Mediterranean diet.

The ATPase activity of the phosphatidylethanolamine flippase TAT-5 inhibits extracellular vesicle budding from the plasma membrane

Cells release extracellular vesicles (EVs) from their surface, but the mechanisms that govern EV release by plasma membrane budding are poorly understood. The lipid flippase TAT-5 inhibits EV release from the plasma membrane in C. elegans , but how the level of flippase activity regulates EV release was unknown. We generated point mutations in the DGET motif of TAT-5 predicted to lead to a partial or complete loss of ATPase activity. We discovered that tat-5(E246Q) mutants were sterile, while tat-5(D244T) mutants produced embryos that arrested during development. Using degron-based reporters, we found that EV release was increased in tat-5(D244T) mutant embryos and that phagocytosis was also disrupted. These data suggest that a low level of flippase activity can promote fertility, while a higher level of flippase activity is required to inhibit EV release, allow phagocytosis, and carry out embryonic development.

C. elegans TAT-6, a putative aminophospholipid translocase, is expressed in sujc cells in the hermaphrodite gonad

In healthy eukaryotic cells, the two leaflets that make up plasma membranes are highly asymmetric with respect to the lipids they contain. In both unicellular eukaryotes and metazoans, the asymmetry in the distribution of aminophospholipids is maintained by P4-family transmembrane ATPases, which catalyze the movement of selected phospholipids from the outer leaflet to the inner. C. elegans has six P4-family ATPases, TAT-1 – TAT-6. TAT-1 – TAT-5 are expressed in many tissues and cells. Here we report that, in contrast, TAT-6 is much less broadly expressed and that, within the somatic gonad, expression of TAT-6 reporters is restricted to the spermathecal-uterine core cell (sujc) cells.

Lipid Transporters Beam Signals from Cell Membranes

Lipid composition in cellular membranes plays an important role in maintaining the structural integrity of cells and in regulating cellular signaling that controls functions of both membrane-anchored and cytoplasmic proteins. ATP-dependent ABC and P4-ATPase lipid transporters, two integral membrane proteins, are known to contribute to lipid translocation across the lipid bilayers on the cellular membranes. In this review, we will highlight current knowledge about the role of cholesterol and phospholipids of cellular membranes in regulating cell signaling and how lipid transporters participate this process.

Identification of a Novel Link between the Intermediate Filament Organizer IFO-1 and Cholesterol Metabolism in the Caenorhabditis elegans Intestine

The intestine is an organ essential to organismal nutrient absorption, metabolic control, barrier function and immunoprotection. The Caenorhabditis elegans intestine consists of 20 cells harboring a dense intermediate filament network positioned below the apical plasma membrane that forms a junction-anchored sheath around the intestinal lumen. This evolutionarily conserved arrangement provides mechanical and overall stress-protection, and it serves as an important model for deciphering the role of intestinal architecture in metazoan biology. We recently reported that the loss-of-function mutation of the intestinal intermediate filament organizer IFO-1 perturbs this architecture, leading to reduced body size and reproduction. Here, we demonstrate that the IFO-1 mutation dramatically affects cholesterol metabolism. Mutants showed an increased sensitivity to cholesterol depletion, reduced cholesterol uptake, and cholesterol transfer to the gonads, which is also observed in worms completely lacking an intermediate filament network. Accordingly, we found striking similarities to transcriptome and lipidome profiles of a nuclear hormone receptor (NHR)-8 mutant. NHR-8 is homologous to mammalian LXR (liver X receptor) that serves as a sterol sensor and transcriptional regulator of lipid metabolism. Remarkably, increasing exogenous cholesterol partially rescues the developmental retardation in IFO-1 mutants. Our results uncover a novel link of the intestinal intermediate filament cytoskeleton to cholesterol metabolism that contributes to compromised growth and reproduction.

