Manduca sexta Perilipin 1B: A new PLIN1 isoform linked to fat storage prior to pupation

Perilipins (PLINs) are proteins that associate with lipid droplets (LDs) and play roles in the control of triglycerides (TG) metabolism. Two types of PLINs - 1 and 2- occur in insects. Following previous work on MsPLIN1A (a 42 kDa protein formerly called MsLsd1), here we report a new PLIN1 isoform, MsPLIN1B. MsPLIN1B cDNA was cloned and the 1835bp cDNA contains an ORF encoding a 47.9 kDa protein whose expression was confirmed by mass spectrometry. Alternative transcripts A and B, which differ in the alternative use of exon 1, were the most abundant PLIN1 transcripts in the fat body. These transcripts encode nearly identical proteins except that the B isoform contains 59 additional residues in its amino terminus. No conserved domain was identified in the extra region of MsPLIN1B. The novel PLIN1 isoform is found in lepidopteran species. In Manduca, PLIN1B was expressed only in the 5th instar larva and its levels correlated with fat storage. Furthermore, PLIN1B levels increased with the fat content of the diet in insects of the same age confirming a direct relationship between PLIN1B and TG storage irrespective of development. The nutritional status impacted PLIN1B levels, which decreased in starvation and increased with subsequent re-feeding. Altogether data support a link between PLIN1B and TG storage in caterpillars prior to pupation. The combined findings suggest distinct roles for PLIN1A, PLIN1B and PLIN2. MsPLIN1A abundance correlates with mobilization of TG stores, MsPLIN2 with the synthesis of new LDs and MsPLIN1B abundance correlates with high levels of TG storage and large LD sizes at the end of the last feeding period.

Clues on the function of Manduca sexta perilipin 2 inferred from developmental and nutrition-dependent changes in its expression

Cellular triglycerides (TG) are stored in cytosolic lipid droplets (LDs). Perilipins (PLIN) are a group of LD-proteins that play important roles in the assembly and transport of LDs and in TG metabolism. Two members of the PLIN family are found in insects (PLIN1 & 2 or Lsd1 & 2). We have cloned and expressed Manduca sexta PLIN2 (MsPLIN2), and studied developmental and nutritional changes in the expression of PLIN2. Nutritional changes induced fast alterations in PLIN2 mRNA and protein levels in fat body and midgut of the feeding larvae. The relationship observed between PLIN2 expression and TG synthesis in both larval fat body and midgut suggests that PLIN2 is needed when tissues are accumulating TG. However, when the fat body was storing TG at maximal capacity, MsPLIN2 levels declined. This unexpected finding suggests the occurrence of alternative mechanism/s to shield TG from the action of lipases in M. sexta LDs. In addition, it implies that the cellular level of lipid storage could be modulating MsPLIN2 expression and/or degradation. The study also confirmed that MsPLIN2 was most abundant in the adult fat body, which is characterized by a high rate of TG hydrolysis and lipid mobilization. Whether MsPLIN2 is directly involved in lipolysis and/or the secretion of lipids in the fat body of adult of M. sexta is unknown at this time. Nonetheless, the coexistence of high PLIN2 and lipolysis levels suggests a complex role for MsPLIN2. Altogether, we found that MsPLIN2 is needed when the synthesis of glycerides, DG and TG, is active even if the insect is accumulating or consuming TG.

Multifaceted biological insights from a draft genome sequence of the tobacco hornworm moth, Manduca sexta

Manduca sexta, known as the tobacco hornworm or Carolina sphinx moth, is a lepidopteran insect that is used extensively as a model system for research in insect biochemistry, physiology, neurobiology, development, and immunity. One important benefit of this species as an experimental model is its extremely large size, reaching more than 10 g in the larval stage. M. sexta larvae feed on solanaceous plants and thus must tolerate a substantial challenge from plant allelochemicals, including nicotine. We report the sequence and annotation of the M. sexta genome, and a survey of gene expression in various tissues and developmental stages. The Msex_1.0 genome assembly resulted in a total genome size of 419.4 Mbp. Repetitive sequences accounted for 25.8% of the assembled genome. The official gene set is comprised of 15,451 protein-coding genes, of which 2498 were manually curated. Extensive RNA-seq data from many tissues and developmental stages were used to improve gene models and for insights into gene expression patterns. Genome wide synteny analysis indicated a high level of macrosynteny in the Lepidoptera. Annotation and analyses were carried out for gene families involved in a wide spectrum of biological processes, including apoptosis, vacuole sorting, growth and development, structures of exoskeleton, egg shells, and muscle, vision, chemosensation, ion channels, signal transduction, neuropeptide signaling, neurotransmitter synthesis and transport, nicotine tolerance, lipid metabolism, and immunity. This genome sequence, annotation, and analysis provide an important new resource from a well-studied model insect species and will facilitate further biochemical and mechanistic experimental studies of many biological systems in insects.

Monoacylglycerol and diacylglycerol acyltransferases and the synthesis of neutral glycerides in Manduca sexta

The insect fat body and the adipose tissue of vertebrates store fatty acids (FA) as triacylglycerols (TG). However, the fat body of most insects has the unique ability to rapidly produce and secrete large amounts of diacylglycerol (DG). Monoacylglycerol acyltransferase (MGAT), which catalyzes the synthesis of DG from MG, and a diacylglycerol acyltransferase (DGAT), which catalyzes the synthesis of TG from DG, are key enzymes in the metabolism of neutral glycerides. However, very little is known about these acyltransferases in insects. In the present study we have cloned two predicted MGATs and a DGAT from Manduca sexta and compared their sequences with predicted MGAT and DGAT homologs from a number of insect species. The comparison suggested that insects may only have a single DGAT gene, DGAT1. The apparent absence of a DGAT2 gene in insects would represent a major difference with vertebrates, which contain DGAT1 and DGAT2 genes. Insects seem to have a single MGAT gene which is similar to the MGAT2 of vertebrates. A number of conserved phosphorylation sites of potential physiological significance were identified among insect proteins and among insect and vertebrate proteins. DGAT1 and MGAT are expressed in fat body, midgut and ovaries. The relative rates of utilization of FAs for the synthesis of DG and TG correlated with the relative expression levels of MGAT and DGAT suggesting that regulation of the expression levels of these acyltransferases could be determining whether the fat body secretes DG or stores fatty acids as TG. The expression patterns of the acyltransferases suggest a role of the monoacylglycerol pathway in the production and mobilization of DG in M. sexta fat body.

Lipid Droplets as Signaling Platforms Linking Metabolic and Cellular Functions

The main cells of the adipose tissue of animals, adipocytes, are characterized by the presence of large cytosolic lipid droplets (LDs) that store triglyceride (TG) and cholesterol. However, most cells have LDs and the ability to store lipids. LDs have a well-known central role in storage and provision of fatty acids and cholesterol. However, the complexity of the regulation of lipid metabolism on the surface of the LDs is still a matter of intense study. Beyond this role, a number of recent studies have suggested that LDs have major functions in other cellular processes, such as protein storage and degradation, infection, and immunity. Thus, our perception of LDs has been radically transformed from simple globules of fat to highly dynamic organelles of unexpected complexity. Here, we compiled some recent evidence supporting the emerging view that LDs act as platforms connecting a number of relevant metabolic and cellular functions.

TGL-mediated lipolysis in Manduca sexta fat body: possible roles for lipoamide-dehydrogenase (LipDH) and high-density lipophorin (HDLp)

Triglyceride-lipase (TGL) is a major fat body lipase in Manduca sexta. The knowledge of how TGL activity is regulated is very limited. A WWE domain, presumably involved in protein-protein interactions, has been previously identified in the N-terminal region of TGL. In this study, we searched for proteins partners that interact with the N-terminal region of TGL. Thirteen proteins were identified by mass spectrometry, and the interaction with four of these proteins was confirmed by immunoblot. The oxidoreductase lipoamide-dehydrogenase (LipDH) and the apolipoprotein components of the lipid transporter, HDLp, were among these proteins. LipDH is the common component of the mitochondrial α-keto acid dehydrogenase complexes whereas HDLp occurs in the hemolymph. However, subcellular fractionation demonstrated that these two proteins are relatively abundant in the soluble fraction of fat body adipocytes. The cofactor lipoate found in typical LipDH substrates was not detected in TGL. However, TGL proved to have critical thiol groups. Additional studies with inhibitors are consistent with the notion that LipDH acting as a diaphorase could preserve the activity of TGL by controlling the redox state of thiol groups. On the other hand, when TG hydrolase activity of TGL was assayed in the presence of HDLp, the production of diacylglycerol (DG) increased. TGL-HDLp interaction could drive the intracellular transport of DG. TGL may be directly involved in the lipoprotein assembly and loading with DG, a process that occurs in the fat body and is essential for insects to mobilize fatty acids. Overall the study suggests that TGL occurs as a multi-protein complex supported by interactions through the WWE domain.

