Lysophosphatidylserine: A Signaling Lipid with Implications in Human Diseases.

Lysophosphatidylserine (lyso-PS) has emerged as yet another important signaling lysophospholipid in mammals, and deregulation in its metabolism has been directly linked to an array of human autoimmune and neurological disorders. It has an indispensable role in several biological processes in humans, and therefore, cellular concentrations of lyso-PS are tightly regulated to ensure optimal signaling and functioning in physiological settings. Given its biological importance, the past two decades have seen an explosion in the available literature toward our understanding of diverse aspects of lyso-PS metabolism and signaling and its association with human diseases. In this Review, we aim to comprehensively summarize different aspects of lyso-PS, such as its structure, biodistribution, chemical synthesis, and SAR studies with some synthetic analogs. From a biochemical perspective, we provide an exhaustive coverage of the diverse biological activities modulated by lyso-PSs, such as its metabolism and the receptors that respond to them in humans. We also briefly discuss the human diseases associated with aberrant lyso-PS metabolism and signaling and posit some future directions that may advance our understanding of lyso-PS-mediated mammalian physiology.

Metabolic transitions regulate global protein fatty acylation.

Intermediary metabolites and flux through various pathways have emerged as key determinants of post-translational modifications. Independently, dynamic fluctuations in their concentrations are known to drive cellular energetics in a bi-directional manner. Notably, intracellular fatty acid pools that drastically change during fed and fasted states act as precursors for both ATP production and fatty acylation of proteins. Protein fatty acylation is well regarded for its role in regulating structure and functions of diverse proteins; however, the effect of intracellular concentrations of fatty acids on protein modification is less understood. In this regard, we unequivocally demonstrate that metabolic contexts, viz. fed and fasted states, dictate the extent of global fatty acylation. Moreover, we show that presence or absence of glucose that influences cellular and mitochondrial uptake/utilization of fatty acids and affects palmitoylation and oleoylation, which is consistent with their intracellular abundance in fed and fasted states. Employing complementary approaches including click-chemistry, lipidomics, and imaging, we show the top-down control of cellular metabolic state. Importantly, our results establish the crucial role of mitochondria and retrograde signaling components like SIRT4, AMPK, and mTOR in orchestrating protein fatty acylation at a whole cell level. Specifically, pharmacogenetic perturbations that alter either mitochondrial functions and/or retrograde signaling affect protein fatty acylation. Besides illustrating the cross-talk between carbohydrate and lipid metabolism in mediating bulk post-translational modification, our findings also highlight the involvement of mitochondrial energetics.

Kupyaphores are zinc homeostatic metallophores required for colonization of Mycobacterium tuberculosis.

Mycobacterium tuberculosis (Mtb) endures a combination of metal scarcity and toxicity throughout the human infection cycle, contributing to complex clinical manifestations. Pathogens counteract this paradoxical dysmetallostasis by producing specialized metal trafficking systems. Capture of extracellular metal by siderophores is a widely accepted mode of iron acquisition, and Mtb iron-chelating siderophores, mycobactin, have been known since 1965. Currently, it is not known whether Mtb produces zinc scavenging molecules. Here, we characterize low-molecular-weight zinc-binding compounds secreted and imported by Mtb for zinc acquisition. These molecules, termed kupyaphores, are produced by a 10.8 kbp biosynthetic cluster and consists of a dipeptide core of ornithine and phenylalaninol, where amino groups are acylated with isonitrile-containing fatty acyl chains. Kupyaphores are stringently regulated and support Mtb survival under both nutritional deprivation and intoxication conditions. A kupyaphore-deficient Mtb strain is unable to mobilize sufficient zinc and shows reduced fitness upon infection. We observed early induction of kupyaphores in Mtb-infected mice lungs after infection, and these metabolites disappeared after 2 wk. Furthermore, we identify an Mtb-encoded isonitrile hydratase, which can possibly mediate intracellular zinc release through covalent modification of the isonitrile group of kupyaphores. Mtb clinical strains also produce kupyaphores during early passages. Our study thus uncovers a previously unknown zinc acquisition strategy of Mtb that could modulate host-pathogen interactions and disease outcome.

