Fatty acids in the lipids of Drosophila heads: effects of visual mutants, carotenoid deprivation and dietary fatty acids

Lipids of Drosophila heads were extracted and separated by high-performance thin-layer chromatography. Fatty acid compositions of major phospholipids as well as of triglycerides were analyzed by gas-liquid chromatography. Proportions of the major fatty acids (14:0, 16:0, 16:1, 18:0, 18:1, 18:2, 18:3) varied depending on the lipid analyzed. Docosahexaenoic acid (22:6), common in vertebrate photoreceptors and brain, and arachidonic acid (20:4), a precursor of eicosanoids, were lacking. A comparison of the fatty acid composition of the diet vs. the head suggested that Drosophila can desaturate but may not be able to elongate fatty acid carbon chains. Fatty acid analyses were carried out after the following visual system alterations: i) the transduction mutant where no receptor potential results from a deficit in phospholipase C; ii) an allele of eyes absent; iii) the mutant outer rhabdomeres absent which lacks visual pigment and rhabdomeres in the predominant type of compound eye receptor, rhabdomeres 1 through 6; and iv) carotenoid deprivation which reduces opsin and rhabdomere size. We also evaluated aging by comparing newly-emerged vs. aged wild-type flies. Alterations in fatty acid composition based on some of these manipulations were found. Based on comparisons between flies reared on media differing in C16 and C18, there is an indication that diet readily affects tissue fatty acid composition.     

Distribution of isomeric octadecenoic fatty acids in milk fat

Milk fat octadecenoic acids were isolated by preparative gas-liquid chromatography of their methyl esters and separated into cis and trans isomers by argentation thin-layer chromatography. The distribution of the double bonds in the two fractions was determined by reductive ozonolysis. Cis octadecenoic acids had double bonds in positions 6 to 14. The cis-9 isomer was the most abundant isomer representing approximately 95% of the total. Trans octadecenoic acids had double bonds in positions 6 to 16, the predominant isomer being trans-11 octadecenoic acid. Data illustrating the range of variation in the distribution of positional isomers of octadecenoic acid for herd milk fat and butter are presented. There was no obvious seasonal variation in the distribution pattern. Fat from perirenal and subcutaneous adipose tissue of a dairy cow had composition similar to that of butter samples.     

Influence of short-term dietary measures on dioxin concentrations in human milk

Breast-feeding may expose infants to high levels of toxic chlorinated dioxins. To diminish intake of these lipophilic compounds by the baby, two diets were tested for their ability to reduce concentrations of dioxins in human milk. The diets were a low-fat/high- carbohydrate/low-dioxin diet. (about 20% of energy intake derived from fat) and a high fat /low-carbohydrate/low-dioxin diet. These diets were tested in 16 and 18 breast-feeding women, respectively. The test diets were followed for 5 consecutive days in the fourth week after delivery. Milk was sampled before and at the end of the dietary regimen, and dioxin concentrations and fatty acid concentrations were determined. Despite significant influences of these diets on the fatty acid profiles, no significant influence on the dioxin concentrations in breast milk could be found. We conclude that short-term dietary measures will not reduce dioxin concentration in human milk.     

The effects of n-6 polyunsaturated fatty acids on the expression of nm-23 in human cancer cells

Over an interval of approximately six months beginning in October 1993, most haemophilia A patients in Canada were switched from a plasma-derived intermediate-purity factor VIII concentrate (i.p. VIII) to a recombinant factor VIII (rVIII). In order to determine the consequence of this change in therapy on progression of HIV infection, we gathered surveillance data on clinical status and CD4 and CD8 cell counts in those patients who were HIV seropositive at the time of switching concentrates. Data were recorded at the time of switchover, annually for 2 years thereafter, and retrospectively at a point 1 year prior to the switch. CD4 cells fell significantly over the study period. Multiple direct comparisons revealed that this decline was restricted to the time intervals which included the final year in which patients received intermediate-purity factor VIII concentrate (i.p. VIII). In the 2 year interval in which rVIII was used exclusively, there was a nonsignificant fall in CD4 cells. Changes in CD4 cells did not correlate with the intensity of exposure to either i.p.VIII or rVIII. CD8 cells did not fall significantly over the study period. There was no obvious reduction in the incidence of death or clinical progression over the 2 years in which rVIII was used. However, we are hopeful that the stabilizing trend in CD4 cell counts which followed the introduction of rVIII will be predictive of corresponding clinical stabilization over the coming years.     