Membrane attachment and structure models of lipid storage droplet protein 1

Neutral lipid triglycerides, a main reserve for fat and energy, are stored in organelles called lipid droplets. The storage and release of triglycerides are actively regulated by several proteins specific to the droplet surface, one of which in insects is PLIN1. PLIN1 plays a key role in the activation of triglyceride hydrolysis upon phosphorylation. However, the structure of PLIN1 and its relation to functions remain elusive due to its insolubility and crystallization difficulty. Here we report the first solid-state NMR study on the Drosophila melanogaster PLIN1 in combination with molecular dynamics simulation to show the structural basis for its lipid droplet attachment. NMR spin diffusion experiments were consistent with the predicted membrane attachment motif of PLIN1. The data indicated that PLIN1 has close contact with the terminal methyl groups of the phospholipid acyl chains. Structure models for the membrane attachment motif were generated based on hydrophobicity analysis and NMR membrane insertion depth information. Simulated NMR spectra from a trans-model agreed with experimental spectra. In this model, lipids from the bottom leaflet were very close to the surface in the region enclosed by membrane attachment motif. This may imply that in real lipid droplet, triglyceride molecules might be brought close to the surface by the same mechanism, ready to leave the droplet in the event of lipolysis. Juxtaposition of triglyceride lipase structure to the trans-model suggested a possible interaction of a conserved segment with the lipase by electrostatic interactions, opening the lipase lid to expose the catalytic center.

Developmental changes in the protein composition of Manduca sexta lipid droplets

The lipid droplets (LDs) are intracellular organelles mainly dedicated to the storage and provision of fatty acids. To accomplish these functions the LDs interact with other organelles and cytosolic proteins. In order to explore possible correlations between the physiological states of cells and the protein composition of LDs we have determined and compared the proteomic profiles of lipid droplets isolated from the fat bodies of 5th-instar larvae and adult Manduca sexta insects and from ovaries. These LD-rich tissues represent three clearly distinct metabolic states in regard to lipid metabolism: 1) Larval fat body synthesizes fatty acids (FA) and accumulates large amounts as triglyceride (TG); 2) Fat body from adult insects provides FA to support reproduction and flight; 3) Ovaries do not synthesize FA, but accumulate considerable amounts of TG in LDs. Major qualitative and semi-quantitative variations in the protein compositions of the LDs isolated from these three tissues were observed by MS/MS and partially validated by immuno-blotting. The differences observed included changes in the abundance of lipid droplet specific proteins, cytosolic proteins, mitochondrial proteins and also proteins associated with the machinery of protein synthesis. These results suggest that changes in the interaction of LDs with other organelles and cytosolic proteins are tightly related to the physiological state of cells. Herein, we summarize and compare the protein compositions of three subtypes of LDs and also describe for the first time the proteomic profile of LDs from an insect ovary. The compositions and compositional differences found among the LDs are discussed to provide a platform for future studies on the role of LDs, and their associated proteins, in cellular metabolism.

Characterization of apoA-I-dependent lipid efflux from adipocytes and role of ABCA1

Adipose tissue is a major reservoir of cholesterol and, as such, it may play a significant role in cholesterol homeostasis. The aims of this study were to obtain a quantitative characterization of apolipoprotein A-I (apoA-I)-dependent lipid efflux from adipocytes and examine the role of ATP-binding cassette transporter A1 (ABCA1) in this process. The rates of apoA-I-induced cholesterol and phospholipid efflux were determined and normalized by cellular protein or ABCA1 levels. In order to allow a comparative analysis, parallel experiments were also performed in macrophages. These studies showed that apoA-I induces cholesterol efflux from adipocytes at similar rates as from macrophages. Enhancement of the expression of ABCA1 increased the rates of cholesterol efflux from both adipocytes and macrophages. The results also suggested that a non-ABCA1-dependent mechanism could make significant contributions to the rate of apoA-I-dependent cholesterol efflux when the expression levels of ABCA1 are low. Furthermore, the study of the effect of inhibitors of lipid efflux showed that glyburide and brefeldin A, which affect ABCA1 function, exerted strong and similar inhibitory effects on lipid efflux from both adipocytes and macrophages, whereas BLT1, an SRB-I inhibitor, only exerted a moderate inhibition. Overall these studies suggest that ABCA1 plays a major role in apoA-I-dependent lipid efflux from adipocytes and showed high similarities between the abilities of adipocytes and macrophages to release cholesterol in an apoA-I-dependent fashion.

Mobilization of lipid stores in Manduca sexta: cDNA cloning and developmental expression of fat body triglyceride lipase, TGL

Fatty acids stored as triglycerides (TG) in the fat body serve as precursor in multiple processes including energy production and synthesis of cellular components. Mobilization of fatty acids from TG depends on the action of lipases. The fat body triglyceride lipase from Manduca sexta, MsTGL, is the only insect lipase that has been purified and characterized, so far. A TGL cDNA from M. sexta fat body encoding a 649 amino acid protein was cloned and its identity confirmed by mass spectrometry and Edman sequencing data of the purified protein. The protein sequence has conserved domains and residues of potential importance for the function and regulation of TGL activity. The expression of TGL and the lipase activity of fat body homogenates were studied in several developmental stages of M. sexta. TG-hydrolase activity of fat body increased as larva grew to the last instar and, then, decreased to minimal levels during pupa stage. Lipase activity was progressively restored in adult insects and reached maximum values at this stage. The fat body lipase activity from adult insects, 1-2 day after emergence, was 9-fold higher than that from 2 to 3 days old 5th-instar larvae. A good correlation was found between the abundance of TGL protein and the lipase activity of fat body homogenates. This correlation and the expression pattern of TGL throughout development are consistent with the notion that TGL is the main fat body TG lipase of M. sexta.

Deep sequencing of small RNA libraries reveals dynamic regulation of conserved and novel microRNAs and microRNA-stars during silkworm development

BACKGROUND

In eukaryotes, microRNAs (miRNAs) have emerged as critical regulators of gene expression. The Silkworm (Bombyx mori L.) is one of the most suitable lepidopteran insects for studying the molecular aspects of metamorphosis because of its large size, availability of mutants and genome sequence. Besides, this insect also has been amply studied from a physiological and biochemical perspective. Deep sequencing of small RNAs isolated from different stages of silkworm is a powerful tool not only for measuring the changes in miRNA profile but also for discovering novel miRNAs.

RESULTS

We generated small RNA libraries from feeding larvae, spinning larvae, pupae and adults of B. mori and obtained approximately 2.5 million reads of 18-30 nt. Sequence analysis identified 14 novel and 101 conserved miRNAs. Most novel miRNAs are preferentially expressed in pupae, whereas more than 95% of the conserved miRNAs are dynamically regulated during different developmental stages. Remarkably, the miRNA-star (miR*) of four miRNAs are expressed at much higher levels than their corresponding miRNAs, and their expression profiles are distinct from their corresponding miRNA profiles during different developmental stages. Additionally, we detected two antisense miRNA loci (miR-263-S and miR-263-AS; miR-306-S and miR-306-AS) that are expressed in sense and antisense directions. Interestingly, miR-263 and miR-306 are preferentially and abundantly expressed in pupae and adults, respectively.

CONCLUSIONS

We identified 101 homologs of conserved miRNAs, 14 species-specific and two antisense miRNAs in the silkworm. Our results provided deeper insights into changes in conserved and novel miRNA and miRNA* accumulation during development.

Deep sequencing of small RNA libraries reveals dynamic regulation of conserved and novel microRNAs and microRNA-stars during silkworm development

BACKGROUND

In eukaryotes, microRNAs (miRNAs) have emerged as critical regulators of gene expression. The Silkworm (Bombyx mori L.) is one of the most suitable lepidopteran insects for studying the molecular aspects of metamorphosis because of its large size, availability of mutants and genome sequence. Besides, this insect also has been amply studied from a physiological and biochemical perspective. Deep sequencing of small RNAs isolated from different stages of silkworm is a powerful tool not only for measuring the changes in miRNA profile but also for discovering novel miRNAs.

RESULTS

We generated small RNA libraries from feeding larvae, spinning larvae, pupae and adults of B. mori and obtained approximately 2.5 million reads of 18-30 nt. Sequence analysis identified 14 novel and 101 conserved miRNAs. Most novel miRNAs are preferentially expressed in pupae, whereas more than 95% of the conserved miRNAs are dynamically regulated during different developmental stages. Remarkably, the miRNA-star (miR*) of four miRNAs are expressed at much higher levels than their corresponding miRNAs, and their expression profiles are distinct from their corresponding miRNA profiles during different developmental stages. Additionally, we detected two antisense miRNA loci (miR-263-S and miR-263-AS; miR-306-S and miR-306-AS) that are expressed in sense and antisense directions. Interestingly, miR-263 and miR-306 are preferentially and abundantly expressed in pupae and adults, respectively.

