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PubMed1986

Evolution of proteins in mammalian cytoplasmic and mitochondrial ribosomes.

The proteins of cytoplasmic and mitochondrial ribosomes from the cow and the rat were analyzed by co-electrophoresis in two dimensional polyacrylamide gels to determine their relative evolutionary rates. In a pairwise comparison of individual ribosomal proteins (r-proteins) from the cow and the rat, over 85% of the cytoplasmic r-proteins have conserved electrophoretic properties in this system, while only 15% of the proteins of mitochondrial ribosomes from these animals fell into this category. These values predict that mammalian mitochondrial r-proteins are evolving about 13 times more rapidly than cytoplasmic r-proteins. Based on actual evolutionary rates for representative cytoplasmic r-proteins, this mitochondrial r-protein evolutionary rate corresponds to an amino acid substitution rate of 40 X 10(-10) per site per year, placing mitochondrial r-proteins in the category of rapidly evolving proteins. The mitochondrial r-proteins are apparently evolving at a rate comparable to that of the mitochondrial rRNA, suggesting that functional constraints act more or less equally on both kinds of molecules in the ribosome. It is significant that mammalian mitochondrial r-proteins are evolving more rapidly than cytoplasmic r-proteins in the same cell, since both sets of r-proteins are encoded by nuclear genes. Such a difference in evolutionary rates implies that the functional constraints operating on ribosomes are somewhat relaxed for mitochondrial ribosomes.

Animals↗
PubMed1990

Cytoplasmic male sterility in beta is associated with structural rearrangements of the mitochondrial DNA and is not due to interspecific organelle transfer.

Chloroplast (ct) and mitochondrial (mt) DNAs from four cytoplasmic male sterile (cms) and 22 normal fertile sugar beet lines and accessions of wild beets from the genus Beta have been compared with restriction analyses and Southern hybridizations. We have used restriction analyses of ctDNA as a phylogenetic marker to confirm the taxonomic relationships between the different cytoplasms. According to the ctDNA data, all four cms cytoplasms belong to the same taxonomic section, Beta. Restriction patterns of ct and mtDNA from fertile accessions produced analogous trees of similarity and showed a close correlation between the organellar DNA diversity and the accepted taxonomic classification of the species studied. However, the mt-DNA restriction profiles of the four cms types differed dramatically from each other and from those of all fertile accessions from the genus. No indication of cytoplasmic introgression was found in any of the four investigated cms types. Southern hybridization to mtDNA revealed variant genomic arrangements in the different fertile and cms cytoplasms, indicating that rearrangement of the mitochondrial genome is a common denominator to the different cms systems in Beta. It may, indeed, be a common property to spontaneously occurring cms in all or most species.

Chloroplasts↗
PubMed1997

Cytoplasmic streaming and gravity sensing in Chara internodal cells.

Since the nineteenth century, the merits of two alternate models for explaining the mechanism of plant gravity perception have been discussed. The gravitational pressure model states that plant cells perceive gravity by sensing their relative buoyancy to that of the surrounding medium, whereas the more popular starch-statolith model states that intracellular sedimenting particles act as gravity sensors. Vertically-oriented Chara internodal cells exhibit a gravity dependent polarity of cytoplasmic streaming such that the downwardly-directed stream moves ca. 10% faster than the upwardly-directed stream. This polarity of cytoplasmic streaming is not simply a consequence of gravity acting directly on the cytoplasm but is rather under physiological control. When Chara internodal cells are placed in a medium more dense than themselves, the gravity-induced polarity of cytoplasmic streaming is reversed. This phenomenon cannot be explained by a model which relies on intracellular sedimenting particles as gravity sensors but is consistent with the gravitational pressure model for gravity sensing. We propose that gravity causes the internodal cells to settle within the confines of the extracellular matrix resulting in a tension between the plasma membrane and the extracellular matrix at the top of the cell and a compression between the plasma membrane and the extracellular matrix at the bottom of the cell. These stresses are proposed to act upon peptides which span the plasma membrane/extracellular matrix interface at the ends of the cells and which subsequently activate Ca2+ channels which in turn may induce a polarity of cytoplasmic streaming.

