Change in Intracellular Calcium Ions upon Maturation in Starfish Oocytes

A transient increase in intracellular Ca²⁺ upon maturation in starfish oocyte was revealed by light emission of aequorin microinjected into the cell. One minute application of 1-methyladenine (1-MeAde) to a limited area of the oocyte surface was sufficient to induce the Ca²⁺ transient over the entire cell though it did not induce the germinal vesicle breakdown (GVBD). Ten minutes application of 1-MeAde induced a similar Ca²⁺ transient followed by GVBD. Even when the transient increase of Ca²⁺ was inhibited by injecting EGTA into the oocyte, 1-MeAde treatment for a long period induced GVBD. These facts indicate that the Ca²⁺ increase is neither necessary nor sufficient for maturation of the starfish oocyte.
When the oocyte, which had been treated with 1-MeAde for 1 min at a limited area around the animal pole, was treated again with 1-MeAde for 10 min starting about 15 min after the first treatment, a Ca²⁺ transient similar to the first one was induced and was followed by GVBD. By contrast, in the oocyte treated with 1-MeAde at an area around the vegetal pole, neither Ca²⁺ transient nor GVBD was induced by the second treatment with 1-MeAde. These results indicate a difference in responsiveness to the hormone between the animal hemisphere and the vegetal hemisphere of the oocyte.     

PROPIONATE AND CYCLOHEXIMIDE REVERSIBLY BLOCK PROGESTERONE INDUCED CALCIUM SURGE IN AMBYSTOMA MEXICANUM OOCYTES

The protoprotein aequorin was used in order to monitor Ca²⁺ transients in conditions where progesterone induced maturation was reversibly inhibited. Propionate but not isethionate Cl-free medium impaired both meiosis reinitiation and the Ca²⁺ transient, unless oocytes were returned to normal Cl-containing medium. Similar results were obtained with the protein synthesis inhibitor cycloheximide. In both cases, the incidence of germinal vesicle breakdown (GVBD) and the time schedule relating it to the Ca²⁺ surge appeared not very different from that found from control oocytes. The evidence suggests that both treatments act on the initial step by which progesterone triggers the intracellular Ca²⁺ release needed for maturation promoting factor (MPF) elaboration. No definitive conclusion can be reached however from these experiments concerning the need for protein synthesis during meiosis reinitiation.     

Effect of Ca2+-free Seawater Treatment on 1-Methyladenine Production in Starfish Ovarian Follicle Cells

When follicle cells from ovaries of the starfish Asterina pectinifera were washed with Ca²⁺-free seawater (CaFSW), the production of 1-methyladenine (1-MeAde) in response to gonad-stimulating substance (GSS) decreased to a large degree. The cyclic AMP content of CaFSW-treated follicle cells was much lower than that of non-treated cells, and the level was increased slightly by GSS, but not to a degree sufficient for production of 1-MeAde. The production of 1-MeAde and cyclic AMP in the presence of GSS was dependent on extracellular Ca²⁺ concentration, and was considerably reduced at a Ca²⁺ concentration below 1 mM. Thus, the decrease of 1-MeAde production by follicle cells after treatment with CaFSW is due to the low levels of cAMP. Furthermore, an ADP-ribosylation experiment using [α-³²P]NAD⁺ in the presence of cholera toxin and pertussis toxin with membrane preparations of follicle cells treated with CaFSW revealed the presence of two types (stimulatory and inhibitory) of G-proteins. However, activity of the adenylyl cyclase was not influenced by GSS regardless of the presence or absence of GTP. These findings may suggest that GSS is unable to bind to its receptor in follicle cells after washing with CaFSW.     

Acid Release during Activation of Barnea candida (Mollusca, Pelecypoda) Oocytes

Barnea caridida oocytes release acid (1.35 pmole H⁺/oocyte) upon fertilization. After artificial activation by an excess of KCl, germinal vesicle breakdown (GVBD) occurs normally and a quite similar, but not identical, acid release is recorded (1.10 pmole H⁺/oocyte). KCl activation of Barnea oocytes is completely inhibited in 100 mM sodium-acetate sea water at pH 6.5 and fertilization does not result in activation when the oocytes are transferred after one minute into 100 mM sodium-acetate sea water at pH 6.3. When D–600, a calcium transmembrane fluxes inhibitor, is added 20 seconds after fertilization, GVBD is inhibited but a normal acid release is recorded. The presence of at least 10 mM sodium ions in the external medium is required for 100% activation of these oocytes by an excess of KCl. These results suggest that while an intracellular pH increase may be a requisite for GVBD, this can not be a sufficient condition to trigger it unless a calcium influx is allowed to occur. Moreover, the acid release does not result from a Ca⁺⁺-H⁺ exchange transport but appears more likely to be due to a Na⁺-H^* exchange as it has been demontrated in sea urchin eggs.     