CONCLUSIONS

We identified 101 homologs of conserved miRNAs, 14 species-specific and two antisense miRNAs in the silkworm. Our results provided deeper insights into changes in conserved and novel miRNA and miRNA* accumulation during development.

Insect fat body: energy, metabolism, and regulation

The fat body plays major roles in the life of insects. It is a dynamic tissue involved in multiple metabolic functions. One of these functions is to store and release energy in response to the energy demands of the insect. Insects store energy reserves in the form of glycogen and triglycerides in the adipocytes, the main fat body cell. Insect adipocytes can store a great amount of lipid reserves as cytoplasmic lipid droplets. Lipid metabolism is essential for growth and reproduction and provides energy needed during extended nonfeeding periods. This review focuses on energy storage and release and summarizes current understanding of the mechanisms underlying these processes in insects.

Structure and stability of crustacean lipovitellin: influence of lipid content and composition

Lipovitellin (LV) is essential in crustacean eggs for embryo viability and development. Two LV were isolated from eggs of Macrobrachium borellii. corresponding to early (LVe ) and late (LVl) embryo developing stages. They differ in lipid composition but not in lipid/protein ratio or apoprotein composition. Structural information was obtained by fluorescence spectroscopy, far-UV circular dichroism, partial trypsinolysis and electron microscopy applied to LVe and LVl and two partially delipidated forms of LVe generated by phospholipase A2 (LVp) or Triton X-100 (LVt) treatment. All LV forms contained two apoprotein subunits of 94 and 112 kDa, being the 112k Da subunit more accessible to trypsinolysis in all. Only in LVp, different cleavage sites appeared. Secondary structure was similar in LVe and LVl, but LVp and LVt showed a small increase in beta-sheet at expense of alpha-helix. Electron microscopy revealed a spheroidal morphology in all LV and a decreased size in LVp. Delipidated LVs were more resistant to denaturation with guanidinium-HCl. Acrylamide quenching of tryptophan fluorescence was more efficient in delipidated LVs, probably due to apolipoprotein rearrangement, as reinforced by fluorescence anisotropy. It is concluded that LV stability, shape, and apoprotein conformation is not largely affected by the changes in lipid composition that take place during embryogenesis.

Purification and characterization of recombinant lipid storage protein-2 from Drosophila melanogaster

Lipid storage protein 2 (Lsd 2) is a conserved insect protein that belongs to the small PAT family of proteins. PAT proteins are found associated to the lipid droplets of adipocytes and play significant roles in the regulation of triacylglycerides metabolism. Here we describe the expression and purification of Lsd2, its reconstitution in lipoprotein particles, the location of putative lipid binding sites and its secondary structure. This study provides the starting point for future studies on the mechanism of function of Lsd2. The similarities and differences between Lsd1 and Lsd2, the only PAT proteins found in insects, are discussed.

The central kink region of fowlicidin-2, an alpha-helical host defense peptide, is critically involved in bacterial killing and endotoxin neutralization

Fowlicidins are a group of newly identified chicken cathelicidin host defense peptides. We have shown that the putatively mature fowlicidin-2 of 31 amino acid residues possesses potent antibacterial and lipopolysaccharide (LPS)- neutralizing activities, but with a noticeable toxicity to mammalian cells. As a first step in exploring the structure-activity relationships of fowlicidin-2, in this study we determined its tertiary structure by nuclear magnetic resonance spectroscopy. Unlike the majority of cathelicidins, which are composed of a predominant alpha-helix with a short hinge sequence near the center, fowlicidin-2 consists of 2 well-defined alpha-helical segments (residues 6-12 and 23-27) connected by a long extensive kink (residues 13-20) induced by proline. To further investigate the functional significance of each of these structural components, several N- and C-terminal deletion analogs of fowlicidin-2 were synthesized and analyzed for their antibacterial, cytotoxic and LPS-neutralizing activities. Our results indicated that neither the N- nor C-terminal alpha-helix alone is sufficient to confer any function. Rather, fowlicidin-2(1-18) and fowlicidin-2(15-31), 2 alpha-helical segments with inclusion of the central cationic kink region, retained substantial capacities to kill bacteria and neutralize the LPS-induced proinflammatory response, relative to the parent peptide. More desirably, these 2 peptide analogs showed substantially reduced toxicity to human erythrocytes and epithelial cells, indicative of improved potential as antibacterial and antisepsis agents. To our knowledge, fowlicidin-2 is the first alpha-helical cathelicidin, with the central kink region shown to be critically important in killing bacteria and neutralizing LPS.

Expression of lipid storage droplet protein-1 may define the role of AKH as a lipid mobilizing hormone in Manduca sexta

Adipokinetic hormone (AKH) is the main hormone involved in the acute regulation of hemolymph lipid levels in several insects. In adult Manduca sexta AKH promotes a rapid phosphorylation of "Lipid storage protein-1", Lsd1, and a concomitant activation of the rate of hydrolysis of triglycerides by the main fat body lipase. In contrast, in the larval stage AKH modulates hemolymph trehalose levels. The present study describes the sequence of a full-length Lsd1 cDNA obtained from M. sexta fat body and investigates a possible link between Lsd1 expression and the distinct effects of AKH in larva and adult insects. The deduced protein sequence showed a high degree of conservation compared to other insect Lsd1s, particularly in the central region of the protein (amino acids 211-276) in which the predicted lipid binding helices are found. Lsd1 was absent in feeding larva and its abundance progressively increased as the insect develops from the non-feeding larva to adult. Contrasting with the levels of protein, Lsd1 transcripts were maximal during the feeding larval stages. The subcellular distribution of Lsd1 showed that the protein exclusively localizes in the lipid droplets. Lsd1 was found in the fat body but it was undetectable in lipid droplets isolated from oocytes or embryos. The present study suggests a link between AKH-stimulated lipolysis in the fat body and the expression of Lsd1.

Brefeldin A inhibits cholesterol efflux without affecting the rate of cellular uptake and re-secretion of apolipoprotein A-I in adipocytes

A possible role of cellular uptake and re-secretion of apoA-I in the mechanism of cholesterol efflux induced by apoA-I was investigated using a novel experimental approach. Incubation of adipocytes with a recombinant human apoA-I containing a consensus PKA phosphorylation site, pka-ApoA-I, leads to the appearance of phosphorylated protein in the cell culture medium unambiguously proving cellular uptake and re-secretion of pka-ApoA-I. Phosphorylation of apoA-I is abolished by PKA inhibitors and enhanced by PKA activators demonstrating the specific involvement of PKA. Studies on the concentration dependence of pka-apoA-I phosphorylation and competition experiments with human apoA-I suggest that apolipoprotein uptake is a receptor mediated process. A possible role of apoA-I recycling in the mechanism of cholesterol efflux was investigated by determining the rates of apoA-I induced cholesterol efflux and apoA-I recycling in the presence and in the absence of Brefeldin A (BFA). The studies showed that BFA strongly inhibits cholesterol efflux without affecting the rate of apoA-I recycling. Since BFA affects vesicular trafficking of ABCA1, this study suggests that the interaction of apoA-I with ABCA1 does not mediate apolipoprotein uptake and re-secretion. This result suggests that lipidation of apoA-I and apolipoprotein uptake/re-secretion are independent processes.

Function and structure of lipid storage droplet protein 1 studied in lipoprotein complexes

Triglycerides (TG) stored in lipid droplets (LDs) are the main energy reserve in all animals. The mechanism by which animals mobilize TG is complex and not fully understood. Several proteins surrounding the LDs have been implicated in TG homeostasis such as mammalian perilipin A and insect lipid storage proteins (Lsd). Most of the knowledge on LD-associated proteins comes from studies using cells or LDs leaving biochemical properties of these proteins uncharacterized. Here we describe the purification of recombinant Lsd1 and its reconstitution with lipids to form lipoprotein complexes suitable for functional and structural studies. Lsd1 in the lipid bound state is a predominately alpha-helical protein. Using lipoprotein complexes containing triolein it is shown that PKA mediated phosphorylation of Lsd1 promoted a 1.7-fold activation of the main fat body lipase demonstrating the direct link between Lsd1 phosphorylation and activation of lipolysis. Serine 20 was identified as the Lsd1-phosphorylation site triggering this effect.

