Introduction

Polyamines (putrescine, spermidine and spermine) are ubiquitous in biological systems. Cell growth, both in vivo and in vitro, is tightly coupled with the enhancement of ODC (ornithine decarboxylase) activity and the polyamines level1, 2.

Fully grown amphibian oocytes were arrested physiologically at the G2/M border of the first meiotic division. Upon exposure to progesterone, oocytes resumed meiotic cycle. The. most prominent event occurred in the process was the appearance of maturation promoting factor (MPF) activity, a nonspecific meiotic or mitotic inducer,which in turn directly triggered the entry of interphasic cell into mitotic phase without any further requirement of protein synthesis 3.

Protein synthesis was enhanced during the progesterone induced amphibian oocyte maturation. The synthesis of cyclins and pp39mos was sufficient to activate oocyte MPF 4. Ribosomal S6 protein phosphorylation has been proposed to regulate the increased protein synthesis 5. On the other hand,it is notable that the augmentation of protein synthesis resulted from special unmasking of mRNA stored in the oocyte cytoplasm 6. The process of mRNA unmasking and its regulation during the oocyte maturation remained unclear.

During the process of progesterone induced amphibian oocyte maturation, the enhancement of ODC activity and the change in polyamine levels took place. No information has been available as regarding to the physiological roles of polyamine in the course of oocyte maturation 7, 8.

This paper was oriented to probe into the effect of polyamine on oocyte maturation and its mechanism. The results revealed that spermine was able to inhibit protesterone induced toad oocyte maturation, probably resulting from spermine enhanced 55 kD protein phosphorylation in toad oocyte. Certainly, it is worthy to prove whether the 55 kD protein was one of the components of messenger ribonucleoprotein particles (mRNP) related to the unmasking process of mRNA.

Materials and Mathods

Materials

Bufo bufo gargarizans were obtained from Jiangsu Province in December, and kept in cold room (4°C) to main tain the full - grown oocytes sensitive to the stimulation of progesterone. Some female toads were reared in an environment of higher temperature (28 - 30°C) all over the year in order to make oocytes inert to progesterone treatment. This means that oocytes from the “high temperature reared toads” will not undergo GVBD after hormone stimulation.

Progesterone, putrescine,spermidine and spermine were obtained from Sigma. γ-32P -ATP and γ-32P -GTP(10 Ci/mmol), 35S - methionine (523 mCi/mmol) were purchased from Amersham.

Oocyte preparation

Full -grown oocytes (about 1.8 mm in diameter) were selected from toad's ovary fragments. Ovarian membrane and follicular membrane around the oocytes were divested manually with a pair of microforceps under a stereoscope. The nude oocytes were incubated in Ringer's solution at 18°C for the experiments.

Criteria of oocyte maturation

Oocyte maturation was judged by the disappearance of germinal vesicle after 8 - 12 h of progesterone treatment or microinjection of 100 nl cytoplasm containinig MPF. The breakdown of the germinal vesicle was ascertained by the dissection of oocytes after 5 min of fixation in 10% trichloroacetic acid. All the experiments were performed with oocytes that have a potency to undergo germinal vesicle breakdown (GVBD) with a frequency of 100%. The time when 50% of oocyte population underwent GVBD was set as 1.0 on the relative time scale, i.e. 1.0 GVBD50, so that different schedules can be compared.

Microinjection assay

Approximately 50–100 nl of sample or cytoplasm were microinjected into oocyte at the animal hemispere near the equatorial region. For microinjection, all the chemical reagents used were dissolved in 5 mM Tris–HCl buffer (pH 7. 4). 0. 1–0.2–Ci of r –32p – ATP was used for each microinjection in the analysis of protein phosphorylation.

32P– labeled protein determination

Oocytes were homogenized in cold homogenization buffer (50 mM Tris – HCl, pH 7.4, 200 mM sucrose, 10 mM β–glycerophosphate, 1 mM MgCl2, 1 m M PMSF), then centrifuged at 2000×g for 20 min at 4°C. Aliquots (15– 40μ) of the 2000×g supernatant were deposited on filler paper discs (Whatman No. 3 MM) and precipitated on the discs by 2–18 h of 10% trichloroacetic acid treatment. For qualitative analysis of 32P–labeling, the 2000×g super– natant samples were diluted with an equal volume of SDS sample buffer and boiled at 100°C for 3 min, then submitted to 5–13% or 7.5 %–15% SDS– polyacrylamide gel electrophoresis and autoradiography.