Breakdown of crystalline cellulose by synergistic action between cellulase components from Clostridium thermocellum and Trichoderma koningii

Abstract Certain isolated components of fungal cellulases, which cannot effect the breakdown of highly ordered cellulose individually, interact together synergistically to do so when recombined. Suprisingly, not all fungal cellulase components exhibit this property, and no such synergism has been observed so far between fungal and bacterial cellulases.
The cellulase complex of Clostridium thermocellum cannot effect the extensive breakdown of highly ordered cellulose unless Ca²⁺ and dithiothreitol (DTT) are present. However, we now report that isolated cellobiohydrolase from Trichoderma koningii can combine with C. thermocellum cellulase to effect the breakdown of cellulose in the absence of Ca²⁺ and DTT. enhanced activity is observed if Ca²⁺ and DTT are present.
This finding may have important applications in industry: it certainly has important implications for those interested in the basic mechanism of cellulase action in C. thermocellum .     

Biochemical characterization of pectate lyases produced by fluorescent pseudomonads associated with spoilage of fresh fruits and vegetables

An improved method for purification of pectate lyases (PLI and PLII) from culture fluids of Pseudomonas fluorescens CY091 and Ps. viridiflava PJ-08-6 by using a phosphocellulose cation exchanger was described. Analysis of purified PLI and PLII by sodium dodecyl sulphate-polyacrylamide and isoelectric focusing gel electrophoresis revealed that both enzymes had been purified to near homogeneity. Optimal Ca²⁺ concentration required for PLI and PLII activity was determined to be 0·5 mmol l⁻¹. The Ca²⁺ requirement could not be replaced by other metal cations such as Mg²⁺, Cu²⁺, Zn²⁺, Fe³⁺ and Co²⁺. Optimal pH for activity was determined to be between 8·5 and 9·0. The K _m values for sodium polygalacturonate were 1·28 and 1·11 mg ml⁻¹ for PLI and PLII, respectively. Both PLI and PLII were stable at low temperatures (25°C or below) for at least 1 month. However, at 37°C, the activity decreased 50% in 36 h. Optimal temperatures for activity were estimated to be 46° and 52°C for PLI and PLII, respectively. Thermal stability of both enzymes at elevated temperatures (48°C or higher) increased when CaCl₂ or a positively charged molecule such as polylysine was present, but decreased when polygalacturonate or a negatively charged molecule such as heparin was present. PLI and PLII exhibit differential degrees of sensitivity to group-specific inhibitors, including iodoacetic acid and diethylpyrocarbonate. This result suggests that both sulphydryl and imidazole groups are important for the catalytic function of PLI and PLII.     

Calcium and plant action potentials

Abstract. Under normal conditions the action potential in Characeae is dependent on the presence of both Cl⁻ and Ca²⁺. Cl⁻ seems to play a straightforward part as a transient depolarizing flow. The role of Ca²⁺, however, is emerging as an increasingly complex one: there are Ca²⁺ concentration changes in the cytoplasm, as well as transient Ca²⁺ currents across the plasmalemma and possibly the tonoplast. In most Characeae Ca²⁺ is necessary for the Cl⁻ channel to function, and it is also involved in the cessation of the cytoplasmic streaming observed at the time of excitation.
The function of Ca²⁺ at the time of the action potential is being revealed by experimental techniques of increasing sophistication. The development of these methods and possible associated artefacts are considered.     