Inner workings and biological impact of phospholipid flippases

The plasma membrane, trans-Golgi network and endosomal system of eukaryotic cells are populated with flippases that hydrolyze ATP to help establish asymmetric phospholipid distributions across the bilayer. Upholding phospholipid asymmetry is vital to a host of cellular processes, including membrane homeostasis, vesicle biogenesis, cell signaling, morphogenesis and migration. Consequently, defining the identity of flippases and their biological impact has been the subject of intense investigations. Recent work has revealed a remarkable degree of kinship between flippases and cation pumps. In this Commentary, we review emerging insights into how flippases work, how their activity is controlled according to cellular demands, and how disrupting flippase activity causes system failure of membrane function, culminating in membrane trafficking defects, aberrant signaling and disease.

The Drosophila melanogaster phospholipid flippase dATP8B is required for odorant receptor function

The olfactory systems of insects are fundamental to all aspects of their behaviour, and insect olfactory receptor neurons (ORNs) exhibit exquisite specificity and sensitivity to a wide range of environmental cues. In Drosophila melanogaster, ORN responses are determined by three different receptor families, the odorant (Or), ionotropic-like (IR) and gustatory (Gr) receptors. However, the precise mechanisms of signalling by these different receptor families are not fully understood. Here we report the unexpected finding that the type 4 P-type ATPase phospholipid transporter dATP8B, the homologue of a protein associated with intrahepatic cholestasis and hearing loss in humans, is crucial for Drosophila olfactory responses. Mutations in dATP8B severely attenuate sensitivity of odorant detection specifically in Or-expressing ORNs, but do not affect responses mediated by IR or Gr receptors. Accordingly, we find dATP8B to be expressed in ORNs and localised to the dendritic membrane of the olfactory neurons where signal transduction occurs. Localisation of Or proteins to the dendrites is unaffected in dATP8B mutants, as is dendrite morphology, suggesting instead that dATP8B is critical for Or signalling. As dATP8B is a member of the phospholipid flippase family of ATPases, which function to determine asymmetry in phospholipid composition between the outer and inner leaflets of plasma membranes, our findings suggest a requirement for phospholipid asymmetry in the signalling of a specific family of chemoreceptor proteins.

The olfactory systems of insects are fundamental to critical behaviours such as finding mates, food and host plants. Insects can detect a wide range of environmental cues using three different families of olfactory receptor proteins. Why insects have three different families of receptor genes, and how they function together, is not fully understood. Here we identified a new gene, dATP8B, which is critically and specifically required for the function of only one of these receptor families in Drosophila. dATP8B is a member of the P4-type ATPases, or phospholipid flippases; these enzymes function in establishing a difference or asymmetry in lipid composition between the outer and inner leaflets of plasma membranes. This is thought to be important for many cellular membrane processes; however, specific functions of individual flippase proteins are not well described. We find that dATP8B is required for the function of the odorant receptor family, but not the ionotropic-like and gustatory receptor families. This further highlights the functional differences between these receptor families and suggests a role for phospholipids in the signalling of a specific family of receptor proteins.

Identification of suppressors of mbk-2/DYRK by whole-genome sequencing

Screening for suppressor mutations is a powerful method to isolate genes that function in a common pathway or process. Because suppressor mutations often do not have phenotypes on their own, cloning of suppressor loci can be challenging. A method combining whole-genome sequencing (WGS) and single nucleotide polymorphism (SNP) mapping (WGS/SNP mapping) was developed to identify mutations with visible phenotypes in C. elegans. We show here that WGS/SNP mapping is an efficient method to map suppressor mutations without the need for previous phenotypic characterization. Using RNA-mediated interference to test candidate loci identified by WGS/SNP mapping, we identified 10 extragenic and six intragenic suppressors of mbk-2, a DYRK family kinase required for the transition from oocyte to zygote. Remarkably, seven suppressors are mutations in cell-cycle regulators that extend the timing of the oocyte-to-zygote transition.