A multi-omics analysis reveals that the lysine deacetylase ABHD14B influences glucose metabolism in mammals.

The sirtuins and histone deacetylases are the best characterized members of the lysine deacetylase (KDAC) enzyme family. Recently, we annotated the "orphan" enzyme ABHD14B (α/ß-hydrolase domain containing protein # 14B) as a novel KDAC and showed this enzyme's ability to transfer an acetyl-group from protein lysine residue(s) to coenzyme-A to yield acetyl-coenzyme-A, thereby, expanding the repertoire of this enzyme family. However, the role of ABHD14B in metabolic processes is not fully elucidated. Here, we investigated the role of this enzyme using mammalian cell knockdowns in a combined transcriptomics and metabolomics analysis. We found from these complementary experiments in vivo that the loss of ABHD14B results in significantly altered glucose metabolism, specifically the decreased flux of glucose through glycolysis and the citric acid cycle. Further, we show that depleting hepatic ABHD14B in mice also results in defective systemic glucose metabolism, particularly during fasting. Taken together, our findings illuminate the important metabolic functions that the KDAC ABHD14B plays in mammalian physiology and poses new questions regarding the role of this hitherto cryptic metabolism-regulating enzyme.

Identification of sequence determinants for the ABHD14 enzymes.

Over the course of evolution, enzymes have developed remarkable functional diversity in catalyzing important chemical reactions across various organisms, and understanding how new enzyme functions might have evolved remains an important question in modern enzymology. To systematically annotate functions, based on their protein sequences and available biochemical studies, enzymes with similar catalytic mechanisms have been clustered together into an enzyme superfamily. Typically, enzymes within a superfamily have similar overall three-dimensional structures, conserved catalytic residues, but large variations in substrate recognition sites and residues to accommodate the diverse biochemical reactions that are catalyzed within the superfamily. The serine hydrolases are an excellent example of such an enzyme superfamily. Based on known enzymatic activities and protein sequences, they are split almost equally into the serine proteases and metabolic serine hydrolases. Within the metabolic serine hydrolases, there are two outlying members, ABHD14A and ABHD14B, that have high sequence similarity, but their biological functions remained cryptic till recently. While ABHD14A still lacks any functional annotation to date, we recently showed that ABHD14B functions as a lysine deacetylase in mammals. Given their high sequence similarity, automated databases often wrongly assign ABHD14A and ABHD14B as the same enzyme, and therefore, annotating functions to them in various organisms has been problematic. In this article, we present a bioinformatics study coupled with biochemical experiments, which identifies key sequence determinants for both ABHD14A and ABHD14B, and enable better classification for them. In addition, we map these enzymes on an evolutionary timescale and provide a much-wanted resource for studying these interesting enzymes in different organisms.

Mapping lipid pathways during phagocytosis.

Phagocytosis is an evolutionarily conserved important immunological process in higher organisms, and acts as the first line of defense against invading pathogenic microbial infections. Additionally, this dynamic innate immune response is also critical for clearing apoptotic cells and/or tissues, is responsible for maintaining homeostasis and acts as a systemic regulator of critical physiological processes such as wound healing and tissue regeneration. Over the past two decades, numerous studies have shown that phagocytosis occurs in three spatiotemporally distinct steps, namely formation, maturation and resolution of the phagosome, and that, both the protein and lipid composition change as a function of the aforementioned steps during this immunological process. While significant knowledge is now available on the proteomic content of a phagosome during the different stages of phagocytosis, the lipidome however, remained lesser explored, until the past few years. In this review, we summarize recent efforts towards mapping the physiological roles and functions of three lipid classes, the phosphatidylinositols, cholesterol and sphingolipids during the various stages of phagocytosis, and discuss strategies evolved by microbes to hijack and/or disrupt these lipid pathways to evade the immune system. We conclude this review with some potential avenues that may be pursued towards mapping hitherto unknown lipid pathways during phagocytosis, and how this research might be beneficial in our ongoing battle to overcome pathogenic infections.