Protein synthesis determination

160 oocytes were incubated in 2 ml of Ringer's solution containing approximately 30 μCi/ml of 35S–labeled methionine for 10 h. After extensive washing with incubation medium, groups of 40 oocytes were injected individually with 100 nl of the indicated concentration (5, 10 or 20 mM) of spermine. 2 h later,each group was subdivided into 2 portions (20 oocytes each), and submitted to the following treatment: incubated in Ringer's solution as control; incubated in Ringer's solution containing 1μ M progesterone. The acid–insoluble radioactivity from 2000 ×g supernatant was measured for the determination of protein synthesis as described by Stith et al. 9.

Quantitative determination of the contents of polyamine by HPLC

A group of 10 oocytes were homogenized in 100 μl of 0. 5N HClO4. The supernatant was obtained after centrifugation and was directly used for quantitative determination of the oocyte polyamine according to the method proposed by Seiler and Knodgen 10.

Determination of protein concentration

Protein concentration was determined by the method described by Bradford 11 with bovine serum albumin serving as standard.

Determination of the levels of oocyte cAMP

Using the method proposed by Thibier et al. 12.

Results

I. Effects of spermine on the process of progesterone induced oocyte maturation

Previous experiments concerning physiological functions of polyamines were all performed under the condition of endogenous depletion of polyamines 13, 14, 15. We were, however,interested in the effects of exogenous spermine on oocyte maturation without any perturbation of endogenous spermine. As shown in Fig. 1, spermine inhibited progesterone–induced oocyte maturation when 100 nl of the polyamine was injected into oocytes 2 h prior to progesterone stimulation. The effective dose of spermine to inhibit 50% of progesterone induced oocyte maturation (ID50) was 6.8 mM (100 nl). The inhibitory effect of spermine on maturation was also observed when the injected spermine was dissolved in either water (pH 7.4) or Ringer's solution, but with a slightly higher ID50 when dissolved in Tris buffer. In all circumstances the oocytes that were unable to undergo GVBD seemed to be still healthy after 20 h of hormone stimulation, as judged by the presence of normal germinal vesicle under stereoscopic analysis.

Fig. 1
figure 1

Effect of polyamines on the progesterone–induced oocyte maturation

Groups of 15–20 oocytes were injected with 100 nl of various concentrations of either putrescine (▵), spermine (•) or spermidine (○) 2 h prior to the stimulation with 1 μM progesterone. GVBD was scored 11–12 h after the addition of hormone. Each point represented the mean of results of 5 groups with its standard deviation.

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Spermidine has also been found to be capable of repressing progesterone induced oocyte maturation. Its ID50 was about 20 mM (Fig 1). However, putrescine could not inhibit the process of maturation induced by progesterone, even when a higher dose was used.

To further explore the inhibition of oocyte maturation by spermine, we made an attempt to characterize the action of spermine in detail on the process of maturation. From the results shown in Fig 2, it was obvious that the later the time of spermine treatment was applied in the course of maturation, the higher the frequency of GVBD was obtained. The time at which 50% oocytes became resistant to spermine inhibition, designated as 50% spermine–sensitive transition (SST50), on average, occurred at 0.68 GVBD50.

Fig. 2
figure 2

Inhibitory effect of spermine in the process of progesterone induced oocyte maturation. Oocytes (15 – 20 per group) were injected with 100 nl of 25 mM spermine at the indicated relative time of GVBD50 after progesterone treatment. GVBD was scored 12 h after exposure to 1 μM progesterone. Vertical bars indicated the mean ± S. E. M. of 5 in dependent observations.

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Progesterone induced maturation was also inhibited when oocytes were cultured in Ringer's solution containing 10 mM spermine (pH 7.4). The efficiency of maturation inhibition caused by spermine was observed to vary among oocytes derived from individual animals, possibly indicating the difference in the capacity of oocytes to uptake exogenous spermine.

II. Effects of spermine on MPF activation and MPF action

MPF activity in maturing oocytes, as determined by the ooplasm transfer method, appeared shortly before 0.50 GVBD50, and reached its maximal level at 0. 85 GVBD50. The SST50 occurred at 0.68 GVBD50 (Fig 2), just in the period when MPF activity developed rapidly. Such an apparent relation forced us to probe into the effect of spermine on MPF activation.