Modulation of exercise-induced immunosuppression by dietary polyunsaturated fatty acids in mice

The possible interaction between intense exercise, known to suppress the immune response, and nutritive factors, such as polyunsaturated fatty acids (PUFA), was examined in inbred female C57Bl/6 mice. The animals received for 8 wk either a natural ingredient diet or a diet supplemented with 10 g/100 g linseed oil containing over 50% of 18:3 (n-3) alpha-linoleic acid. Other groups received PUFA containing only traces of 18:3 (n-3) fatty acid; beef tallow, containing mostly 18:1 (n-9) saturated fat, safflower oil, an 18:2 (n-6) PUFA, and fish oil, containing longer chain (n-3) PUFA. Each dietary group was divided into two subgroups: sedentary diet controls and exercised animals. Exercise consisted of continuous swimming at high intensity until exhaustion. It was shown in three separate experiments that (1) the primary humoral response to sheep red blood cells, determined by the plaque-forming cell (PFC) assay, was affected by PUFA diet in sedentary animals in the order beef tallow > control diet > safflower oil > fish oil > linseed oil, and (2) the PFC response was suppressed by the exhaustive exercise, as compared to sedentary controls, except for animals fed 18:3 (n-3) linseed oil, where the normal response was noted. Phagocytosis of fluorescent microspheres by peritoneal macrophages, determined by flow cytometry, was significantly lower in exercised animals receiving the linseed oil diet, whereas other diets either increased or did not significantly change the macrophage phagocytic activity, compared to the sedentary diet controls. Spleen lymphocyte subsets were unchanged in exercised animals except for a marked shift from the lymphoid peak toward the erythroid peak. Generally, our data showed a marked immunomodulatory effect of 18-3 (n-3) alpha-linoleic acid on the exhaustive exercise-related immunosuppression, as compared to the effects of other selected PUFA.     

Effect of dietary forage concentration and buffer addition on duodenal flow of trans-C18:1 fatty acids and milk fat production in dairy cows

The objective of the study was to determine the effects of dietary fat source on duodenal flow, apparent absorption, and milk fat incorporation of trans-C18:1 fatty acids. Four ruminally and duodenally cannulated multiparous Holsteins cows in mid to late lactation were fed a basal diet containing 36% corn silage, 24% alfalfa haylage, and 40% concentrate (dry matter basis). Diets contained 0% supplemental fat (control diet), 3.7% high oleic sunflower oil, 3.7% high linoleic sunflower oil, or 3.7% partially hydrogenated vegetable shortening; treatments were administered in a 4 x 4 Latin square design with 3-wk experimental periods. The flow of trans-C18:1 to the duodenum was higher for cows fed diets supplemented with fat than for cows fed the control diet (283 vs. 64 g/d). Incomplete biohydrogenation accounted for the increased flow of trans-C18:1 to the duodenum in cows fed diets containing high oleic and high linoleic sunflower oil. Increased flow of trans-C18:1 in cows fed the diet containing partially hydrogenated vegetable shortening most likely originated from the trans-C18:1 in the diet. Milk fat percentages were 3.48, 3.07, 3.18, and 3.38% for cows fed the control diet and diets containing high oleic sunflower oil, high linoleic sunflower oil, and vegetable shortening, respectively. Milk trans-C18:1 increased from 2.9 to 11.2% of the total fatty acids for cows fed the control diet and the diets supplemented with fat, respectively. Milk trans-C18:1 were equal across all diets supplemented with fat. Ruminal and total tract digestion of organic matter, neutral detergent fiber, and N did not change with fat supplementation. Results showed that trans-C18:1 is increased in the milk of cows with reduced milk fat; however, excessive amounts of trans-C18:1 in milk do not necessarily correspond directly to milk fat depression.     

Trans fatty acid content of human milk in Spain

The C18 fatty acid fraction of 38 samples of mature human milk from Spanish women was analyzed using capillary gas chromatography. The average content of trans fatty acids found in these samples represented 0.95% of total fatty acids. This value is lower than the percentage found in human milk from other developed countries in which the consumption of hydrogenated fat is higher. Trans fatty acid content in human milk has been related to the types of fats and oils present in the diets of the nursing mothers. According to the results obtained in this survey, we can also conclude that the amount of trans forms in breast milk lipids is lower than the trans content found in infant formulas in Spain.     