Calcium↗
PubMed2004

Modulation of SMN nuclear foci and cytoplasmic localization by its C-terminus.

The survival of motor neuron ( SMN1) gene product, SMN, is detected both in the cytoplasm and in nuclear gems and cajal bodies. We show here that SMN exon 6 is essential both for formation of its nuclear foci and for its cytoplasmic localization. However, exon 7 inhibits the formation of SMN nuclear foci but promotes SMN cytoplasmic localization. More interestingly, we find that a random C-terminal tag of five or more amino acids downstream of exon 6 is sufficient to inhibit the occurrence of multiple nuclear foci and to promote cytoplasmic localization of SMNDelta7, the primary product of the SMN2 gene. Moreover, SMNDelta7 proteins that bear spinal muscular atrophy mutations in exon 6 either showed defects in nuclear foci formation or enhanced cytoplasmic localization. We conclude that exon 6 and exon 7 synergistically regulate SMN distribution that may require specific exon 6 motifs but is independent of specific sequences in exon 7.

Amino Acid Motifs↗
PubMed1999

Cardiac ryanodine receptor activity is altered by oxidizing reagents in either the luminal or cytoplasmic solution.

The location of reactive cysteine residues on the ryanodine receptor (RyR) calcium release channel was assessed from the changes in channel activity when oxidizing or reducing reagents were added to the luminal or cytoplasmic solution. Single sheep cardiac RyRs were incorporated into lipid bilayers with 10(-7) m cytoplasmic Ca2+. The thiol specific-lipophilic-4,4'-dithiodipyridine (4,4'-DTDP, 1 mm), as well as the hydrophilic thimerosal (1 mm), activated and then inhibited RyRs from either the cis (cytoplasmic) or trans (luminal) solutions. Activation was associated with an increase in the (a) mean channel open time and (b) number of exponential components in the open time distribution from one ( approximately 2 msec) to three (approximately 1 msec; approximately 7 msec; approximately 15 msec) in channels activated by trans 4,4'-DTDP or cis or trans thimerosal. A longer component (approximately 75 msec) appeared with cis 4, 4'-DTDP. Activation by either oxidant was reversed by the thiol reducing agent, dithiothreitol. The results suggest that three classes of cysteines are available to 4,4'-DTDP or thimerosal, SHa or SHa* activating the channel and SHi closing the channel. SHa is either distributed over luminal and cytoplasmic RyR domains, or is located within the channel pore. SHi is also located within the transmembrane domain. SHa* is located on the cytoplasmic domain of the protein.

Animals↗
PubMed2005

Mutation of the pore glutamate affects both cytoplasmic and external dequalinium block in the rat olfactory CNGA2 channel.

Dequalinium has recently been reported to block CNGA1 and CNGA2 channels expressed in Xenopus laevis. Using the inside-out configuration of the patch-clamp technique, we examined the effects of dequalinium on rat olfactory CNGA2 channels expressed in human embryonic kidney (HEK293) cells and studied aspects of its molecular mechanism of action. We found that cytoplasmic dequalinium blocked wild-type (WT) CNGA2 channels in a voltage-dependent manner with an IC(50) of approximately 1.3 muM at a V(m) of + 60 mV, and an effective fractional charge, zdelta, of +0.8 (z=2, delta=+0.4), suggesting that cytoplasmic dequalinium interacts with a binding site that is about two fifths of the way along the membrane electric field (from the intracellular side). Neutralizing the negatively charged pore lining glutamate acid residue (E342Q) still allows effective channel block by cytoplasmic dequalinium with an IC(50) of approximately 2.2 muM at a V(m) of +60 mV but now having a zdelta of +0.1 (delta=+0.05), indicating a profoundly decreased level of voltage-dependence. In addition, by comparing the extent of block under different levels of channel activation, we show that the block by cytoplasmic dequalinium displayed clear state-dependence in WT channels by interacting predominantly with the closed channel, whereas the block in E342Q channels was state-independent. Application of dequalinium to the external membrane surface also blocked currents through WT channels and the E342Q mutation significantly increased the IC(50) for external block approximately fivefold. These results confirm dequalinium as a potent, voltage-dependent and state-dependent blocker of cyclic-nucleotide-gated channels, and show that neutralization of the E342 residue profoundly affects the block by both cytoplasmic and external application of dequalinium.