The toad oocyte maturation was blocked by 96% when the oocytes were microinjected with 100 nl of 25 mM spermine 2 h before progesterone stimulation (Tab 1). Then one would ask: Is MPF activation repressed, or its action to elicit GVBD is inhibited by spermine ? To elucidate these questions, oocytes treated with spermine and progesterone were microinjected with cytoplasm containing MPF. The recipient oocytes underwent GVBD with the same frequency as that of the donor oocytes(Tab 2). The results were in favor of our suggestion that progesterone induced MPF activation other than the action of MPF is inhibited by spermine treatment.

Tab 1 Effect of spermine on the appearance of MPF activity
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Tab 2 Release of spermine inhibitory effect by MPF on progesterone–induced oocyte maturation
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It has been found, however, that spermine could repress MPF–induced oocyte maturation, but the ID50 is approximately 25 mM. much higher than that of spermine required for inhibiting progesterone induced maturation (unpublished data). a fact which suggested that different mechanisms may be involved in these two kinds of spermine inhibition.

III. The level of cAMP in spermine treated oocytes

The level of cAMP has been shown to fall in the early stage of progesterone induced amphibian oocyte maturation 16. Some agents that elevate the cAMP level of oocytes, for example, cholera toxin, could inhibit the hormone induced maturation and activation. We have investigated the effcet of spermine on cAMP of the toad oocytes. It has been found that spermine significantly decreased the level of cAMP in intact oocytes (Fig 3). The results strengthened the assumption that the inhibitory effect of spermine on oocyte maturation did not result from the change of cAMP content.

Fig. 3
figure 3

Effect of spermine on endogenous level of oocyte cAMP

•– – –• oocytes without any treatment

○– – –○ oocytes stimulated by lμM progesterone

▴– – –▴ oocytes microinjected with 100 nl of 25 mM spermine

Each point represented the average value of 2 independent determinations.

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IV. Analysis of protein synthesis in spermine treated oocytes

Spermine has been shown to repress protein synthesis in mammalian cells 17. It has also been shown that progesterone enhanced protein synthesis was coupled with MPF activation in the course of oocyte maturation. We have analyzed the changes of protein synthesis in spermine–treated oocytes in order to further clarify the mechanism of its inhibition.

The amount of 35S–labeled methionine incorporated into proteins was 21,000 cpm per 100μg of 2,000 ×g supernatant from unstimulated toad oocytes. The magnitude of incorporation fell considerrably in spermine–treated oocytes, and a decrease of 49% as compared to the control was obtained in oocytes injected with 20 mM spermine (Fig 4) which seemed almost a saturated dose. Protein synthesis was elevated by more than 9 0% in progesterone–stimulated oocytes than that of untreated oocytes . Spermine blocked the augmentation of protein synthesis promoted by progesterone in a dose dependent manner, and its ID50(5.0 mM) was close to the ID50(6.8 mM) of spermine inhibition on GVBD. The experimental results suggested that the inhibitory effect of spermine on protein synthesis may result in the failure of GVBD.

Fig. 4
figure 4

The inhibitory effect of spermine on oocyte protein synthesis

(○–––○) the amount of 35S–methionine incorporated into proteins in the control oocytes

(•–––•) the net augmentation of 35S–methionine incorporated into oocytes after progesterone treatment as compared with the control

(□–––□) GVBD%, scored 10 h after hormone treatment.

Each point represented the average value of two independent measurements.

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V. Protein phosphorylation in spermine treated oocytes

It is necessary to study the inhibitory mechanism of spermine on oocyte protein synthesis from the point of view of protein phosphorylation, since it has been shown that some phosphorylated proteins could regulate mRNA translation in the process of maturation. In order to characterize 32P–labeled proteins, the 2,000 ×g supernatant fractions were submitted to SDS polyacrylamide gel electrophoresis. Several proteins were readily phosphorylated in oocytes treated with spermine (Fig 5, lanes A–D). Also,a series of proteins were phosphorylated in the course of oocyte maturation (Fig 5, lanes A and E), but no significant change was observed about the pattern of phosphorylation as compared with those of spermine treated oocyte, suggesting that their mode of phosphorylation was independent of oocyte maturation. It seems that the effect of spermine on these proteins is not a prerequisite for the inhibition of oocyte maturation.