Changes in dietary fatty acids alter phospholipid fatty acid composition in selected regions of rat brain

1. Eighty rats were randomized into four groups receiving one of the following diets: rat chow containing (1) 6% soybean oil, (2) 6% primrose oil, (3) 6% fish oil, (4) a combination of 4.5% primrose and 1.5% fish oil. 2. Following two months of each regimen, the rats were sacrificed by microwave irradiation and the brain's fatty acid composition was analysed with gas chromatography for each of the following regions: frontal cortex, striatum, occipital cortex, hippocampus, hypothalamus, cerebellum and pituitary. 3. Linoleic acid was decreased by both primrose and fish oil supplementations. The fish oil substitution resulted in a significant elevation of 20:3n-6, a decrease of 22:4n-6 and a non-significant decrease of 20:4n-6, probably reflecting inhibition of delta-5-desaturation. At the same time the fish oil diet significantly elevated 22:5n-3 while 22:5n-6 was decreased. 4. The primrose oil diet lowered the n-3/n-6 ratio in all regions except in the cerebellum. In contrast, the fish oil diet elevated the n-3/n-6 ratio in all regions. 5. The results demonstrate that changes in dietary fat composition can alter the fatty acid composition of the adult rat brain and that these effects are region specific. 6. This is of interest since metabolites of essential fatty acids may be involved in physiological and pathological processes in the brain and it has been hypothesized that dietary intake of fats may influence the outcome of psychiatric disorders such as schizophrenia.     

Depression of lipogenesis in swine adipose tissue by specific dietary fatty acids

The objective of this study was to document the influence of specific dietary fatty acids on rates of lipid synthesis and sensitivity to insulin in porcine adipose tissue. Weanling pigs were assigned to one of six groups, and each group was fed diets containing 10 g/100 g of added cornstarch or 10 g/100 g of added fatty acid. The fatty acid-enriched diets contained either a combination of 14:1 plus 16:1 (14:1/16:1 diet), 16:0, 18:0, 18:1, or 18:2 (n-6). With the exception of the cornstarch diet, all diets contained approximately 35% 14:0. Subcutaneous adipose tissue samples were collected at slaughter from the area overlying the first cranial vertebra. Fresh samples were incubated for 2 h in 20 mM glucose and 0, 10, 100 or 1,000 microU/mL of porcine insulin. The smallest adipocytes were observed in adipose tissue from pigs fed the 16:0 or 18:2 diets. Glucose incorporation into lipids was greater (P < .05) in adipose tissue from cornstarch-fed pigs than in adipose tissue from the other treatment groups. Lipogenesis was 67, 53, 35, 32, and 20% lower (P < .05) in adipose tissue from 16:0-, 14:1/16:1-, 18:0-, 18:2-, and 18:1-fed pigs, respectively, than in adipose tissue from the cornstarch-fed pigs. Insulin increased lipogenesis by 19% (P < .05) in adipose tissue from the cornstarch-fed pigs and by 15 to 40% (P < .05) in adipose tissue from the 14:1/16:1-fed pigs. Insulin did not stimulate lipogenesis (P > .4) in adipose tissue from pigs fed the 16:0, 18:0, or 18:1 diets. The data suggest that fatty acid chain length and unsaturation are determinants in the effects of dietary fat and insulin on de novo lipogenesis.     

Divergent incorporation of dietary trans fatty acids in different serum lipid fractions