Substrate and inhibitor specificity of Mycobacterium avium dihydrofolate reductase

Dihydrofolate reductase (EC 1.5.1.3) is a key enzyme in the folate biosynthetic pathway. Information regarding key residues in the dihydrofolate-binding site of Mycobacterium avium dihydrofolate reductase is lacking. On the basis of previous information, Asp31 and Leu32 were selected as residues that are potentially important in interactions with dihydrofolate and antifolates (e.g. trimethoprim), respectively. Asp31 and Leu32 were modified by site-directed mutagenesis, giving the mutants D31A, D31E, D31Q, D31N and D31L, and L32A, L32F and L32D. Mutated proteins were expressed in Escherichia coli BL21(DE3)pLysS and purified using His-Bind resin; functionality was assessed in comparison with the recombinant wild type by a standard enzyme assay, and growth complementation and kinetic parameters were evaluated. All Asp31 substitutions affected enzyme function; D31E, D31Q and D31N reduced activity by 80-90%, and D31A and D31L by > 90%. All D31 mutants had modified kinetics, ranging from three-fold (D31N) to 283-fold (D31L) increases in K(m) for dihydrofolate, and 12-fold (D31N) to 223 077-fold (D31L) decreases in k(cat)/K(m). Of the Leu32 substitutions, only L32D caused reduced enzyme activity (67%) and kinetic differences from the wild type (seven-fold increase in K(m); 21-fold decrease in k(cat)/K(m)). Only minor variations in the K(m) for NADPH were observed for all substitutions. Whereas the L32F mutant retained similar trimethoprim affinity as the wild type, the L32A mutation resulted in a 12-fold decrease in affinity and the L32D mutation resulted in a seven-fold increase in affinity for trimethoprim. These findings support the hypotheses that Asp31 plays a functional role in binding of the substrate and Leu32 plays a functional role in binding of trimethoprim.

Stimulation of lipolysis enhances the rate of cholesterol efflux to HDL in adipocytes

Adipose tissue constitutes a major location for cholesterol storage and, as such, it may play a role in the regulation of circulating cholesterol levels. A possible metabolic link between the lipolytic activity of adipocytes and their ability to release cholesterol to reconstituted human high density lipoprotein, HDL, was investigated in 3T3-L1 adipocytes. In the presence of HDL, composed of human apoA-I and phosphatidylcholine, adipocytes release cholesterol in a lipoprotein-dose and time dependent fashion. beta-adrenergic activation of the lipolysis promotes a 22% increase in the extent of cholesterol efflux to reconstituted discoidal HDL particles. Activation of lipolysis promotes a rapid decrease in the cholesterol content of the plasma membrane and a concomitant increase in lipid droplet cholesterol. This change is independent of the presence of HDL. Activation of the lipolysis does not affect the levels of ABCA1 and SR-BI. Therefore, the enhancement of cholesterol efflux is not due to the level of plasma membrane cholesterol, or to the levels of the cholesterol transporters ABCA1 and scavenger receptor SR-BI. Brefeldin A did not affect the rate of cholesterol efflux under basal lipolytic conditions, but it abolished the lipolysis-dependent enhancement of cholesterol efflux to HDL. This study suggests that activation of lipolysis is accompanied by an increase in BFA-sensitive vesicular transport that in turn enhances cholesterol efflux to HDL. The study supports a metabolic link between the lipolytic activity of adipocytes and the rate of cellular cholesterol efflux to HDL.

Fowlicidin-3 is an alpha-helical cationic host defense peptide with potent antibacterial and lipopolysaccharide-neutralizing activities

Cathelicidins are an important family of cationic host defense peptides in vertebrates with both antimicrobial and immunomodulatory activities. Fowlicidin-1 and fowlicidin-2 are two newly identified chicken cathelicidins with potent antibacterial activities. Here we report structural and functional characterization of the putatively mature form of the third chicken cathelicidin, fowlicidin-3, for exploration of its therapeutic potential. NMR spectroscopy revealed that fowlicidin-3 comprises 27 amino-acid residues and adopts a predominantly alpha-helical structure extending from residue 9 to 25 with a slight kink induced by a glycine at position 17. It is highly potent against a broad range of Gram-negative and Gram-positive bacteria in vitro, including antibiotic-resistant strains, with minimum inhibitory concentrations in the range 1-2 microM. It kills bacteria quickly, permeabilizing cytoplasmic membranes immediately on coming into contact with them. Unlike many other host defense peptides with antimicrobial activities that are diminished by serum or salt, fowlicidin-3 retains bacteria-killing activities in the presence of 50% serum or physiological concentrations of salt. Furthermore, it is capable of suppressing lipopolysaccharide-induced expression of proinflammatory genes in mouse macrophage RAW264.7 cells, with nearly complete blockage at 10 microM. Fowlicidin-3 appears to be an excellent candidate for future development as a novel antimicrobial and antisepsis agent, particularly against antibiotic-resistant pathogens.

Canine cathelicidin (K9CATH): gene cloning, expression, and biochemical activity of a novel pro-myeloid antimicrobial peptide

Cathelicidins, a group of cationic peptides found in leukocytes and epithelial cells, play a central role in the early innate immune defense against infection. Although these host defense peptides have been reported in several mammalian species, including primates, no cathelicidins have been identified in carnivores. Here we report the cloning, tissue expression and biological activity of a novel canine cathelicidin (K9CATH). The full-length cDNA sequence of K9CATH encodes a predicted 172 amino acid pre-propeptide that is 60-70% similar to other mammalian cathelicidins. Mass spectrometry analysis confirmed that the 38 aa mature K9CATH peptide was present in neutrophil granule contents. Synthetic K9CATH displayed broad antimicrobial activity against Gram-positive bacteria (Listeria monocytogenes, and Staphylococcus aureus; MICs (minimal inhibitory concentrations) 0.5 and 50 microM, respectively), Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae, Salmonella serotype Typhimurium, Pseudomonas aeruginosa, Proteus mirabilis; MICs 1.25 microM, Salmonella serotype Enteritidis; MIC 0.5 microM, and Neisseria gonorrhoeae; MIC 0.06 microM), and yeast (Candida albicans; MIC 12.5-50 microM). K9CATH demonstrated high antimicrobial activity against Ureaplasma canigenitalium, and lower activity against Ureaplasma urealyticum (MIC 0.06 and 50 microM, respectively). Similar to its ovine congener SMAP-29, K9CATH possesses salt-independent antimicrobial activity and LPS binding capacity. K9CATH displayed minimal hemolytic activity against human, dog and chicken erythrocytes. The potency and broad antimicrobial activity of K9CATH suggest that this peptide may act as a fundamental contributor to the innate immune responses in this carnivore species.

Structural characterization of the lipovitellin from the shrimp Macrobrachium borellii

In oviparous species, proteins and lipids are found in the vitellus forming lipoproteins called lipovitellins. They are an important energy source for embryos development and larvae growth and survival. We have previously isolated and partially characterized the sole egg cytosolic lipovitellin from the freshwater shrimp Macrobrachium borellii. It is a native protein of 440 kDa, composed of two subunits of 94 and 112 kDa. In the present work we studied size, shape and structure of M. borellii lipovitellin using electron microscopy, crosslinking reagents, MALDI-TOF, circular dichroism, fluorescence and partial proteolysis. The results showed that lipovitellin has a quasi spherical morphology with an estimated diameter of 18.5+/-3.5 nm. It appears to be composed of two subunits of 94 kDa, and one of 112 kDa. The larger subunit is more susceptible to trypsinolysis, indicating that it is less compactly folded and/or more exposed to the aqueous medium than the 94 kDa subunits. The hetero-trimer is held together by non-covalent interactions. Peptide mass fingerprinting by MALDI-TOF, produced 42 polypeptides matching to a vitellogenin of a related species (Macrobrachium rosenbergii). Circular dichroism indicated that this protein contains 35.7% alpha-helix, 16.6% beta-sheet and 20% turns. Tryptophan fluorescence emission, at a maximum of 334 nm, indicated that the environment polarity of these aromatic residues is similar to that of other crustacean lipoproteins.

Adipokinetic hormone-induced mobilization of fat body triglyceride stores in Manduca sexta: role of TG-lipase and lipid droplets

Triglycerides (TG) stores build up in the insect fat body as lipid droplets at times of excess of food. The mobilization of fat body triglyceride (TG) is stimulated by adipokinetic hormones (AKH). The action of AKH involves a rapid activation of cAMP-dependent protein kinase (PKA). Recent in vitro studies have shown that PKA phosphorylates and activates the TG-lipase substrate, the lipid droplets. Conversely, purified TG-lipase from Manduca sexta fat body is phosphorylated by PKA in vitro but is not activated. This study was directed to learn whether or not AKH promotes a change in the state of phosphorylation of the lipase in vivo, and what are the relative contributions of cytosol and lipid droplets to the overall increase of lipolysis triggered by AKH. TG-lipase activity of fat body cytosols isolated from control and AKH-treated insects was determined against the native substrate, in vivo [3H]-TG radiolabeled lipid droplets, obtained from control and AKH-treated insects. The lipase activity of the system composed of AKH-cytosol and AKH-lipid droplets (11.1 +/- 2.1 nmol TG/min-mg) was 3.1-fold higher than that determined with control cytosol and lipid droplets (3.6 +/- 0.5 nmol TG/min-mg). Evaluation of the role of AKH-induced changes in the lipid droplets on lipolysis showed that changes in the lipid droplets are responsible for 70% of the lipolytic response to AKH. The remaining 30% appears to be due to AKH-dependent changes in the cytosol. However, the phosphorylation level of the TG-lipase was unchanged by AKH, indicating that phosphorylation of the TG-lipase plays no role in the activation of lipolysis induced by AKH.