Fig. 5
figure 5

One -dimentional gel analysis of the oocyte protein phosphorylation changes after the microinjection of 100 nl different concentration of spermine

Lane A: 0 mM spermine

Lane B: 5 mM spermine

Lane C: 10 mM spermine

Lane D: 20 mM spermine

Lane E: 1 μM progesterone stimulation

Lane F: 5 mM spermine plus 1 μM progesterone stimulation

Lane G: 10 mM spermine plus 1 μM progesterone stimulation

Lane H: 20 mM spermine plus 1 μM progesterone stimulation

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On the other hand, a 55 kD protein vas dephosphorylated in mature oocytes induced by progesterone (Fig 5, lane E). Moreover, the extent of phosphorylation of this protein was correlated with the dose of spermine injected (Fig 5, lanes A to D), and the inhibition of the percentage of GVBD in spermine–treated oocytes was correlated with its injected concentration (Fig 1). It is interesting to note that the protein underwent dephosphorylation during the period of 0.40–0.70 GVBD50 (Fig 6), which just coincided with the moment that maturing oocytes were going to become insensitive to spermine inhibition (Fig 2). Such results gave strong support to the view that the inhibition by spermine on progesterone induced maturation was probably due to the spermine dependent phosphorylation of 55 kD protein.

Fig. 6
figure 6

The change of protein phosphorylation and dephosphorylation in oocytes during the course of progesterone induced maturation

Lane A: injected with Tris HCl buffer

Lane B: injected with 100 nl of 20 mM spermine

Lane C: treated with 1 μM progesterone until 1, 2 GVBD50

Lane D: treated with 1 μM progesterone 2 h after the injection of spermine until 1.2 GVBD50

Lane E: treated with the hormone until 0.2 GVBD50

Lane F: treated with the hormone until 0.4 GVBD50

Lane G: treated with the hormone until 0. 7 GVBD50

Lane H: treated with the hormone until 0.9 GVBD50

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We have further investigated the possible relationship between the maintenan of phosphorylation of 55 kD protein and the inhibition of spermine on progesterone induced MPF activity. Trichosanthin, an inhibitor of protein synthesis at translation level, has been found to inhibit the appearance of MPF activity in the toad oocytes stimulated by progesterone (unpublished data). Trichosanthin was shown to inhibit progesterone induced 55 kD protein phosphorylation (Fig 7, lanes C and D).

Fig. 7
figure 7

55 kD protein dephosphorylation in progesterone stimulated oocytes derived from“hibernant toad”

Lanes A–D: Supernatants of oocytes derived from hibernated toad

Lane A: from hormone untreated oocytes

Lane B: treated with 1μM progesterone

Lane C: microinjected with 50 ng Trichosanthin

Lane D: microinjected with the Trichosanthin,then submitted to the treatment of progesterone

Lanes E–F: Supernatants of oocytes derived from toad reared at high temperature

Lane E: form hormone untreated oocytes

Lane F: treated with 1 μM progesterone

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Previous experiment results from our laboratory revealed that progesterone never induded MPF activity in oocytes derived from “high temperature reared toads”, although it stimulated the appearance of an active substance which was able to promote the activation of MPF in the hormone untreated oocytes obtained from “hibernated females” 18. As we have prejudged, the 55 kD protein was prominently phosphorylated in the former kind of oocytes, and was not dephosphorylated when they were submitted to the hormone treatment (Fig 7, lanes E and F), which was in good agreement with the absence of active MPF in the oocytes. A conclusion may thus derived that dephosphorylation of 55 kD protein coupled tightly with the appearance of MPF activity.

VI. Intracellular levels of polyamines in the process of progesterone induced oocyte maturation .

In each hormone untreated toad oocyte, 1.08 nmol of HClO4 soluble putrescine, 0.42 nmol of HClO4 soluble spermidine and 0. 28 nmol of HClO4 soluble spermine were estimated using HPLC method 10 (Tab 3). During the course of progesterone induced oocyte maturation, a consecutive increase of intracellular putrescine level was observed as illustrated in Fig 8a. The largest amount found in mature oocytes,was higher than that of hormone untreated oocyces. The level of spermidine did not change in the process of maturation (Fig 8 b) indicating that the metabolism in oocytes of this polyamine was insusceptible to the hormone stimulation. However,the intracellular level of spermine dropped by 28% during the period of 0.40–0. 60 GVBD50 (Fig 8c).

Tab 3 The polyamine contents in hormone - untreated and mature oocytes
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Fig. 8
figure 8

Change of the level of polyamines in Bufo oocytes during the course of progesterone. induced maturation.

(•) oocytes treated with progesterone

(○) hormone untreated oocytes Vertical bars indicated the mean +S. E. M. of 3 independent determinations.