Mitochondrial oxidative stress alters a pathway in Caenorhabditis elegans strongly resembling that of bile acid biosynthesis and secretion in vertebrates

Mammalian bile acids (BAs) are oxidized metabolites of cholesterol whose amphiphilic properties serve in lipid and cholesterol uptake. BAs also act as hormone-like substances that regulate metabolism. The Caenorhabditis elegans clk-1 mutants sustain elevated mitochondrial oxidative stress and display a slow defecation phenotype that is sensitive to the level of dietary cholesterol. We found that: 1) The defecation phenotype of clk-1 mutants is suppressed by mutations in tat-2 identified in a previous unbiased screen for suppressors of clk-1. TAT-2 is homologous to ATP8B1, a flippase required for normal BA secretion in mammals. 2) The phenotype is suppressed by cholestyramine, a resin that binds BAs. 3) The phenotype is suppressed by the knock-down of C. elegans homologues of BA–biosynthetic enzymes. 4) The phenotype is enhanced by treatment with BAs. 5) Lipid extracts from C. elegans contain an activity that mimics the effect of BAs on clk-1, and the activity is more abundant in clk-1 extracts. 6) clk-1 and clk-1;tat-2 double mutants show altered cholesterol content. 7) The clk-1 phenotype is enhanced by high dietary cholesterol and this requires TAT-2. 8) Suppression of clk-1 by tat-2 is rescued by BAs, and this requires dietary cholesterol. 9) The clk-1 phenotype, including the level of activity in lipid extracts, is suppressed by antioxidants and enhanced by depletion of mitochondrial superoxide dismutases. These observations suggest that C. elegans synthesizes and secretes molecules with properties and functions resembling those of BAs. These molecules act in cholesterol uptake, and their level of synthesis is up-regulated by mitochondrial oxidative stress. Future investigations should reveal whether these molecules are in fact BAs, which would suggest the unexplored possibility that the elevated oxidative stress that characterizes the metabolic syndrome might participate in disease processes by affecting the regulation of metabolism by BAs.

Cholesterol metabolism, in particular the transport of cholesterol in the blood by lipoproteins, is an important determinant of human cardiovascular health. Bile acids are breakdown products of cholesterol that have detergent properties and are secreted into the gut by the liver. Bile acids carry out three distinct roles in cholesterol metabolism: 1) Their synthesis from cholesterol participates in cholesterol elimination. 2) They act as detergents in the uptake of dietary cholesterol from the gut. 3) They regulate many aspects of metabolism, including cholesterol metabolism, by molecular mechanisms similar to that of steroid hormones. We have found that cholesterol uptake and lipoprotein metabolism in the nematode Caenorhabditis elegans are regulated by molecules whose activities, biosynthesis, and secretion strongly resemble that of bile acids and which might be bile acids. Most importantly we have found that oxidative stress upsets the regulation of the synthesis of these molecules. The metabolic syndrome is a set of cardiovascular risk factors that include obesity, high blood cholesterol, hypertension, and insulin resistance. Given the function of bile acids as metabolic regulators, our findings with C. elegans suggest the unexplored possibility that the elevated oxidative stress that characterizes the metabolic syndrome may participate in mammalian disease processes by affecting the regulation of bile acid synthesis.

Membrane phospholipid asymmetry counters the adverse effects of sterol overloading in the Golgi membrane of Drosophila

Cholesterol and phospholipids serve as structural and functional components of cellular membranes in all eukaryotes. Heterogeneity in cholesterol and phospholipid content both within and between different organelles is an important characteristic of eukaryotic membranes. How this heterogeneity is achieved and orchestrated to maintain proper cellular physiology remains poorly understood. We previously found that overexpression of the Drosophila oxysterol-binding protein (OSBP) leads to sterol accumulation in the Golgi apparatus. Here, we show that Osbp overexpression in a set of neuroendocrine neurons compromises the function of the Golgi apparatus. It impairs trafficking of the neuropeptide bursicon and results in post-eclosion behavior defects characterized by unexpanded wings. We performed a genetic screen to identify modifiers that suppress the unexpanded wing phenotype. A putative phospholipid flippase-encoding gene, CG33298, was validated, suggesting that a membrane-asymmetry-directed mechanism balances cholesterol chaos within the Golgi membranes. Since the functional connection between cholesterol metabolism and the activity of phospholipid flippase has been implicated in studies in yeast and worms, our findings here support an evolutionarily conserved causal link between cholesterol homeostasis and phospholipid asymmetry that maintains normal cellular physiology.