The main triglyceride-lipase from the insect fat body is an active phospholipase A(1): identification and characterization

The main triglyceride-lipase (TG-lipase) from the fat body of Manduca sexta has been identified as the homolog of Drosophila melanogaster CG8552. This protein is conserved among insects and also shares significant sequence similarity with vertebrate phospholipases (PLs) from the phosphatidic acid preferring-phospholipase A1 (PA-PLA(1)) family. It is shown here that the TG-lipase is also a PL. TG-lipase and PL activities copurify and are inhibited by, or resistant to, the same lipase inhibitors, indicating that both activities are catalyzed by the same enzyme and active site. The PL activity of TG-lipase corresponded to PL type A(1). The concentration dependence of lipase activity with TG and PL micellar substrates showed saturation kinetics, with apparent K(m) values of 152 +/- 11 and 7.8 +/- 1.1 muM, respectively. TG-lipase was able to hydrolyze the major phospholipid components of the lipid droplets, phosphatidylcholine and phosphatidylethanolamine. The enzyme hydrolyzes 77 molecules of TG for every molecule of PL contained in the lipid droplets. It was observed that the activation of lipolysis in vivo is accompanied by activation of the hydrolysis of phospholipids of the lipid droplets. These results suggest that the PL activity of the insect TG-lipase could be required to allow access of the lipase to TG molecules contained in the core of the lipid droplets.

Structure-activity relationships of fowlicidin-1, a cathelicidin antimicrobial peptide in chicken

Cationic antimicrobial peptides are naturally occurring antibiotics that are actively being explored as a new class of anti-infective agents. We recently identified three cathelicidin antimicrobial peptides from chicken, which have potent and broad-spectrum antibacterial activities in vitro (Xiao Y, Cai Y, Bommineni YR, Fernando SC, Prakash O, Gilliland SE & Zhang G (2006) J Biol Chem281, 2858-2867). Here we report that fowlicidin-1 mainly adopts an alpha-helical conformation with a slight kink induced by glycine close to the center, in addition to a short flexible unstructured region near the N terminus. To gain further insight into the structural requirements for function, a series of truncation and substitution mutants of fowlicidin-1 were synthesized and tested separately for their antibacterial, cytolytic and lipopolysaccharide (LPS)-binding activities. The short C-terminal helical segment after the kink, consisting of a stretch of eight amino acids (residues 16-23), was shown to be critically involved in all three functions, suggesting that this region may be required for the peptide to interact with LPS and lipid membranes and to permeabilize both prokaryotic and eukaryotic cells. We also identified a second segment, comprising three amino acids (residues 5-7) in the N-terminal flexible region, that participates in LPS binding and cytotoxicity but is less important in bacterial killing. The fowlicidin-1 analog, with deletion of the second N-terminal segment (residues 5-7), was found to retain substantial antibacterial potency with a significant reduction in cytotoxicity. Such a peptide analog may have considerable potential for development as an anti-infective agent.

Activation of the lipid droplet controls the rate of lipolysis of triglycerides in the insect fat body

The hydrolysis of triglyceride (TG) stored in the lipid droplets of the insect fat body is under hormonal regulation by the adipokinetic hormone (AKH), which triggers a rapid activation cAMP-dependent kinase cascade (protein kinase A (PKA)). The role of phosphorylation on two components of the lipolytic process, the TG-lipase and the lipid droplet, was investigated in fat body adipocytes. The activity of purified TG-lipase determined using in vivo TG-radiolabeled lipid droplets was unaffected by the phosphorylation of the lipase. However, the activity of purified lipase was 2.4-fold higher against lipid droplets isolated from hormone-stimulated fat bodies than against lipid droplets isolated from unstimulated tissue. In vivo stimulation of lipolysis promotes a rapid phosphorylation of a lipid droplet protein with an apparent mass of 42-44 kDa. This protein was identified as "Lipid Storage Droplet Protein 1" (Lsdp1). In vivo phosphorylation of this protein reached a peak approximately 10 min after the injection of AKH. Supporting a role of Lsdp1 in lipolysis, maximum TG-lipase activity was also observed with lipid droplets isolated 10 min after hormonal stimulation. The activation of lipolysis was reconstituted in vitro using purified insect PKA and TG-lipase and lipid droplets. In vitro phosphorylation of lipid droplets catalyzed by PKA enhanced the phosphorylation of Lsdp1 and the lipolytic rate of the lipase, demonstrating a prominent role PKA and protein phosphorylation on the activation of the lipid droplets. AKH-induced changes in the properties of the substrate do not promote a tight association of the lipase with the lipid droplets. It is concluded that the lipolysis in fat body adipocytes is controlled by the activation of the lipid droplet. This activation is achieved by PKA-mediated phosphorylation of the lipid droplet. Lsdp1 is the main target of PKA, suggesting that this protein is a major player in the activation of lipolysis in insects.

cAMP-dependent protein kinase of Manduca sexta phosphorylates but does not activate the fat body triglyceride lipase

cAMP-dependent-protein kinase (PKA) is a central player of the adipokinetic signal that controls the mobilization of stored lipids in the fat body. Previous studies showed that adipokinetic hormone (AKH) rapidly activates PKA from the fat body of Manduca sexta (Arrese et al. (J. Lipid. Res. 40(3): 556)). As a part of our investigation on lipolysis in insects, here we report the purification and characterization of the catalytic subunit of PKA from the fat body of M. sexta and its role in the direct activation of the TG lipase in vitro. PKA was purified to apparent homogeneity and the identity of the protein was confirmed by MALDI-TOF and Western blot analysis. The enzyme showed a high affinity for Mg-ATP (Km = 39 microM) and Kemptide (Km = 31 microM) and was strongly inhibited by the PKA specific inhibitors PKI 5-24 and H89. Manduca sexta PKA only recognized serine residues as phosphate acceptor; theronine or tyrosine containing peptides were not phosphorylated. Purified fat body TG-lipase proved to be a good substrate of the purified kinase. However, phosphorylation of the lipase did not enhance the lipolytic activity of the enzyme in vitro. These results suggest that, besides lipase phosphorylation, the mechanism of AKH-induced activation of the lipolysis requires the involvement of other proteins and/or signals.

In vivo lipoprotein binding assay of the insect exchangeable apolipoprotein, apolipophorin-III

An original method for the study of the lipid binding properties of exchangeable apolipoproteins is reported. Binding of Locusta migratoria apolipophorin-III to Manduca sexta low-density lipophorin (LDLp) and high-density lipophorin (HDLp) was studied in vivo. This assay could be used useful to investigate the effect of mutations in the lipid binding properties of exchangeable apolipoproteins under physiological conditions.

Role of helices and loops in the ability of apolipophorin-III to interact with native lipoproteins and form discoidal lipoprotein complexes

The structure of Locusta migratoria apolipophorin-III consists of a five-helix bundle connected by four short loops. The role of the conformational flexibility of helices and loops on the lipid-binding activity of this apolipoprotein was investigated by disulfide mediated tethering experiments. One disulfide mutant tethering the second and fourth loops (L2-L4), and two disulfide mutants restricting the flexibility of the neighboring alpha-helices 3 and 4 (H3-H4) and 1 and 5 (H1-H5), were studied. The ability of the disulfide mutants to interact with phospholipid vesicles, mixed micelles of phosphatidylcholine and cholate, and in vivo with native spherical lipoprotein particles was studied. The L2-L4 mutant was active with native lipoproteins as well as being able to form discoidal lipoproteins upon incubation with either liposomes or discoidal micelles. The H3-H4 mutant was not able to interact with liposomes or native lipoproteins but interacted with discoidal micelles. The H1-H5 mutant was unable to interact with lipid in any of the three systems. Three conclusions were reached: (1) opening of the helix bundle does not require the separation of loops 2 and 4 as recently proposed by others and (2) alpha-helices 3 and/or 4 are involved in the insertion of apoLp-III in both phospholipid bilayers and monolayers. The conformational flexibility of helices 3 and 4 is required for the lipid-binding activity of apoLp-III. (3) Interaction of helices 1 and/or 5 with the lipid surface is required to the formation of stable lipoprotein complexes of any kind.