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As shown in Tab 4, 60% of 14C–labeled spermine was soluble in 0.5 N HClO4 which was out of our expectation. The radioactivity which escaped from the oocytes and entered into the incubation medium represented less than 10% of the total. An estimated amount of 30% of the radioactive spermine injected into oocytes bound tightly in the pellets, obtained from oocyte homogenate after centrifugation, and composed mainly of yolk platelets. In the meantime, the binding amount of labeled spermine was found to be slightly enhanced in oocytes injected with 100 nl of 5 mM spermine. As from data shown in Tab 4, HClO4 –soluble radioactivity of spermine stimulated oocyte and the hormone–untreated oocyte was comparable.

Tab 4 The amount of spermine in oocytes and culture medium after injection*.
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Discussion

I. Spermine inhibits progesterone induced oocyte maturation

It is obvious from our data that spermine was the most potent polyamine capable of repressing the progesterone induced oocyte maturation (Fig 1). 0. 28 nmol of HClO4 soluble spermine could be evaluated in each resting oocyte by HPlC method (Fig 8) which represents 60% of the total endogenous spermine (Tab 3). Since the amount of the total endogenous spermine per oocyte was 0.39 nmol and the ID50 of injected spermine inhibition was 0.34 nmol (Fig 1), it is difficult to imagine that the inhibition of exogenous spermine on oocyte maturation was an effect caused by non physiological dose. This viewpoint was strengthened by the fact that MPF could relieve the inhibitory effect of spermine on oocyte maturation (Tab 2).

II. Spermine blocks progesterone induced MPF activation and acts as an intracellular regulator to control oocyte maturation

The occurrence of SST50 at 0.68 GVBD50(Fig 2) indicated that the putative endogenous target for spermine inhibition might play a crucial role in the midst of the course of oocyte maturation. The decline of oocyte cAMP content, as one of the most important event observed in the early stage of maturation, was promoted rather than inhibited by spermine treatment (Fig 3). Accordingly, we paid much attention to the effect of spermine on MPF activity. The MPF activity was hardly detectable in oocytes whose GVBD was totally inhibited by spermine. In cytoplasm microtransfer experiments, the percentage of GVBD in spermine treated donor oocytes was almost the same as that in the recipient oocytes (Tab 1), showing that spermine inhibition of progesterone induced maturation was due to the lack of MPF activation instead of GVBD triggering action of MPF. In consistent with the conclusion, MPF was found to be able to release the inhibition of maturation by spermine (Tab 2). However, we have indeed found that spermine could inhibit MPF induced oocyte maturation, but the ID50(25 mM) was much higher than that of progesterone induced oocyte maturation, indicating that different mechanisms might exist in these 2 different cases of inhibition.

Sunkara et al. 8 have detected a 5 fold reduction of spermine content in mature Xenopus oocytes induced by HCG. However, they did not find any change of spermine level in the process of progesterone induced maturation. Now we have disclosed the fact that endogenous level of spermine was reduced by 28% in the course of progesterone induced toad oocyte maturation (Fig 8). As putrescine inhibited spermine synthesis at its physiological concentration, any increased level of putrescine could lead to a drop of spermine level19. The same situation, however,did not occur in the course of normal oocyte maturation, since a significant increase of putrescine was observed at 0.90 GVBD50, nor microinjection of 100 nl 20 mM putrescine changed the basal level of oocyte spermine significantly (unpublished data).

We have found that the level of spermine dropped in the process of progesterone induced toad oocyte maturation (Fig 8) and exogenous spermine has been shown to inhibit oocyte maturation (Fig 1). We have also found that the reduced level of endogenous spermine could lower casein kinase G activity which, in turn, resulted in decrease of its substrate (the 55 kD protein) phosphorylation (unpublished data). Therefore, spermine may act as a negative intracellular regulator in modulating the toad oocyte maturation induced by progesterone. As evidence supporting this tentative hypothesis, Russell and Durie 20 suggested that the rate of cell growth parallels with the ratio of intracellular content of spermidine to that of spermine. Further investigation might ascertain whether or not an agent which could specifically lower endogenous level of spermine could accelerate progesterone induced toad oocyte maturation.

III. The failure of 55 kD protein dephosphorylation causes inhibition of progesterone induced maturation

55 kD protein was dephosphorylated in the process of progesterone induced toad oocyte maturation. Spermine selectively enhanced the basal level of phosphorylated 55 kD protein so that the hormone -stimulated dephosphorylation was incapable of reaching the same extent as observed in the control oocytes. The extent of dephosphorylation of 55 kD protein coupled positively with the percentage of GVBD in the polyamine treated toad oocytes (Fig 5 and Fig 1). We have shown that casein kinase G could inhibit progesterone induced oocyte maturation by phosphorylating its substrate 55 kD protein (unpublished data). It is worthy to notice that 55 kD protein dephosphorylation and the SST50 occurred at the same stage of maturation around 0. 7 GVBD50 (Fig 2 and Fig 6).