Spatiotemporal gating of SIRT1 functions by O-GlcNAcylation is essential for liver metabolic switching and prevents hyperglycemia.

Inefficient physiological transitions are known to cause metabolic disorders. Therefore, investigating mechanisms that constitute molecular switches in a central metabolic organ like the liver becomes crucial. Specifically, upstream mechanisms that control temporal engagement of transcription factors, which are essential to mediate physiological fed-fast-refed transitions are less understood. SIRT1, a NAD+-dependent deacetylase, is pivotal in regulating hepatic gene expression and has emerged as a key therapeutic target. Despite this, if/how nutrient inputs regulate SIRT1 interactions, stability, and therefore downstream functions are still unknown. Here, we establish nutrient-dependent O-GlcNAcylation of SIRT1, within its N-terminal domain, as a crucial determinant of hepatic functions. Our findings demonstrate that during a fasted-to-refed transition, glycosylation of SIRT1 modulates its interactions with various transcription factors and a nodal cytosolic kinase involved in insulin signaling. Moreover, sustained glycosylation in the fed state causes nuclear exclusion and cytosolic ubiquitin-mediated degradation of SIRT1. This mechanism exerts spatiotemporal control over SIRT1 functions by constituting a previously unknown molecular relay. Of note, loss of SIRT1 glycosylation discomposed these interactions resulting in aberrant gene expression, mitochondrial dysfunctions, and enhanced hepatic gluconeogenesis. Expression of nonglycosylatable SIRT1 in the liver abrogated metabolic flexibility, resulting in systemic insulin resistance, hyperglycemia, and hepatic inflammation, highlighting the physiological costs associated with its overactivation. Conversely, our study also reveals that hyperglycosylation of SIRT1 is associated with aging and high-fat-induced obesity. Thus, we establish that nutrient-dependent glycosylation of SIRT1 is essential to gate its functions and maintain physiological fitness.

A Superfamily-wide Activity Atlas of Serine Hydrolases in Drosophila melanogaster.

The serine hydrolase (SH) superfamily is, perhaps, one of the largest functional enzyme classes in all forms of life and consists of proteases, peptidases, lipases, and carboxylesterases as representative members. Consistent with the name of this superfamily, all members, without any exception to date, use a nucleophilic serine residue in the enzyme active site to perform hydrolytic-type reactions via a two-step ping-pong mechanism involving a covalent enzyme intermediate. Given the highly conserved catalytic mechanism, this superfamily has served as a classical prototype in the development of several platforms of chemical proteomics techniques, activity-based protein profiling (ABPP), to globally interrogate the functions of its different members in various native, yet complex, biological settings. While ABPP-based proteome-wide activity atlases for SH activities are available in numerous organisms, including humans, to the best of our knowledge, such an analysis for this superfamily is lacking in any insect model. To address this, we initially report a bioinformatics analysis toward the identification and categorization of nonredundant SHs in Drosophila melanogaster. Following up on this in silico analysis, leveraging discovery chemoproteomics, we identify and globally map the full complement of SH activities during various developmental stages and in different adult tissues of Drosophila. Finally, as a proof of concept of the utility of this activity atlas, we highlight sexual dimorphism in SH activities across different tissues in adult D. melanogaster, and we propose new research directions, resources, and tools that this study can provide to the fly community.

The loss of enzymatic activity of the PHARC-associated lipase ABHD12 results in increased phagocytosis that causes neuroinflammation.