Conformation of a group 2 late embryogenesis abundant protein from soybean. Evidence of poly (L-proline)-type II structure

Late embryogenesis abundant (LEA) proteins are members of a large group of hydrophilic, glycine-rich proteins found in plants, algae, fungi, and bacteria known collectively as hydrophilins that are preferentially expressed in response to dehydration or hyperosmotic stress. Group 2 LEA (dehydrins or responsive to abscisic acid) proteins are postulated to stabilize macromolecules against damage by freezing, dehydration, ionic, or osmotic stress. However, the structural and physicochemical properties of group 2 LEA proteins that account for such functions remain unknown. We have analyzed the structural properties of a recombinant form of a soybean (Glycine max) group 2 LEA (rGmDHN1). Differential scanning calorimetry of purified rGmDHN1 demonstrated that the protein does not display a cooperative unfolding transition upon heating. Ultraviolet absorption and circular dichroism spectroscopy revealed that the protein is in a largely hydrated and unstructured conformation in solution. However, ultraviolet absorption and circular dichroism measurements collected at different temperatures showed that the protein exists in equilibrium between two extended conformational states: unordered and left-handed extended helical or poly (L-proline)-type II structures. It is estimated that 27% of the residues of rGmDHN1 adopt or poly (L-proline)-type II-like helical conformation at 12 degrees C. The content of extended helix gradually decreases to 15% as the temperature is increased to 80 degrees C. Studies of the conformation of the protein in solution in the presence of liposomes, trifluoroethanol, and sodium dodecyl sulfate indicated that rGmDHN1 has a very low intrinsic ability to adopt alpha-helical structure and to interact with phospholipid bilayers through amphipathic alpha-helices. The ability of the protein to remain in a highly extended conformation at low temperatures could constitute the basis of the functional role of GmDHN1 in the prevention of freezing, desiccation, ionic, or osmotic stress-related damage to macromolecular structures.

Structure of apolipophorin-III in discoidal lipoproteins. Interhelical distances in the lipid-bound state and conformational change upon binding to lipid

The structure of apolipophorin III in the lipid-bound state and the extent of the conformational change that takes place when the five-helix bundle apolipoprotein binds to a lipoprotein lipid surface were investigated by fluorescence resonance energy transfer in discoidal lipoproteins. Four intramolecular interhelical distances between helix pairs 1-4, 2-4, 3-4, and 5-4 were estimated by fluorescence resonance energy transfer in both the lipid-free and the lipid-bound states. Depending on the helices pairs, the intramolecular interhelical distances increased between 15 and > or = 20 A upon binding of the apolipoprotein to lipid, demonstrating for the first time that binding to lipid is accompanied by a major change in interhelical distances. Using discoidal lipoproteins made with a combination of apolipophorin III molecules containing donor and acceptor groups and apolipophorin III molecules containing neither donor nor acceptor groups, it was possible to obtain information about intermolecular interhelical distances between the helix 4 of one apolipoprotein and the helices 1, 2, 3, and 5 of a second apolipoprotein residing in the same discoidal lipoprotein. Altogether, the estimated intermolecular and intramolecular interhelical distances suggest a model in which the apolipoprotein arranges in pairs of antiparallel and fully extended polypeptide chains surrounding the periphery of the bilayer disc.

Temperature-induced extended helix/random coil transitions in a group 1 late embryogenesis-abundant protein from soybean

Group 1 late embryogenesis-abundant (LEA) proteins are a subset of hydrophilins that are postulated to play important roles in protecting plant macromolecules from damage during freezing, desiccation, or osmotic stress. To better understand the putative functional roles of group 1 LEA proteins, we analyzed the structure of a group 1 LEA protein from soybean (Glycine max). Differential scanning calorimetry of the purified, recombinant protein demonstrated that the protein assumed a largely unstructured state in solution. In the presence of trifluoroethanol (50% [w/v]), the protein acquired a 30% alpha-helical content, indicating that the polypeptide is highly restricted to adopt alpha-helical structures. In the presence of sodium dodecyl sulfate (1% [w/v]), 8% of the polypeptide chain adopted an alpha-helical structure. However, incubation with phospholipids showed no effect on the protein structure. Ultraviolet absorption and circular dichroism spectroscopy revealed that the protein existed in equilibrium between two conformational states. Ultraviolet absorption spectroscopy studies also showed that the protein became more hydrated upon heating. Furthermore, circular dichroism spectral measurements indicated that a minimum of 14% of amino acid residues existed in a solvent-exposed, left-handed extended helical or poly (L-proline)-type (PII) conformation at 20 degrees C with the remainder of the protein being unstructured. The content of PII-like structure increased as temperature was lowered. We hypothesize that by favoring the adoption of PII structure, instead of the formation of alpha-helical or beta-sheet structures, group 1 LEA proteins retain a high content of surface area available for interaction with the solvent. This feature could constitute the basis of a potential role of LEA proteins in preventing freezing, desiccation, or osmotic stress damage.

Interaction of the alpha-helices of apolipophorin III with the phospholipid acyl chains in discoidal lipoprotein particles: a fluorescence quenching study

Quenching of tryptophan fluorescence by nitroxide-labeled phospholipids and nitroxide-labeled fatty acids was used to investigate the lipid-binding domains of apolipophorin III. The location of the Trp residues relative to the lipid bilayer was investigated in discoidal lipoprotein particles made with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and five different single-Trp mutants of apoLp-III. A comparison of the quenching efficiencies of phospholipids containing nitroxide groups at the polar head, and at positions 5 and 16 of the sn-2 acyl chain, indicated that the protein is interacting with the acyl chains of the phospholipid along the periphery of the bilayer of the discoidal lipoprotein. N-Bromosuccinimide readily abolished 100% of the fluorescence of all Trp residues in the lipid-bound state. Larger quenching rates were observed for the Trp residues in helices 1, 4, and 5 than for those located in helices 2 and 3, suggesting differences between the interaction of these two groups of helices. However, the extent of Trp fluorescence quenching observed in lipoproteins made with any of the mutants was comparable to that reported for deeply embedded Trp residues, suggesting that all Trp residues interact with the phospholipid acyl chains. This study provides the first experimental evidence of a massive interaction of the alpha-helices of apoLp-III with the phospholipid acyl chains in discoidal lipoproteins. The extent of interaction deduced is consistent with the apolipoprotein adopting a highly extended conformation.

Essential role of the conformational flexibility of helices 1 and 5 on the lipid binding activity of apolipophorin-III

It has been recently postulated that the conformational flexibility of helices 1 and 5 of Locusta migratoria apoLp-III could play an important role in early steps of binding of this apolipoprotein to a lipid surface (Soulages, J. L., and Arrese, E. L. (2000) J. Biol. Chem. 275, 17501-17509). To test this model, we have designed a double Cys mutant in which a disulfide bond linking helices 1 and 5 could be formed, resulting in an apolipoprotein with reduced conformational flexibility of its N- and C-terminal helices. Substitution of Thr(18) and Ala(147) by Cys residues provided a protein that under nonreducing conditions was fully oxidized. The far-UV CD spectra of this mutant in the reduced and oxidized states indicated that their secondary structures were identical to the structure of the wild type recombinant apoLp-III, which contains no Cys residues. Near-UV CD studies confirmed the formation of a disulfide bond and the absence of structural perturbations. The lipid binding activity of the reduced mutant, as determined by its ability to form discoidal lipoproteins, was nearly identical to that of the wild type protein. Contrarily, the disulfide form of the mutant was not able to form discoidal lipoproteins with liposomes of either dimirystoylphosphatidylcholine or dimyristoylphosphatidylglycerol. It is concluded that the separation of the helices 1 and 5 constitutes one of the key steps along the complex pathway for the formation of the final apolipoprotein lipid-bound state. It is inferred that the conformational flexibility of helices 1 and 5 is a key property of apoLp-III, allowing the exposure of hydrophobic protein regions and the interaction of the hydrophobic faces of the amphipathic alpha-helices with the lipoprotein lipid surface.

Diacylglycerol transport in the insect fat body: evidence of involvement of lipid droplets and the cytosolic fraction

In this report we show the existence of a distinct pool of fat body diacylglycerol (DG) that can be distinguished from the bulk DG. This is a dynamic pool of DG that uses FA entering the fat body from the hemolymph, whereas the bulk DG uses the fatty acids stored in the fat body fat droplets. Using a dual labeling technique, it was possible to compare the effect of hormone-stimulated DG synthesis and secretion on the distribution of radiolabeled FA among the lipids of the dynamic pool (short-term radiolabeling), with the hormonal effect on the total complement of fat body lipids (long-term radiolabeling). We observed that, whereas DG represents 2% to 3% of the fat body lipid mass, about 20% of the short-term radiolabeled lipids are represented by DG. Stimulation of lipolysis produces a fast decrease in the fraction of short-term radiolabeled DG, whereas there is an increase in the mass of fat body DG. The subcellular distribution of bulk DG showed that its majority (62%) was in the fat cake whereas only 2.9% was in the cytosol. On lipolysis stimulation, the largest changes in specific activities of newly synthesized DG were detected in the cytosol and the fat cake, suggesting that newly synthesized DG localized in the lipid droplets and the cytosol is preferentially mobilized.