Phospholipid flippases: building asymmetric membranes and transport vesicles

Phospholipid flippases in the type IV P-type ATPase family (P4-ATPases) are essential components of the Golgi, plasma membrane and endosomal system that play critical roles in membrane biogenesis. These pumps flip phospholipid across the bilayer to create an asymmetric membrane structure with substrate phospholipids, such as phosphatidylserine and phosphatidylethanolamine, enriched within the cytosolic leaflet. The P4-ATPases also help form transport vesicles that bud from Golgi and endosomal membranes, thereby impacting the sorting and localization of many different proteins in the secretory and endocytic pathways. At the organismal level, P4-ATPase deficiencies are linked to liver disease, obesity, diabetes, hearing loss, neurological deficits, immune deficiency and reduced fertility. Here, we review the biochemical, cellular and physiological functions of P4-ATPases, with an emphasis on their roles in vesicle-mediated protein transport. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

The P4-ATPase TAT-5 inhibits the budding of extracellular vesicles in C. elegans embryos

BACKGROUND

Cells release extracellular vesicles (ECVs) that can influence differentiation, modulate the immune response, promote coagulation, and induce metastasis. Many ECVs form by budding outwards from the plasma membrane, but the molecules that regulate budding are unknown. In ECVs, the outer leaflet of the membrane bilayer contains aminophospholipids that are normally sequestered to the inner leaflet of the plasma membrane, suggesting a role for lipid asymmetry in ECV budding.

RESULTS

We show that loss of the conserved P4-ATPase TAT-5 causes the large-scale shedding of ECVs and disrupts cell adhesion and morphogenesis in Caenorhabditis elegans embryos. TAT-5 localizes to the plasma membrane and its loss results in phosphatidylethanolamine exposure on cell surfaces. We show that RAB-11 and endosomal sorting complex required for transport (ESCRT) proteins, which regulate the topologically analogous process of viral budding, are enriched at the plasma membrane in tat-5 embryos, and are required for ECV production.

CONCLUSIONS

TAT-5 is the first protein identified to regulate ECV budding. TAT-5 provides a potential molecular link between loss of phosphatidylethanolamine asymmetry and the dynamic budding of vesicles from the plasma membrane, supporting the hypothesis that lipid asymmetry regulates budding. Our results also suggest that viral budding and ECV budding may share common molecular mechanisms.

Caenorhabditis elegans numb inhibits endocytic recycling by binding TAT-1 aminophospholipid translocase

Numb regulates endocytosis in many metazoans, but the mechanism by which it functions is not completely understood. Here we report that the Caenorhabditis elegans Numb ortholog, NUM-1A, a regulator of endocytic recycling, binds the C isoform of transbilayer amphipath transporter-1 (TAT-1), a P4 family adenosine triphosphatase and putative aminophospholipid translocase that is required for proper endocytic trafficking. We demonstrate that TAT-1 is differentially spliced during development and that TAT-1C-specific splicing occurs in the intestine where NUM-1A is known to function. NUM-1A and TAT-1C colocalize in vivo. We have mapped the binding site to an NXXF motif in TAT-1C. This motif is not required for TAT-1C function but is required for NUM-1A's ability to inhibit recycling. We demonstrate that num-1A and tat-1 defects are both suppressed by the loss of the activity of PSSY-1, a phosphatidylserine (PS) synthase. PS is mislocalized in intestinal cells with defects in tat-1 or num-1A function. We propose that NUM-1A inhibits recycling by inhibiting TAT-1C's ability to translocate PS across the membranes of recycling endosomes.

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.

P-type ATPase TAT-2 negatively regulates monomethyl branched-chain fatty acid mediated function in post-embryonic growth and development in C. elegans

Monomethyl branched-chain fatty acids (mmBCFAs) are essential for Caenorhabditis elegans growth and development. To identify factors acting downstream of mmBCFAs for their function in growth regulation, we conducted a genetic screen for suppressors of the L1 arrest that occurs in animals depleted of the 17-carbon mmBCFA C17ISO. Three of the suppressor mutations defined an unexpected player, the P-type ATPase TAT-2, which belongs to the flippase family of proteins that are implicated in mediating phospholipid bilayer asymmetry. We provide evidence that TAT-2, but not other TAT genes, has a specific role in antagonizing the regulatory activity of mmBCFAs in intestinal cells. Interestingly, we found that mutations in tat-2 also suppress the lethality caused by inhibition of the first step in sphingolipid biosynthesis. We further showed that the fatty acid side-chains of glycosylceramides contain 20%–30% mmBCFAs and that this fraction is greatly diminished in the absence of mmBCFA biosynthesis. These results suggest a model in which a C17ISO-containing sphingolipid may mediate the regulatory functions of mmBCFAs and is negatively regulated by TAT-2 in intestinal cells. This work indicates a novel connection between a P-type ATPase and the critical regulatory function of a specific fatty acid.