Inhibition of Starfish Oocyte Maturation by Tumor-Promoting Phorbol Esters*

Effect of tumor promoters including phorbol esters and teleocidin on 1-methyladenine (1-MeAde)-induced oocyte maturation was studied in the starfish. When isolated immature oocytes were treated with 1-MeAde and 12-O-tetradecanoylphorbol-13-acetate (TPA), 1-MeAde-induced maturation was completely inhibited at more than 2.5 μg/ml. However, if TPA was added after the hormone-dependent period (the minimum period wherein 1-MeAde is required), such maturation-inhibiting effect was no longer observed. Pretreatment with TPA for 5 min showed that its inhibitory action is irreversible. However, when TPA-injected oocytes were treated with 1-MeAde, all oocytes underwent germinal vesicle breakdown (GVBD). GVBD was induced in TPA-treated oocytes upon injection of the cytoplasm of maturing oocytes containing maturation-promoting factor (MPF). These facts show that TPA acts on the oocyte surface to inhibit the production of MPF. Retinoids including retinal, retinol and retinoic acid reversed the inhibitory effect of TPA on 1-MeAde-induced maturation. Experiments with various phorbol esters showed a good correlation between their maturation-inhibiting activity and their known tumor-promoting activity. Further, telecoidin, which is structurally unrelated to phorbol esters, inhibited 1-MeAde action. Since both tumor-promoting phorbol esters and teleocidin are known to activate Ca²⁺ -activated, phospholipid-dependent protein kinase (protein kinase C) and their activation effect is inhibited by retinoids, it appears that the activation of protein kinase C by tumor promoters is involved in blocking of 1-MeAde action.     

Effects of Microinjected Cations on the Early Event of Fertilization in the Medaka Egg

Effects of microinjected cations on the early events of fertilization were examined using eggs of Oryzias latipes . Microinjection of either Ca²⁺, Ba²⁺ or Sr²⁺ into the thin cortical cytoplasm induced breakdown of cortical alveoli (vesicles) (CABD) under Ca-Mg-free conditions, but microinjection of Mg²⁺, Mn²⁺ or Co²⁺ prevented CABD at the injected region when the eggs were inseminated in regular saline. Under Ca-Mg-free conditions, CABD could also be induced by microinjection of various solutions (NaCl, choline chloride, sucrose, pH buffer) without any divalent cations or ionophore A23187. Ca²⁺ microinjected into the cortical cytoplasm did not play a role in sperm penetration. Upon microinjection with either Ca²⁺, Mg²⁺ or K⁺, the resting membrane potential leakage was transiently observed. However, depolarization of the membrane followed by slow hyperpolarization was observed only upon microinjection of Ca²⁺. From these experiments, it was inferred that microinjected divalent cations such as Ca²⁺, Ba²⁺ or Sr²⁺ do not act directly upon the cortical alveolus membrane, but trigger the induction of CABD via depolarization of the membrne and increase in intracellular Ca²⁺.     

ROLE OF MALATE TRANSPORT IN REGULATING METABOLISM IN MITOCHONDRIA ISOLATED FROM RABBIT BRAIN

Abstract— Partly purified chromaffin granules were incubated in vitro with Ca²⁺ (with trace amounts of ⁴⁵Ca²⁺) in concentrations ranging from 4 μm to 1 mm. After incubation the granules were washed with media containing EDTA and then subjected to density gradient centrifugation (1.3 to 2.0 m-sucrose solutions) in order to characterize the particles which had taken up ⁴⁵Ca²⁺. By using marker enzymes and various inhibitors of Ca²⁺ uptake into such cell particles as mitochondria it was established that under the conditions of the experiments chromaffin granules took up Ca²⁺ from the incubation medium. To characterize this uptake a simplified density gradient procedure was tested and found to be suitable. The uptake of Ca²⁺ into chromaffin granules was strongly dependent on temperature. It was not activated by ATP. The uptake was linear up to 10 min. At high calcium concentrations (above 200 μm) the rate of uptake levelled off. The uptake at 37°C was 1 nmol Ca²⁺/mg protein/min at a Ca²⁺ concentration of 500 μm. Mg²⁺ had no influence on Ca²⁺ uptake, whereas Sr²⁺ (1 mm) inhibited it. The methods established in this study should prove useful for a further characterization of this Ca²⁺ uptake into chromaffin granules which is likely to represent a useful model for the Ca²⁺ uptake occurring in the intact gland.     