Phagocytosis is an important evolutionary conserved process, essential for clearing pathogens and cellular debris in higher organisms, including humans. This well-orchestrated innate immunological response is intricately regulated by numerous cellular factors, important amongst which are the immunomodulatory lysophosphatidylserines (lyso-PSs) and the pro-apoptotic oxidized phosphatidylserines (PSs) signalling lipids. Interestingly, in mammals, both these signalling lipids are physiologically regulated by the lipase ABHD12, mutations of which cause the human neurological disorder PHARC. Despite the biomedical significance of this lipase, detailed mechanistic studies and the specific contribution of ABHD12 to innate processes like phagocytosis remain poorly understood. Here, by immunohistochemical and immunofluorescence approaches, using the murine model of PHARC, we show, that upon an inflammatory stimulus, activated microglial cells in the cerebellum of mice deficient in ABHD12 have an amoeboid morphology, increased soma size and display heightened phagocytosis activity. We also report that upon an inflammatory stimulus, cerebellar levels of ABHD12 increase to possibly metabolize the heightened oxidized PS levels, temper phagocytosis and, in turn, control neuroinflammation during oxidative stress. Next, to complement these findings, with the use of biochemical approaches in cultured microglial cells, we show that the pharmacological inhibition and/or genetic deletion of ABHD12 results in increased phagocytic uptake in a fluorescent bead uptake assay. Together, our studies provide compelling evidence that ABHD12 plays an important role in regulating phagocytosis in cerebellar microglial cells and provides a possible explanation, as to why human PHARC subjects display neuroinflammation and atrophy in the cerebellum.

A chemical-genetic screen identifies ABHD12 as an oxidized-phosphatidylserine lipase.

Reactive oxygen species (ROS) are transient, highly reactive intermediates or byproducts produced during oxygen metabolism. However, when innate mechanisms are unable to cope with sequestration of surplus ROS, oxidative stress results, in which excess ROS damage biomolecules. Oxidized phosphatidylserine (PS), a proapoptotic 'eat me' signal, is produced in response to elevated ROS, yet little is known regarding its chemical composition and metabolism. Here, we report a small molecule that generates ROS in different mammalian cells. We used this molecule to detect, characterize and study oxidized PS in mammalian cells. We developed a chemical-genetic screen to identify enzymes that regulate oxidized PS in mammalian cells and found that the lipase ABHD12 hydrolyzes oxidized PS. We validated these findings in different physiological settings including primary peritoneal macrophages and brains from Abhd12-/- mice under inflammatory stress, and in the process, we functionally annotated an enzyme regulating oxidized PS in vivo.

Mapping the Neuroanatomy of ABHD16A, ABHD12, and Lysophosphatidylserines Provides New Insights into the Pathophysiology of the Human Neurological Disorder PHARC.

Lysophosphatidylserine (lyso-PS), a lysophospholipid derived from phosphatidylserine (PS), has emerged as a potent signaling lipid in mammalian physiology. In vivo, the metabolic serine hydrolases ABHD16A and ABHD12 are major lipases that biosynthesize and degrade lyso-PS, respectively. Of biomedical relevance, deleterious mutations to ABHD12 cause accumulation of lyso-PS in the brain, and this deregulated lyso-PS metabolism leads to the human genetic neurological disorder PHARC (polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract). While the roles of ABHD16A and ABHD12 in lyso-PS metabolism in the mammalian brain are well established, the anatomical and (sub)cellular localizations of both lipases and the functional cross-talk between them with respect to regulating lyso-PS lipids remain under investigated. Here, using subcellular organelle fractionation, biochemical assays, and immunofluorescence-based high-resolution microscopy, we show that the PS lipase ABHD16A is an endoplasmic reticulum-localized enzyme, an organelle intricately regulating cellular PS levels. In addition, leveraging immunohistochemical analysis using genetic ABHD16A and ABHD12 knockout mice as important controls, we map the anatomical distribution of both of these lipases in tandem in the murine brain and show for the first time the distinct localization of these lipases to different regions and cells of the cerebellum. We complement the aforementioned immunohistochemical studies by quantitatively measuring lyso-PS concentrations in various brain regions using mass spectrometry and find that the cerebellar lyso-PS levels are most affected by deletion of ABHD16A (decreased) or ABHD12 (increased). Taken together, our studies provide new insights into lyso-PS signaling in the cerebellum, the most atrophic brain region in human PHARC subjects.