We isolated from the endogenous polyprenyl-phospho-sugar pool of Mycobacterium smegmatis two mannose-containing compounds, i.e., a partially saturated C35-octahydroheptaprenyl-P-mannose and a fully unsaturated C50-decaprenyl-P-mannose. The relative amount of C35-polyprenyl-P-mannose in mycobacterial cells was comparable to that of decaprenyl- P-pentoses and, at least, an order of magnitude higher than that of C50-decaprenyl-P-mannose. The major form of mycobacterial polyprenyl-P-mannose was structurally characterized by combined gas chromatography-mass spectrometry, fast-atom bombardment tandem mass spectrometry and proton-nuclear magnetic resonance spectroscopy as beta-d-mannopyranosyl-monophospho-(C35)octahydroheptapren ol of which all three isoprene units have Z ( cis ) configuration. The differences in the structure and cellular concentrations of the mycobacterial mannosyl-P-polyprenols reflect distinct biochemical pathways of the two compounds and suggest the existence of specific GDP-Man:polyprenyl-P mannosyltransferases (synthetases) able to distinguish between C35-octahydroheptaprenyl- and C50-decaprenyl- phosphates of mycobacteria. Since the 6'-O-mycoloylated form of C35-octahydroheptaprenyl-P-mannose isolated from M. smegmatis is apparently involved in mycolate rather than mannosyl transfer reactions, we speculate that a catabolic pathway responsible for degradation of C35-P-mannose and recycling C35-octahydroheptaprenyl phosphate might exist in mycobacteria.     

Influence of fatty acids on the binding of calcium to human serum albumin

Fatty acids have been shown to influence the binding of calcium to human serum albumin. The calcium binding to albumin was enhanced when long-chain fatty acids were added to albumin prepared by two different methods and decreased when fatty acids were removed from albumin. Palmitic, stearic and oleic acids all exhibited this phenomenon. The effects of long-chain fatty acids on the binding of calcium to albumin in vitro appears to be of sufficient magnitude to have in vivo implications in calcium homeostasis and in determining the ratio of free to total calcium. Preliminary in vivo experiments have confirmed the calcium binding of fatty acids in serum and suggest a physiological role for this phenomena.     

Comparison between dietary monounsaturated and polyunsaturated fatty acids as regards diet-related diseases

We have studied the effects of dietary fatty acid (FA) composition on lipids and lipoproteins, platelet function and other hemostatic variables as well as on the endogenous formation of DNA adducts of malonaldehyde (MA) in healthy subjects in controlled dietary experiments. The FAs studied were monounsaturated oleic acid (OA, 18:1 n-9), n-6-polyunsaturated linoleic acid (LA, 18:2 n-6), n-3 polyunsaturated alpha-linolenic acid (ALA, 18:3 n-3), and two long-chain, n-3 polyunsaturated FAs, eicosapentaenoic acid (EPA, 20:5 n-3) and docosahexaenoic acid (DHA, 22:6 n-3). The results indicated that a high OA and high LA diet had comparable effects on lipids and lipoproteins when they replaced saturated FAs in a diet. Furthermore, the effect of ALA did not differ from that of LA in this respect. Both diets also similarly increased in vitro platelet aggregation when compared with high saturated FA baseline diet. In another study the effect of LA and ALA on platelet function was studied. In this study ALA decreased in vitro platelet aggregation when compared with LA. When ALA was compared with EPA + DHA it was found that platelet function and some coagulation and fibrinolysis parameters were mainly affected in a similar manner by ALA and EPA + DHA treatments. The high LA diet increased the levels of DNA adducts of MA when compared with the effect of the high OA diet. Our findings indicate that the interpretation of the effect of diet, dietary fat or a specific FA on the development of chronic disease is extremely complex.     

Regulation of plasma lipoprotein levels by dietary triglycerides enriched with different fatty acids

Saturated vegetable oils (coconut, palm, and palm kernel oil) containing predominantly saturated fatty acids, lauric (12:0) or myristic (14:0 and palmitic (16:0), raise plasma total cholesterol (TC) and low density lipoprotein cholesterol (LDL-C) levels in animals and humans, presumably by decreasing LDL receptor activity and/or increasing LDL-C production rate. Although stearic acid (18:0) is chemically a saturated fatty acid, both human and animal studies suggest it is biologically neutral (neither raising nor lowering) blood cholesterol levels. Although earlier studies indicated that medium chain fatty acids (8:0-10:0) were also thought to be neutral, more recent studies in animals and humans suggest otherwise. Unsaturated vegetable oils such as corn, soybean, olive, and canola oil, by virtue of their predominant levels of either linoleic acid (18:2) or oleic acid (18:1), are hypocholesterolemic, probably as a result of their ability to upregulate LDL receptor activity and/or decrease LDL-C production rate. Whether trans fatty acids such as trans oleate (t18:1), in hydrogenated products such as margarine, are hypercholesterolemic remains controversial. Studies in humans suggest that their cholesterol-raising potential falls between the native nonhydrogenated vegetable oil and the more saturated dairy products such as butter. Assessment of the magnitude of the cholesterolemic response of trans 18:1 is difficult because in most diet studies its addition is often at the expense of cholesterol-lowering unsaturated fatty acids, making an independent evaluation almost impossible.     