Fatty acids serve diverse functions in organisms, including roles at the cell membrane to coordinate cell signaling processes. Monomethyl branched-chain fatty acids (mmBCFAs) are a special type of fatty acid that is commonly present in animals. Because mmBCFAs are a small component of the total fatty acid pool, their functions have not been a major research focus and are largely unclear. We tackled the problem using the nematode C. elegans. Our laboratory previously found that without mmBCFAs, worms cannot develop normally and die. To understand how these obscure fatty acids perform such important roles, we searched for other factors involved in the process by conducting a mutagenesis screen to uncover mutant worms that can recover the ability to grow without the presence of mmBCFAs. We found several such mutations in a single gene that codes for a protein called TAT-2. TAT-2 is one of several poorly understood P-type ATPases that likely help maintain the proper lipid structure in cell membranes. Our work indicates that TAT-2 antagonizes the growth regulatory function of mmBCFAs in intestinal cells. Studies on how mmBCFAs and this protein functionally interact explore a novel, interesting, and important problem that is only beginning to be understood.

Linking phospholipid flippases to vesicle-mediated protein transport

Type IV P-type ATPases (P4-ATPases) are a large family of putative phospholipid translocases (flippases) implicated in the generation of phospholipid asymmetry in biological membranes. P4-ATPases are typically the largest P-type ATPase subgroup found in eukaryotic cells, with five members in Saccharomyces cerevisiae, six members in Caenorhabditis elegans, 12 members in Arabidopsis thaliana and 14 members in humans. In addition, many of the P4-ATPases require interaction with a noncatalytic subunit from the CDC50 gene family for their transport out of the endoplasmic reticulum (ER). Deficiency of a P4-ATPase (Atp8b1) causes liver disease in humans, and studies in a variety of model systems indicate that P4-ATPases play diverse and essential roles in membrane biogenesis. In addition to their proposed role in establishing and maintaining plasma membrane asymmetry, P4-ATPases are linked to vesicle-mediated protein transport in the exocytic and endocytic pathways. Recent studies have also suggested a role for P4-ATPases in the nonvesicular intracellular trafficking of sterols. Here, we discuss the physiological requirements for yeast P4-ATPases in phospholipid translocase activity, transport vesicle budding and ergosterol metabolism, with an emphasis on Drs2p and its noncatalytic subunit, Cdc50p.

The yeast plasma membrane P4-ATPases are major transporters for lysophospholipids

The transbilayer movement of phospholipids plays an essential role in establishing and maintaining the asymmetric distribution of lipids in biological membranes. The P4-ATPase family has been implicated as the major transporters of the aminoglycerophospholipids in both surface and endomembrane systems. Historically, fluorescent lipid analogs have been used to monitor the lipid transport activity of the P4-ATPases. Recent evidence now demonstrates that lyso-phosphatidylethanolamine (lyso-PtdEtn) and lyso-phosphatidylcholine (lyso-PtdCho) are bona fide biological substrates transported by the yeast plasma membrane ATPases, Dnf1p and Dnf2p, in consort with a second protein Lem3p. Subsequent to transport, the lysophospholipids are acylated by the enzyme Ale1p to produce PtdEtn and PtdCho. The transport of the lysophospholipids occurs at rates sufficient to support all the PtdEtn and PtdCho synthesis required for rapid cell growth. The lysophospholipid transporters also utilize the anti-neoplastic and anti-parasitic ether lipid substrates related to edelfosine. The identification of biological substrates for the plasma membrane ATPases coupled with the power of yeast genetics now provides new tools to dissect the structure and function of the aminoglycerophospholipid transporters.