Functional Annotation of ABHD14B, an Orphan Serine Hydrolase Enzyme.

The metabolic serine hydrolase family is, arguably, one of the largest functional enzyme classes in mammals, including humans, comprising 1-2% of the total proteome. This enzyme family uses a conserved nucleophilic serine residue in the active site to perform diverse hydrolytic reactions and consists of proteases, lipases, esterases, amidases, and transacylases, which are prototypical members of this family. In humans, this enzyme family consists of >250, of which approximately 40% members remain unannotated, in terms of both their endogenous substrates and the biological pathways that they regulate. The enzyme ABHD14B, an outlying member of this family, is also known as CCG1/TAFII250-interacting factor B, as it was found to be associated with transcription initiation factor TFIID. The crystal structure of human ABHD14B was determined more than a decade ago; however, its endogenous substrates remain elusive. In this paper, we annotate ABHD14B as a lysine deacetylase (KDAC), showing this enzyme's ability to transfer an acetyl group from a post-translationally acetylated lysine to coenzyme A (CoA), to yield acetyl-CoA, while regenerating the free amine of protein lysine residues. We validate these findings by in vitro biochemical assays using recombinantly purified human ABHD14B in conjunction with cellular studies in a mammalian cell line by knocking down ABHD14B and by identification of a putative substrate binding site. Finally, we report the development and characterization of a much-needed, exquisitely selective ABHD14B antibody, and using it, we map the cellular and tissue distribution of ABHD14B and prospective metabolic pathways that this enzyme might biologically regulate.

The Lysophosphatidylserines-An Emerging Class of Signalling Lysophospholipids.

Lysophospholipids are potent hormone-like signalling biological lipids that regulate many important biological processes in mammals (including humans). Lysophosphatidic acid and sphingosine-1-phosphate represent the best studied examples for this lipid class, and their metabolic enzymes and/or cognate receptors are currently under clinical investigation for treatment of various neurological and autoimmune diseases in humans. Over the past two decades, the lysophsophatidylserines (lyso-PSs) have emerged as yet another biologically important lysophospholipid, and deregulation in its metabolism has been linked to various human pathophysiological conditions. Despite its recent emergence, an exhaustive review summarizing recent advances on lyso-PSs and the biological pathways that this bioactive lysophospholipid regulates has been lacking. To address this, here, we summarize studies that led to the discovery of lyso-PS as a potent signalling biomolecule, and discuss the structure, its detection in biological systems, and the biodistribution of this lysophospholipid in various mammalian systems. Further, we describe in detail the enzymatic pathways that are involved in the biosynthesis and degradation of this lipid and the putative lyso-PS receptors reported in the literature. Finally, we discuss the various biological pathways directly regulated by lyso-PSs in mammals and prospect new questions for this still emerging biomedically important signalling lysophospholipid.

Peri-natal growth retardation rate and fat mass accumulation in mice lacking Dip2A is dependent on the dietary composition.

Disco-interacting protein 2 is a highly conserved three-domain protein with two tandem Adenylate-forming domains. It is proposed to influence the processes involved in neuronal development by influencing lipid metabolism and remains to be characterized. In this study, we show that Disco-interacting protein 2a null mice do not exhibit overt phenotype defects. However, the body composition differences were observed in these mice under different dietary regimens. The neutral lipid composition of two different diets was characterized, and it was observed that the new-born mice grow relatively slower than the wild-type mice with delayed appearance of features such as dentition when fed with high-triacylglycerol NIN-formulation diet. The high-diacylglycerol Safe-formulation diet was found to accumulate more fat mass in mice than those fed with high-triacylglycerol NIN-formulation diet beyond 10 months. These findings point to a proposed relationship between dietary components (particularly the lipid composition) and body composition along with the growth of neonates in mice lacking the gene Disco-interacting protein 2a.