Effect of infused volatile fatty acids and caseinate on milk composition and coagulation in dairy cows

Milk protein secretion is changed by increasing the proportion of energy, mainly as propionic acid, or the availability of AA. Whether associative effects exist between energy nature and protein amounts is unknown. Therefore, ruminal isoenergetic infusions of low or high propionate mixtures were combined factorially with duodenal infusion of sodium caseinate or control. Four ruminally and duodenally fistulated Holstein cows were used. The diet was limited and consisted of 70% forage and 30% concentrate. Caseinate infusion increased milk yield and protein and casein contents and decreased milk fat content; curd yields and coagulation properties of milk were improved. The infusion of propionic acid caused a large increase in rumen propionate. Milk yield tended to decrease, and milk fat decreased, but protein, casein, and curd yields were unchanged; milk-coagulating properties were improved. No interaction existed between energy and protein amounts. Alteration of VFA had little effect on milk composition, but increasing the protein supply to the duodenum increased milk protein.     

Growth inhibitory effects of aromatic fatty acids on ovarian tumor cell lines

Epithelial ovarian cancer is a major cause of cancer-related mortality in women, making the search for new treatment modalities essential. Sodium phenylacetate (NaPa), a phenylalanine derivative, has been shown to induce cytostasis and differentiation by inhibiting protein isoprenylation. Similar effects have been observed with phenylbutyrate, a phenylacetate congener. We examined in parallel the growth inhibitory activity against human ovarian carcinoma cell lines of phenylacetate, phenylbutyric acid (PB), and certain related compounds, and comparisons were made with lovastatin. On a molar basis, hydroxykynurenine and kynurenine showed the highest activity followed by PB and NaPa. Ovarian carcinoma cell lines were also sensitive to lovastatin in micromolar concentrations. Additive effects were observed when PB was combined with cisplatin or when NaPa or PB were combined with lovastatin. NaPa and PB, but not kynurenine, inhibited incorporation of [3H]mevalonate into ovarian carcinoma cells. An immune modulatory role might also be suggested for PB because it resulted in increased ovarian tumor cell expression of human leukocyte antigen class I and the cluster of differentiation molecule CD58, whereas transforming growth factor-beta2 expression was decreased. Phenylbutyrate, which is the ester form of PB, has shown acceptable pharmacological properties and clinical responses in patients with other malignancies, and might be considered for evaluation in ovarian cancer.     

Immunoregulatory and anti-inflammatory effects of n-3 polyunsaturated fatty acids

1. Fish oils are rich in the long-chain n-3 polyunsaturated fatty acids (PUFAs), eicosapentaenoic (20:5n-3) and docosahexaenoic (22:6n-3) acids. Linseed oil and green plant tissues are rich in the precursor fatty acid, alpha-linolenic acid (18:3n-3). Most vegetable oils are rich in the n-6 PUFA linoleic acid (18:2n-6), the precursor of arachidonic acid (20:4n-6). 2. Arachidonic acid-derived eicosanoids such as prostaglandin E2 are pro-inflammatory and regulate the functions of cells of the immune system. Consumption of fish oils leads to replacement of arachidonic acid in cell membranes by eicosapentaenoic acid. This changes the amount and alters the balance of eicosanoids produced. 3. Consumption of fish oils diminishes lymphocyte proliferation, T-cell-mediated cytotoxicity, natural killer cell activity, macrophage-mediated cytotoxicity, monocyte and neutrophil chemotaxis, major histocompatibility class II expression and antigen presentation, production of pro-inflammatory cytokines (interleukins 1 and 6, tumour necrosis factor) and adhesion molecule expression. 4. Feeding laboratory animals fish oil reduces acute and chronic inflammatory responses, improves survival to endotoxin and in models of autoimmunity and prolongs the survival of grafted organs. 5. Feeding fish oil reduces cell-mediated immune responses. 6. Fish oil supplementation may be clinically useful in acute and chronic inflammatory conditions and following transplantation. 7. n-3 PUFAs may exert their effects by modulating signal transduction and/or gene expression within inflammatory and immune cells.     