Fluorescence spectroscopy of single tryptophan mutants of apolipophorin-III in discoidal lipoproteins of dimyristoylphosphatidylcholine

The structure of the exchangeable apolipoprotein, apolipophorin-III from Locusta migratoria, apoLp-III, is described as a bundle of five amphipathic alpha-helices. To study the interaction of each of the helices of apoLp-III with a lipid surface, we designed five single-Trp mutants, each containing a Trp residue in a different alpha-helix. The Trp residues were located in the nonpolar domains of the amphipathic alpha-helices. The kinetics of the spontaneous interaction of the mutants with dimyristoylphosphatidylcholine (DMPC) indicated that all mutants behaved as typical exchangeable apolipoproteins. Circular dichroism in the far-UV indicated that all proteins have a high and similar helical content in the lipid-bound state. The interaction of the Trp residues with the lipid surface was investigated in recombinant lipoprotein particles made with DMPC. The properties of the Trp residues were investigated by fluorescence spectroscopy. These studies showed major changes in the spectroscopic properties of the Trp residues upon binding to lipid. These changes are observed with all single-Trp mutants, indicating that a major conformational change, which affects the properties of all helices, takes place upon binding to lipid. The position of the fluorescence maximum, the quenching efficiency of acrylamide as determined by steady-state and time-resolved fluorescence, and the fluorescence lifetimes of the single-Trp mutants suggest that helices 1, 4, and 5 interact with the nonpolar domains of the lipid. The properties of the Trp in helices 2 and 3 suggest that these helices adopt a different binding configuration than helices 1, 4, and 5. Helices 2 and 3 appear to be interacting with the polar headgroups of the phospholipids or constitute a different domain that does not interact with the lipid surface.

Dynamics and hydration of the alpha-helices of apolipophorin III

Apolipophorin III (apoLp-III) is an exchangeable apolipoprotein whose structure is represented as a bundle of five amphipathic alpha-helices. In order to study the properties of the helical domains of apolipophorin III, we designed and obtained five single-tryptophan mutants of Locusta migratoria apoLp-III. The proteins were studied by UV absorption spectroscopy, time-resolved and steady-state fluorescence spectroscopy, and circular dichroism. Fluorescence anisotropy, near-UV CD and solute fluorescence quenching studies indicate that the Trp residues in helices 1 (N-terminal) and 5 (C-terminal) have the highest conformational flexibility. These two residues also showed the highest degree of hydration. Trp residues in helices 3 and 4 display the lowest mobility, as assessed by fluorescence anisotropy and near UV CD. The Trp residue in helix 2 is protected from the solvent but shows high mobility. As inferred from the properties of the Trp residues, helices 1 and 5 appear to have the highest conformational flexibility. Helix 2 has an intermediate mobility, whereas helices 3 and 4 appear to constitute a highly ordered domain. From the configuration of the helices in the tertiary structure of the protein, we estimated the relative strength of the five interhelical interactions of apoLp-III. These interactions can be ordered according to their apparent stabilizing strengths as: helix 3-helix 4 > helix 2-helix 3 > helix 4-helix 1 approximately helix 2-helix 5 > helix 1-helix 5. A new model for the conformational change that is expected to occur upon binding of the apolipoprotein to lipid is proposed. This model is significantly different from the currently accepted model (Breiter, D. R., Kanost, M. R., Benning, M. M., Wesemberg, G., Law, J. H., Wells, M. A., Rayment, I., and Holden, M. (1991) Biochemistry 30, 603-608). The model presented here predicts that the relaxation of the tertiary structure and the concomitant exposure of the hydrophobic core take place through the disruption of the weak interhelical contacts between helices 1 and 5. To some extent, the weakness of the helix 1-helix 5 interaction would be due to the parallel arrangement of these helices.

Purification and properties of a small lipid-binding protein from the hemolymph of Triatoma infestans

A small lipid-binding protein (sLBP) was purified from the hemolymph of the blood-sucking bug Triatoma infestans. Its isolation involved size exclusion-high performance liquid chromatography (HPLC) followed by anion exchange chromatography-HPLC. The molecular weight of the protein, as determined by gel permeation chromatography, was 20 kDa. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) resolved the protein into a single polypeptide with M(r) approximately equal to 16 kDa. The sLBP contains 6% lipids. Diacylglycerols represent the major lipid class, whereas phosphatidyl-choline, phosphatidyl-ethanolamine, free fatty acids and hydrocarbons were found in minor amounts. The amino acid composition indicated a high content of aspartic and glutamic acids and non-polar aliphatic amino acids. The N-terminal sequence did not resemble the sequence of any other previously reported insect hemolymph protein. Far-UV circular dichroism suggested that sLBP adopts a conformation rich in beta-sheet structure. The presence of this protein in hemolymph, fat body and unfertilized eggs was explored throughout the last nymphal and adult stages of the insect by Western blot assays. These assays indicated that sLBP is particularly abundant in hemolymph. A high concentration of sLBP was also detected in the fat body of the nymphs.

The lipid binding activity of the exchangeable apolipoprotein apolipophorin-III correlates with the formation of a partially folded conformation

Manduca sexta apolipophorin-III, apoLp-III, is an exchangeable apolipoprotein of 17 kDa that contains no Trp, one Tyr, and eight Phe. The effect of pH on the kinetics of association of apoLp-III with dimyristoylphosphatidylcholine was studied. The pH dependence of the kinetics showed three distinct regions. Above pH 7, the reaction rate is slow and slightly affected by pH. A approximately 40-fold increase in the rate constant is observed when the pH is decreased from 8 to 4, and a decrease in rate is observed below pH 4. Far-UV CD spectroscopy indicated that the secondary structure of the protein is not affected when decreasing the pH from 8 to 4.5. The pH dependence of the Tyr fluorescence showed three pH regions that resemble the regions observed in the kinetics. Comparison of the far-UV CD and fluorescence studies indicated the formation of a folding intermediate between pHs 4 and 7. This intermediate was also characterized by near-UV CD and fluorescence quenching. Fluorescence quenching studies with I- and Cs+ indicated a very low exposure of the Tyr residue in both native and intermediate conformations. The pH dependence of the near-UV CD spectra indicated that the native --> intermediate transition is accompanied by a loss in the packing constrains of the Tyr residue. UV absorption spectroscopy of the Phe and Tyr residues indicated that the native --> intermediate transition is also accompanied by the hydration of the Tyr residue and approximately 4 Phe residues. This report shows, for the first time, the correlation between the increase in lipid binding activity of an exchangeable apolipoprotein and the formation of a compact but hydrated conformation near physiological conditions. These results suggest a direct correlation between the lipid binding activity and the internal hydration of the apolipoprotein. The similarity between the insect exchangeable apolipoprotein and the human counterparts, apoA-I, apoA-II, etc., and the recent demonstration of the presence of a molten globular like-state of human apoA-I near physiological conditions [Gursky, O., and Atkinson, D. (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 2991-2995] suggest that this highly hydrated and compact state may play an important physiological role as the most active lipid binding state of the apolipoproteins in general.

Chemical denaturation: potential impact of undetected intermediates in the free energy of unfolding and m-values obtained from a two-state assumption

The chemical unfolding transition of a protein was simulated, including the presence of an intermediate (I) in equilibrium with the native (N) and unfolded (U) states. The calculations included free energies of unfolding, DeltaGuw, in the range of 1.4 kcal/mol to 10 kcal/mol and three different global m-values. The simulations included a broad range of equilibrium constants for the N left arrow over right arrow I process. The dependence of the N <--> I equilibrium on the concentration of denaturant was also included in the simulations. Apparent DeltaGuw and m-values were obtained from the simulated unfolding transitions by fitting the data to a two-state unfolding process. The potential errors were calculated for two typical experimental situations: 1) the unfolding is monitored by a physical property that does not distinguish between native and intermediate states (case I), and 2) the physical property does not distinguish between intermediate and unfolded states (case II). The results obtained indicated that in the presence of an intermediate, and in both experimental situations, the free energy of unfolding and the m-values could be largely underestimated. The errors in DeltaGuw and m-values do not depend on the m-values that characterize the global N <--> U transition. They are dependent on the equilibrium constant for the N <--> I transition and its characteristic m1-value. The extent of the underestimation increases for higher energies of unfolding. Including no random error in the simulations, it was estimated that the underestimation in DeltaGuw could range between 25% and 35% for unfolding transitions of 3-10 kcal/mol (case I). In case II, the underestimation in DeltaGuw could be even larger than in case I. In the same energy range, a 50% error in the m-value could also take place. The fact that most of the mutant proteins are characterized by both a lower m-value and a lower stability than the wild-type protein suggests that in some cases the results could have been underestimated due to the application of the two-state assumption.