β-Amylase from Clostridium thermocellum SS8 - a thermophilic, anaerobic, cellulolytic bacterium

The extracellular amylase produced by Clostridium thermocellum strain SS8 on starch was characterized as a β-amylase based on blue value reduction test and the production of maltose from starch. The enzyme had a temperature and pH optima of 60°C and 6.0, respectively. Of the metal ions tested, Ca^{2 +} and Mg^{2 +} had little effect on enzyme activity, but their presence increased its thermal stability. Ca^{2 +} displayed a higher stabilizing effect and at 10 mmol 1^-1 Ca^{2 +}, the enzyme retained 86% activity even after exposure at 70°C for 30 min. The amylase was induced on starch or maltose but was repressed strongly by glucose.     

The Role of Extracellular Ca2+ in the Activation of Pectinaria Oocytes

Several events are associated with fertilization in oocytes. Two such events are an increase in cytoplasmic Ca²⁺ concentration and the resumption of meiosis. Oocytes of the marine annelid, Pectinaria gouldii , are in metaphase I arrest when they are spawned. In this report we investigate the relationship between Ca²⁺ and resumption of meiosis in this species. Meiosis in unfertilized oocytes could be re-initiated with the divalent cation ionophore, A23187. Oocytes in Ca²⁺ free sea water, however, did not resume meiosis in the presence of the ionophore. Furthermore, it was observed that Ca²⁺ must be present for at least 15 min following ionophore treatment for meiosis to resume. These results suggest that extracellular Ca²⁺ is required for the re-initiation of meiosis in this species.     

Calcium ion transport through tissue discs of the cortical flesh of apple fruit

Tissue discs cut from the cortical flesh of apple fruit (Malus domestica Borkh. ev. Granny Smith) were clamped between two chambers, and the transport of ⁴⁵Ca²⁺ from one chamber to the other was followed. After initial transport associated with partial infiltration of air spaces by the Ca²⁺ -containing solution, steady-state transport rates were achieved over several hours. Transprt was by diffusion through the apoplast, faciliated by exchange with binding sites on the cell walls. Cation competition was observed during Ca²⁺ loading, transport and unloading, suggesting that the presence of other cations and pH will be important in modifying Ca²⁺ transport through non-vascular tissue and in xylem unloading. Modification of the extracellular volume of solution by vacuum infiltration increased Ca²⁺ transport at high concentrations, suggesting that diffusion is the prime motive force when Ca²⁺ is abundant. When low concentrations were infiltrated, there was little effect on Ca²⁺ transport, and exchange had a strong influence. Transport was reduced at 1°C but this could be accounted for by physical effects of low temperature on diffusion and viscosity. The results are discussed in relation to the nature of the apoplast and the transport of Ca²⁺ in non-vascular plant tissue.     

Breakdown of starfish ovarian follicle induced by maturation-promoting factor

Immature starfish oocytes are surrounded by envelopes consisting of follicular cells. These cells adhere to each other and to the oocyte, immobilizing the latter within the ovary. When isolated oocytes in their follicles are treated with 1-methyladenine (1-MeAde), germinal vesicle breakdown (GVBD) and follicular envelope breakdown (FEBD) occur simultaneously. The 1-MeAde acts on the oocyte surface to produce a maturation-promoting factor (MPF) in the cytoplasm, which brings about GVBD. In the present study, MPF was found to induce FEBD as well as GVBD when injected into immature oocytes with their follicles in Asterina pectinifera. Although GVBD was induced by MPF in the presence of cytochalasin D, this drug prevented MPF-induced FEBD, and each follicular cell remained in situ on the surface of the oocyte. However, desmosomes connecting the processes of the follicle cell with the oocyte surface were disrupted following MPF injection even in the presence of cytochalasin D, and the processes became detached from the oocyte. FEBD occurred in these oocytes when cytochalasin D was removed, resulting in the formation of a small follicular clump by microfilament-mediated contraction of the follicle cells. These results show that FEBD is not brought about by the direct action of 1-MeAde but by the action of MPF. Therefore, in starfish, spawning as well as oocyte maturation is directly triggered by MPF produced under the influence of 1-MeAde.     