Kinesin-dependent mechanism for controlling triglyceride secretion from the liver.

Despite massive fluctuations in its internal triglyceride content, the liver secretes triglyceride under tight homeostatic control. This buffering function is most visible after fasting, when liver triglyceride increases manyfold but circulating serum triglyceride barely fluctuates. How the liver controls triglyceride secretion is unknown, but is fundamentally important for lipid and energy homeostasis in animals. Here we find an unexpected cellular and molecular mechanism behind such control. We show that kinesin motors are recruited to triglyceride-rich lipid droplets (LDs) in the liver by the GTPase ARF1, which is a key activator of lipolysis. This recruitment is activated by an insulin-dependent pathway and therefore responds to fed/fasted states of the animal. In fed state, ARF1 and kinesin appear on LDs, consequently transporting LDs to the periphery of hepatocytes where the smooth endoplasmic reticulum (sER) is present. Because the lipases that catabolize LDs in hepatocytes reside on the sER, LDs can now be catabolized efficiently to provide triglyceride for lipoprotein assembly and secretion from the sER. Upon fasting, insulin is lowered to remove ARF1 and kinesin from LDs, thus down-regulating LD transport and sER-LD contacts. This tempers triglyceride availabiity for very low density lipoprotein assembly and allows homeostatic control of serum triglyceride in a fasted state. We further show that kinesin knockdown inhibits hepatitis-C virus replication in hepatocytes, likely because translated viral proteins are unable to transfer from the ER to LDs.

Biochemical characterization of the PHARC-associated serine hydrolase ABHD12 reveals its preference for very-long-chain lipids.

Polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract (PHARC) is a rare genetic human neurological disorder caused by null mutations to the Abhd12 gene, which encodes the integral membrane serine hydrolase enzyme ABHD12. Although the role that ABHD12 plays in PHARC is understood, the thorough biochemical characterization of ABHD12 is lacking. Here, we report the facile synthesis of mono-1-(fatty)acyl-glycerol lipids of varying chain lengths and unsaturation and use this lipid substrate library to biochemically characterize recombinant mammalian ABHD12. The substrate profiling study for ABHD12 suggested that this enzyme requires glycosylation for optimal activity and that it has a strong preference for very-long-chain lipid substrates. We further validated this substrate profile against brain membrane lysates generated from WT and ABHD12 knockout mice. Finally, using cellular organelle fractionation and immunofluorescence assays, we show that mammalian ABHD12 is enriched on the endoplasmic reticulum membrane, where most of the very-long-chain fatty acids are biosynthesized in cells. Taken together, our findings provide a biochemical explanation for why very-long-chain lipids (such as lysophosphatidylserine lipids) accumulate in the brains of ABHD12 knockout mice, which is a murine model of PHARC.

The catalytic mechanism for aerobic formation of methane by bacteria.