Role of glycosylation in the lipid-binding activity of the exchangeable apolipoprotein, apolipophorin-III

Non-glycosylated recombinant Locusta migratoria apolipophorin-III, apoLp-III, was expressed in E. coli and its physical-chemical properties were compared to those of the glycosylated native apoLp-III. Fluorescence quantum yield and acrylamide quenching studies indicated a slightly higher accessibility of the Trp residues in the recombinant apoLp-III. Far-UV CD spectroscopy indicated that the recombinant apoLp-III has a lower alpha-helical content than the glycosylated apoLp-III. Both proteins spontaneously formed discoidal recombinant lipoprotein particles when incubated with dimyristoylphosphatidylcholine (DMPC). Interaction with lipid promotes an increase in alpha-helical content. CD and fluorescence studies indicate that both proteins adopt the same conformation in the lipid-bound state. However, the kinetics of association of the recombinant protein with DMPC is 5-fold faster than that of the native protein. The results suggest that glycosylation inhibits the lipid binding activity by preventing the exposure of hydrophobic domains and/or decreasing the conformational flexibility of the protein.

Purification and properties of a lipid transfer particle from Bombyx mori: comparison to the lipid transfer particle from Manduca sexta

A lipid transfer particle (LTP) was purified from the hemolymph of the silkworm Bombyx mori. Like other insect LTPs, the B. mori LTP is a very high density lipoprotein containing 21% lipid and three apoproteins of mass approximately 350 kDa, approximately 85 kDa, and approximately 60 kDa. B. mori LTP catalyzes the exchange of lipids between different density class lipoproteins found in adult hemolymph and between adult lipoproteins and vitellogenin. However, in no case was net lipid transfer observed. Manduca sexta LTP also catalyzed exchange of lipids, but not net transfer of lipids, between different density class lipoproteins found in adult hemolymph.

Surface plasmon resonance spectroscopy studies of membrane proteins: transducin binding and activation by rhodopsin monitored in thin membrane films

Surface plasmon resonance (SPR) spectroscopy can provide useful information regarding average structural properties of membrane films supported on planar solid substrates. Here we have used SPR spectroscopy for the first time to monitor the binding and activation of G-protein (transducin or Gt) by bovine rhodopsin incorporated into an egg phosphatidylcholine bilayer deposited on a silver film. Rhodopsin incorporation into the membrane, performed by dilution of a detergent solution of the protein, proceeds in a saturable manner. Before photolysis, the SPR data show that Gt binds tightly (Keq approximately equal to 60 nM) and with positive cooperativity to rhodopsin in the lipid layer to form a closely packed film. A simple multilayer model yields a calculated average thickness of about 57 A, in good agreement with the structure of Gt. The data also demonstrate that Gt binding saturates at a Gt/rhodopsin ratio of approximately 0.6. Moreover, upon visible light irradiation, characteristic changes occur in the SPR spectrum, which can be modeled by a 6 A increase in the average thickness of the lipid/protein film caused by formation of metarhodopsin II (MII). Upon subsequent addition of GTP, further SPR spectral changes are induced. These are interpreted as resulting from dissociation of the alpha-subunit of Gt, formation of new MII-Gt complexes, and possible conformational changes of Gt as a consequence of complex formation. The above results clearly demonstrate the ability of SPR spectroscopy to monitor interactions among the proteins associated with signal transduction in membrane-bound systems.

Role of diacylglycerol and apolipophorin-III in regulation of physiochemical properties of the lipophorin surface: metabolic implications

Manduca Sexta adults insects have two defined lipophorin species of densities 1.09 g/mL, [high-density lipophorin (HDLp)] and 1.02 g/mL [low-density lipophorin (LDLp)], respectively, and a continuous broad range of lipophorin particles of intermediate size and density, intermediate-density lipophorin (IDLp). The transformation of HDLp into IDLp and LDLp is the result of the progressive loading of HDLp with diacylglycerol (DG) and an exchangeable apolipoprotein, apolipophorin-III (apoLp-III). In this paper, we describe the physiochemical changes which occur in the lipophorin surface as a result of the transformation of HDLp into LDLp. (1) The increase in apoLp-III content, from 0 to 16 molecules per particle, is accompanied by a gradual increase in the zeta-potential which, at pH 8.6 ranges from /1.02 mV for lipophorins without apoLp-III to -7.76 mV for lipophorins containing 16 molecules of apoLp-III. (2) As judged by the changes in the partition constant for trimethylammonium diphenylhexatriene and oleic acid, an average 2-fold increase in the size of the lipophorin lipid surface takes place when HDLp is loaded with Dg and transformed into LDLp. (3) These data, as well as the results obtained by end point lipolysis with a triacylglycerol (TG) lipase, indicated that the accessible DG content increases 4-7 times when HDLp is converted in LDLp. (4) Fluorescence polarization of the cationic and anionic lipid probes, trimethylammonium diphenylhexatriene and cis-parinaric acid, embedded in eight different subspecies of lipophorin, containing from 12 to 50% DG, showed a small decrease in the surface lipid order when going from HDLp (25% DG) to LDLp (50% DG). (5) Porcine pancreatic phospholipase A2 was used as a probe of the lipoprotein surface. As the DG content of the lipoprotein increased, a higher enzyme activity against the lipoprotein-phospholipids was observed, with a maximum activity 5-fold higher against LDLp than against HDLp. Overall, the changes observed as the lipoprotein particles are loaded with DG and apoLp-III provide a link between the structure and properties of the lipophorin surface and the physiological roles of HDLp and LDLp particles.

Low concentrations of diacylglycerol promote the binding of apolipophorin III to a phospholipid bilayer: a surface plasmon resonance spectroscopy study

The binding of the exchangeable apolipoprotein apolipophorin III (apoLp-III) to an egg phosphatidylcholine bilayer as a function of the concentration of diacylglycerol (DG) in the bilayer was studied by surface plasmon resonance spectroscopy. At a DG concentration of 2 mol % in the bilayer, the binding of apoLp-III reached saturation. Under saturating conditions, apoLp-III forms a closely packed monolayer approximately 55 A thick, in which each molecule of protein occupies approximately 500 A2 at the membrane surface. These dimensions are consistent with the molecular size of the apoLp-III molecule determined by x-ray crystallography, if apoLp-III binds to the bilayer with the long axis of the apoLp-III normal to the membrane surface. In the absence of protein, the overall structure of the lipid bilayer was not significantly changed up to 2.5 mol% DG. However, at 4 and 6 mol % DG, the presence of nonbilayer structures was observed. The addition of apoLp-III to a membrane containing 6 mol % DG promoted the formation of large lipid-protein complexes. These data support a two-step sequential binding mechanism for binding of apoLp-III to a lipid surface. The first step is a recognition process, consisting of the adsorption of apoLp-III to a nascent hydrophobic defect in the phospholipid bilayer caused by the presence of DG. This recognition process might depend on the presence of a hydrophobic sensor located at one of the ends of the long axis of the apoLp-III molecule but would be consolidated through H-bond and electrostatic interactions. Once primary binding is achieved, subsequent enlargement of the hydrophobic defect in the lipid surface would trigger the unfolding of the apolipoprotein and binding via the amphipathic alpha-helices. This two-step sequential binding mechanism could be a general mechanism for all exchangeable apolipoproteins. A possible physiological role of the ability of apoLp-III to bind to lipid structures in two orientations is also proposed.

Hydration and localization of diacylglycerol in the insect lipoprotein lipophorin. A 13C-NMR study

In order to probe the organization of diacylglycerol (DG) in lipophorin, 13C-enriched lipophorin was prepared for NMR investigations. We obtained 13C-enriched lipophorin labeled exclusively in DG by feeding insects tobacco leaves coated with [1-13C]palmitic acid or [1-13C]oleic acid. Lipophorins enriched up to 5% with a [13C]fatty acid were obtained by this procedure. NMR studies of the isolated lipophorin DG showed that palmitic acid accumulates almost entirely (> 90%) in the sn-1 position. Oleic acid was found equally distributed between the sn-1 and sn-2 positions, yielding a DG enriched equally at both positions. The 13C-NMR spectra of both [13C]palmitate- and [13C]oleate-enriched lipophorins showed that DG had one narrow carbonyl resonance indicative of rapid motion. A comparative analysis of the 13C carbonyl chemical shift data for DG in organic solvents, aqueous solutions, and dispersions with the DG carbonyl chemical shift of native lipophorin enriched in [13C]palmitate or [13C]oleate shows a high degree of water exclusion from the DG carbonyls in lipophorin. This result is consistent with the existence of a lipophorin lipid core containing most of the lipophorin DG. This study represents the first attempt to elucidate the organization of DG in lipophorin. The possibility of obtaining [13C]DG-enriched lipophorins, selectively enriched in one or both acyl chains of DG, should provide a powerful tool for further analysis of the organization and the dynamic properties of DG in native lipoproteins.