A Reevaluation of the Role of Mitochondria in Neuronal Ca2+ Homeostasis

Abstract: The ability of mitochondrial Ca²⁺ transport to limit the elevation in free cytoplasmic Ca²⁺ concentration in neurones following an imposed Ca²⁺ load is reexamined. Cultured cerebellar granule cells were monitored by digital fura-2 imaging. Following KCI depolarization, addition of the protonophore carbonylcyanide m -chlorophenylhydrazone (CCCP) to depolarize mitochondria released a pool of Ca²⁺ into the cytoplasm in both somata and neurites. No CCCP-releasable pool was found in nondepolarized cells. Although the KCI-evoked somatic and neurite Ca²⁺ concentration elevations were enhanced when CCCP was present during KCI depolarization, this was associated with a collapsed ATP/ADP ratio. In the presence of the ATP synthase inhibitor oligomycin, glycolysis maintained high ATP/ADP ratios for at least 10 min. The further addition of the mitochondrial complex I inhibitor rotenone led to a collapse of the mitochondrial membrane potential, monitored by rhodamine-123, but had no effect on ATP/ADP ratios. In the presence of rotenone/oligomycin, no CCCP-releasable pool was found subsequent to KCI depolarization, consistent with the abolition of mitochondrial Ca²⁺ transport; however, paradoxically the KCI-evoked Ca²⁺ elevation is decreased. It is concluded that the CCCP-induced increase in cytoplasmic Ca²⁺ response to KCI is due to inhibition of nonmitochondrial ATP-dependent transport and that mitochondrial Ca²⁺ transport enhances entry of Ca²⁺, perhaps by removing the cation from cytoplasmic sites responsible for feedback inhibition of voltage-activated Ca²⁺ channel activity.     

Role of external calcium in homeostasis of intracellular pH in the cyanobacterium Anabaena sp. strain PCC7120 exposed to low pH

The role of external Ca²⁺ in the homeostasis of intracellular pH (pHi) of Anabaena sp. strain PCC7120 in response to a decrease in the external pH (pHex) has been studied in cell suspensions. Increase in cytoplasmic pH after acid shock is dependent on the presence of Ca²⁺ in the medium. The observed Ca²⁺-mediated alkalization of the cytoplasm depends on the extent of the shift in external pH. Acid pH shifts resulted in an increased permeability of the cytoplasmic membrane to protons, which could be reversed by increasing the concentration of Ca²⁺ in the medium. Thus, the ability of Ca²⁺ to increase cytoplasmic pH might be correlated with an inhibition of net proton uptake by increasing concentrations of external Ca²⁺ under these conditions. This combined response resulted in the generation and maintenance of a larger pH gradient (ΔpH) at acid external pH values. All Ca²⁺ channel blockers tested, such as verapamil and LaCl₃, inhibited the observed Ca²⁺-mediated response. On the other hand, the Ca ionophore calcimycin (compound A23187) was agonistic, and stimulated both cytoplasmic alkalization and inhibition of net proton uptake. The protonophorous uncoupler carbonylcyanide m -chlorophenyl hydrazone, inhibited this Ca²⁺-mediated response, whereas monensin, an inhibitor of the Na⁺/H⁺ antiporter, had no significant effect. The results of the present study suggest that an influx of Ca²⁺ from the extracellular space is required for the regulation of cytoplasmic pH in Anabaena sp. strain PCC7120 exposed to low external pH values.     

Cytoplasmic free Ca2+ dynamics in single tomato (Lycopersicon esculentum) protoplasts subjected to chilling temperatures

Protoplasts were isolated from leaves of tomato seedlings ( Lycopersicon esculentum Mill., cv. Marmande) at the 2nd to 4th true leaf stage and were loaded with the calcium binding tetra[acetoxymethyl⁺] ester of the fluorescent stilbene chromophore, Fura 2. Although the loading efficiency of the dye in these protoplasts was low, many protoplasts loaded only in the cytosol were always obtained. Changes in the cytosolic calcium concentration ([Ca²⁺]_cyt) were determined in single protoplasts in a temperature-controlled perfusion chamber by use of fluorescence photometry microscopy after excitation at 340 and 380 nm. When the protoplasts were subjected to chilling temperatures (10–15°C) by a circulating solution, the [Ca²⁺]_cyt increased in 64% of the analysed protoplasts. Depending on the initial resting level of [Ca²⁺]_cyt, three main types of kinetics were obtained in these protoplasts: (1) In 21% of the protoplasts, [Ca²⁺]_cyt increased to a maximum within 10–20 s from the start of temperature decrease, followed by a fast decrease; (2) in 11% of the protoplasts, the [Ca²⁺]_cyt both increased and decreased somewhat slower; and (3) in 32% a constant increase of [Ca²⁺]_cyt was obtained 1 min after the start of temperature decrease.     