Methane is a potent greenhouse gas that is produced in significant quantities by aerobic marine organisms. These bacteria apparently catalyse the formation of methane through the cleavage of the highly unreactive carbon-phosphorus bond in methyl phosphonate (MPn), but the biological or terrestrial source of this compound is unclear. However, the ocean-dwelling bacterium Nitrosopumilus maritimus catalyses the biosynthesis of MPn from 2-hydroxyethyl phosphonate and the bacterial C-P lyase complex is known to convert MPn to methane. In addition to MPn, the bacterial C-P lyase complex catalyses C-P bond cleavage of many alkyl phosphonates when the environmental concentration of phosphate is low. PhnJ from the C-P lyase complex catalyses an unprecedented C-P bond cleavage reaction of ribose-1-phosphonate-5-phosphate to methane and ribose-1,2-cyclic-phosphate-5-phosphate. This reaction requires a redox-active [4Fe-4S]-cluster and S-adenosyl-L-methionine, which is reductively cleaved to L-methionine and 5'-deoxyadenosine. Here we show that PhnJ is a novel radical S-adenosyl-L-methionine enzyme that catalyses C-P bond cleavage through the initial formation of a 5'-deoxyadenosyl radical and two protein-based radicals localized at Gly 32 and Cys 272. During this transformation, the pro-R hydrogen from Gly 32 is transferred to the 5'-deoxyadenosyl radical to form 5'-deoxyadenosine and the pro-S hydrogen is transferred to the radical intermediate that ultimately generates methane. A comprehensive reaction mechanism is proposed for cleavage of the C-P bond by the C-P lyase complex that uses a covalent thiophosphate intermediate for methane and phosphate formation.

A calcium-dependent acyltransferase that produces N-acyl phosphatidylethanolamines.

More than 30 years ago, a calcium-dependent enzyme activity was described that generates N-acyl phosphatidylethanolamines (NAPEs), which are precursors for N-acyl ethanolamine (NAE) lipid transmitters, including the endocannabinoid anandamide. The identity of this calcium-dependent N-acyltransferase (Ca-NAT) has remained mysterious. Here, we use activity-based protein profiling to identify the poorly characterized serine hydrolase PLA2G4E as a mouse brain Ca-NAT and show that this enzyme generates NAPEs and NAEs in mammalian cells.

AIG1 and ADTRP are atypical integral membrane hydrolases that degrade bioactive FAHFAs.

Enzyme classes may contain outlier members that share mechanistic, but not sequence or structural, relatedness with more common representatives. The functional annotation of such exceptional proteins can be challenging. Here, we use activity-based profiling to discover that the poorly characterized multipass transmembrane proteins AIG1 and ADTRP are atypical hydrolytic enzymes that depend on conserved threonine and histidine residues for catalysis. Both AIG1 and ADTRP hydrolyze bioactive fatty acid esters of hydroxy fatty acids (FAHFAs) but not other major classes of lipids. We identify multiple cell-active, covalent inhibitors of AIG1 and show that these agents block FAHFA hydrolysis in mammalian cells. These results indicate that AIG1 and ADTRP are founding members of an evolutionarily conserved class of transmembrane threonine hydrolases involved in bioactive lipid metabolism. More generally, our findings demonstrate how chemical proteomics can excavate potential cases of convergent or parallel protein evolution that defy conventional sequence- and structure-based predictions.

Drosophila DNA/RNA methyltransferase contributes to robust host defense in aging animals by regulating sphingolipid metabolism.

Drosophila methyltransferase (Mt2) has been implicated in the methylation of both DNA and tRNA. In this study, we demonstrate that loss of Mt2 activity leads to an age-dependent decline of immune function in the adult fly. A newly eclosed adult has mild immune defects that are exacerbated in a 15 day old Mt2-/- fly. The age-dependent effects appear to be systemic, including disturbances in lipid metabolism, changes in cell shape of hemocytes and significant fold-changes in levels of transcripts related to host defense. Lipid imbalance, as measured by quantitative lipidomics, correlates with immune dysfunction, with high levels of immunomodulatory lipids, sphingosine-1-phosphate (S1P) and ceramides, along with low levels of storage lipids. Activity assays on fly lysates confirm the age-dependent increase in S1P and concomitant reduction of S1P lyase activity. We hypothesize that Mt2 functions to regulate genetic loci such as S1P lyase and this regulation is essential for robust host defense as the animal ages. Our study uncovers novel links between age--dependent Mt2 function, innate immune response and lipid homeostasis.