Visual attention in infant monkeys: Effects of dietary fatty acids and age.

Effects of dietary essential fatty acids on visual attention were explored longitudinally in infant rhesus monkeys with a visual paired-comparison paradigm. Sets of primate faces and of patterns were presented at Weeks 2, 5, 9, and 13 to 9 infants deficient in ω-3 fatty acids and 8 fed a standard nursery diet. Familiarization to 1 member of each pair preceded simultaneous presentation of both stimuli. Infants fed the deficient diet showed longer individual looks in both immediate and 24-hr tests. Duration of looks decreased with age to familiar but not to novel stimuli. The proportion of time looking at the novel stimulus (% novel) increased with age but was not affected by diet. Look duration and % novel were differentially affected and may reflect different underlying processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effect of dietary trans fatty acids on cholinesterase and monoamine oxidase activities in different organs of rats

The effect of trans fatty acids and essential fatty acid deficiency upon the activities of cholinesterase and monoamine oxidase in livers, hearts, brains and lungs of rats was studied. This study was performed using three groups of male albino rats. Two out of these three groups were fed essential fatty acid low diets containing 10% hydrogenated coconut oil (HCNO) or margarine stock (MS, partially hydrogenated soybean oil) while the third group was fed an adequate supply of essential fatty acids through a diet containing 10% corn oil (CO). In the group of rats fed HCNO the activities of cholinesterase and monoamine oxidase in their livers, hearts, brains and lungs were not significantly different from those of the control group fed CO. In the group of rats fed MS, the activity of cholinesterase was significantly decreased in the livers, hearts and brains, but not affected in the lungs, while the activity of monoamine oxidase was significantly decreased in the livers and hearts but not affected in the brains and lungs as compared to the control group fed CO. The levels of serum total lipids, total cholesterol and triglycerides were elevated in both groups of rats fed HCNO or MS than in the group fed CO, but this elevation was more highly significant in the group fed MS than in the group fed HCNO.     

Profiles of the fatty acids in the plasma membrane of human brain tumors

The ionic channels and signal transduction pathways underlying the 5-hydroxytryptamine (5-HT)-induced hyperpolarization in neurons of the rat dorsolateral septal nucleus (DLSN) were examined by using intracellular and voltage-clamp recording techniques. Application of 5-HT (1-50 microM) caused a hyperpolarizing response associated with a decreased membrane resistance in DLSN neurons. The hyperpolarization induced by 5-HT was blocked by Ba2+ (1 mM) but not by tetraethylammonium (TEA, 3 mM), glibenclamide (100 microM) and extracellular Cs+ (2 mM). 8-Hydroxy-di-n-propylamino tetralin (8-OH-DPAT; 3 microM), a selective agonist for the 5-HT1A receptor, mimicked 5-HT in producing the hyperpolarization. The 5-HT hyperpolarization was blocked by NAN-190 (5 microM), a 5-HT1A receptor antagonist. CP93129 (100 microM), a 5-HT1B receptor agonist, and L-694-247 (100 microM), a 5-HT1B/1D receptor agonist, also produced hyperpolarizing responses. The order of agonist potency was 8-OH-DPAT > CP93129 > or = L-694-247. (+/-)-2,5-Dimethoxy-4-iodoamphetamine hydrochloride (DOI, 100 microM), a 5-HT2 receptor agonist, and RS67333 (100 microM), a 5-HT4 receptor agonist, caused no hyperpolarizing response. The voltage-clamp study showed that 5-HT caused an outward current (I5-HT) in a concentration-dependent manner. I5-HT was associated with an increased membrane conductance. I5-HT reversed the polarity at the equilibrium potential for K+ calculated by the Nernst equation. I5-HT showed inward rectification at membrane potentials more negative than-70 mV. Ba2+ (100 microM) blocked the inward rectifier K+ current induced by 5-HT. I5-HT was irreversibly depressed by intracellular application of guanosine 5'-O-(3-thiotriphosphate)(GTP-gamma S) but not by guanosine 5'-O-(2-thiodiphosphate) (GDP beta S). These results suggest that in rat DLSN neurons activation of 5-HT1A receptors causes a hyperpolarizing response by activating mainly the inward rectifier K+ channels through a GTP-binding protein.