Mitochondria, Calcium Regulation, and Acute Glutamate Excitotoxicity in Cultured Cerebellar Granule Cells

Abstract: Exposure of cultured cerebellar granule cells to 100 µ M glutamate plus glycine in the absence of Mg²⁺ causes calcium loading of the in situ mitochondria and is excitotoxic, as demonstrated by a collapse of the cellular ATP/ADP ratio, cytoplasmic Ca²⁺ deregulation (the failure of the cell to maintain a stable cytoplasmic free Ca²⁺ concentration), and extensive cell death. Glutamate-evoked Ca²⁺ deregulation is exacerbated by the mitochondrial respiratory chain inhibitor rotenone. Cells maintained by glycolytic ATP, i.e., in the presence of the mitochondrial ATP synthase inhibitor oligomycin, remain viable for several hours but are still susceptible to glutamate; thus, disruption of mitochondrial ATP synthesis is not a necessary step in glutamate excitotoxicity. In contrast, the combination of rotenone (or antimycin A) plus oligomycin, which collapses the mitochondrial membrane potential, therefore preventing mitochondrial Ca²⁺ transport, allows glutamate-exposed cells to maintain a high ATP/ADP ratio while accumulating little ⁴⁵Ca²⁺ and maintaining a low bulk cytoplasmic free Ca²⁺ concentration determined by fura-2. It is concluded that mitochondrial Ca²⁺ accumulation is a necessary intermediate in glutamate excitotoxicity, whereas the decreased Ca²⁺ flux into cells with depolarized mitochondria may reflect a feedback inhibition of the NMDA receptor mediated by localized Ca²⁺ accumulation in a microdomain accessible to the mitochondria.     

Some Properties of the Hardening Process in Chorions Isolated from Unfertilized Eggs of Medaka, Oryzias latipes

Chorions isolated from unfertilized eggs of medaka, Oryzias latipes , harden during incubation with Ca²⁺ ions (Masuda et al. , 1991). In this process, i.e. in vitro Ca²⁺-hardening, the amounts of the major proteins of unfertilized egg chorions (83 K, 78 K and 51 K, corresponding to ZI-1, 2 and 3 of oocyte chorions reported by Hamazaki et al , 1987) decreased and new proteins having molecular weights of 148 K or more appeared. Immunoblotting analysis using anti-ZI-1, 2 antisera and anti-ZI-3 antisera showed that the 148 K protein was an intermediate formed during polymerization of the original proteins.
The mechanism of in vitro Ca²⁺-hardening was studied by examining the decrease in ZI-1, 2, and 3, the formation of 148 K protein, and the change in solubility of chorions in 6% sodium dodecylsulfate-1% 2-mercaptoethanol-15% glycerol-0.2 M Tris-HCl (pH 6.8). In vitro Ca²⁺-hardening was inhibited at temperatures higher than 70°C and its optimum pH was about 5.5. It was inhibited by neither aminotriazole nor cadaverine. The results suggested that in vitro Ca²⁺-hardening was generated by some factor(s) other than ovoperoxidase and transglutaminase.     

Tris stimulates ecdysteroid secretion via Ca2+ messenger system in the prothoracic glands of Locusta migratoria

Abstract.  Secretion of ecdysteroids by the prothoracic glands of the migratory locust, Locusta migratoria L., is enhanced in vitro by tris(hydroxymethyl)aminomethane (tris) in a dose-dependent manner. Glands from larvae on the second day of the penultimate stadium are most sensitive. This action of tris depends on the uptake of calcium; increased production of ecdysteroid does not occur in the presence of cadmium, verapamil or TMB-8, or in calcium-free media. The concentration of unbound Ca²⁺, [Ca²⁺]_i, in the cytoplasm is measured with the aid of FURA 2/AM. Tris causes a rise of [Ca²⁺]_i that is fully suppressed by lanthanum and partially by nitrendipine. Two antagonists of IP₃ receptors elicit mutually opposite effects: heparin blocks, whereas 2-APB accelerates, the rise in [Ca²⁺]_i. Ryanodine exerts only a slight effect. It is proposed that tris activates locust prothoracic glands in a Ca²⁺-dependent manner that exhibits many similarities to the transduction pathway of prothoracicotropic hormone.