We have shown that spermine inhibited progesterone induced MPF activation. Then, what is the relation between 55 kD protein dephosphorylation and MPF activation? Neither 55 kD protein dephosphorylation nor MPF activity was detected in Tri-chosanthin treated oocyte (Fig 7) Progesterone was unable to induce these 2 events in toad oocytes derived from “high temperature reared females”(Fig 7), although it stimulated the production of an active substance which could induce MPF activity in oocytes obtained from“hibernated females”18.

However, 55 kD protein has been sufficiently dephosphorylated early before MPF activity reached to its maximal level (Fig 6). It can be considered, therefore, that 55 kD protein dephosphorylation may be involved in MPF activation.

IV. A possible relationship between 55 kD protein dephosphorylation and the regulation of protein synthesis in the process of oocyte maturation

We are very interested in the mechanism with which 55 kD protein dephosphorylation resulted in MPF activation. It has recently been found that cyclin,which was synthesized during meiotic maturation and early embryonic cell cycle, activated MPF activity in cell free system 4. Otherwise, c- mos oncoprotein was shown to be synthesized during progesterone induced oocyte maturation 21. We do not know whether spermine inhibited cyclin and/or c–mos oncoprotein synthesis in toad oocytes, but we have indeed found that spermine interfered with protein synthesis both in progesterone stimulated and unstimulated toad oocytes (Fig 4). The effect of spermine on protein synthesis may cause failure of GVBD since progesterone induced increment of the amount of protein synthesis paralleled with the percentage of GVBD in spermine treated oocyte (Fig 4). Although the ID50 was higher for the spermine inhibition on GVBD than for the reduction of progesterone induced protein synthesis, the discrepancy may well be explained by the synthesis of some other proteins, e.g. that of ODC 22 which was not involved in the inhibition of progesterone induced oocyte maturation by spermine.

Concerning the augmentation of protein synthesis resulted from specific unmasking of mRNP particles of ooplasm, there was evidence that the protein components of mRNP particles were oocyte specific and inhibited reversibly the translation of the mRNA 23. The recruitment of mRNA might be predicted to require the removal of proteins from stored mRNA particles. The inhibitory proteins have been investigated from the angle of phosphorylation. Dearsly et al. 24 have found that a pp60 and pp56 of mRNP particles were heavily phosphorylated at developmental stages of Xenopus oocytes when mRNA translation was maximally repressed. Dephosphorylation of pp60 destabilized the mRNA/protein complex making mRNA accessible for translation25. In amphibian oocytes and eggs, both casein kinase G and its endogenous substrate pp54 are RNA binding proteins, and pp54 was phosphorylated in partially purified kinase preparation26. Casein kinase G may be ubiquitous in mRNP or informosomes27. We also demonstrated that 55 kD protein was dephosphorylated during progesterone induced toad oocyte maturation and it was an endogenous substrate for casein kinase G of toad oocytes (unpublished data). Moreover, microinjection of the kinase inhibited progesterone induced toad oocyte maturation and progesterone stimulated protein synthesis (unpublished data). It has recently been discovered by Kandrov et al. 28 that a considerable amount of microinjected casein kinase G was tightly associated with mRNP particles in frog oocytes. Altogether the data prompted us to speculate that 55 kD protein of toad oocytes may inhibit mRNA recruitment when they are phosphorylated, and may unmask mRNA for translation when dephosphorylated. In fact, the amount of a 56 kD masking protein of mRNP particles decreased during developmental stages of full grown Xenopus oocytes to 4 cell stage embryos29. The decrease in the magnitude of a protein in prosomes has been observed during axolotl meiotic maturation29, which contained some components inhibiting protein synthesis in vitro30.

The present study has, for the first time, demonstrated that spermine inhibited the process of progesterone induced amphibian oocyte maturation. In this process, the level of endogenous spermine was reduced, an event which may be considered as a negative intracellular control to trigger the recruitment of mRNA for protein synthesis via the way of 55 kD protein dephosphorylation. A significant fraction of endogenous spermine was most probably sequestered from the sites of protein synthetic machinery which, when occupied by a polyamine, permitted optimal protein synthesis31.