Animals↗
PubMed2006

Cytoplasmic Clb2 is required for timely inactivation of the mitotic inhibitor Swe1 and normal bud morphogenesis in Saccharomyces cerevisiae.

Subcellular localization is an important determinant of substrate and functional specificity for cyclin-cyclin dependent kinase (CDK) complexes. This work addresses the cytoplasmic function of the budding yeast mitotic cyclin Clb2, which is mostly nuclear but is also present in the bulk cytoplasm and at the mother-bud neck. Clb2 contains two leucine-rich nuclear export signals (NESs)--one of which we newly describe here--that maintain its presence in the cytoplasm. Yeast strains bearing mutations in one or both of these NESs have elongated buds, indicative of a G2/M cell cycle delay. A small number of these cells exhibit a filamentous-like morphology under conditions that do not normally induce filamentous growth. These phenotypes are enhanced by deletion of the other three mitotic cyclins (CLB1,3,4) and are dependent on expression of Swe1, the yeast Cdk1 inhibitory kinase. Deltaclb1,3,4 Deltabud3 cells, which fail to localize Clb2 to the bud neck, also exhibit a Swe1-dependent elongated bud phenotype. Our results support a model in which cytoplasmic Clb2-Cdk1 is required for timely inactivation of Swe1 at the G2/M transition and bud neck targeting of Clb2 contributes to the efficiency of this process. Cytoplasmic Clb2 may also be important for repression of filamentous growth.

Binding Sites↗
PubMed1997

Run-down of the cardiac Ca2+ channel: characterization and restoration of channel activity by cytoplasmic factors.

Possible mechanisms for run-down in the Ca2+ channel, such as proteolysis or dephosphorylation of the channel, were examined in guinea-pig ventricular myocytes. The Ca2+ channel current, recorded in inside-out patches using a pipette solution containing 50 mM Ba2+ and 3 microM Bay K 8644, ran down with a mean survival time of 2.35 min. The survival time was not significantly affected by adenosine triphosphate (ATP) (3 mM), 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid (BAPTA) (2 mM), isoprenaline (l-5 microM), phosphate (l20 mM) and leupeptin (l0 microM). Stimulation of guanosine triphosphate (GTP)-binding proteins was also ineffective. The catalytic subunit of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA, 0.5-2 microM) slightly and transiently increased channel activity, but had minimal effects on the channel when applied after complete run-down. On the other hand, cytoplasm from the heart, skeletal muscle, brain and liver, but not kidney, induced channel activity. There was a positive correlation between NPo (the product of the number of channels N and the open probability Po) value before run-down and that after the application of cytoplasm, suggesting that the activity of once-active channels was restored ba the exogenous cytoplasm. The potency of cytoplasm in tissues in inducing channel activity was not related to PKA activity nor to the number of dihydropyridine binding sites. These results suggest that the run-down of the cardiac Ca2+ channel is not mediated by dephosphorylation or proteolysis of the channel, but involves other factor(s), possibly interaction of the channel protein with a cytoplasmic regulatory protein.

Adenosine Triphosphate↗
PubMed2001

Characterization of Hydrogenobacter thermophilus cytochromes c(552 )expressed in the cytoplasm and periplasm of Escherichia coli.

Hydrogenobacter thermophilus cytochrome c(552) ( Ht cyt c(552)) is a small monoheme protein in the cytochrome c(551) family. Ht cyt c(552) is unique because it is hypothesized to undergo spontaneous cytoplasmic maturation (covalent heme attachment) when expressed in Escherichia coli. This is in contrast to the usual maturation route for bacterial cytochromes c that occurs in the cellular periplasm, where maturation factors direct heme attachment. Here, the expression of Ht cyts c(552) in the periplasm as well as the cytoplasm of E. coli is reported. The products are characterized by absorption, circular dichroism, and NMR spectroscopy as well as mass spectrometry, proteolysis, and denaturation studies. The periplasmic product's properties are found to be indistinguishable from those reported for protein isolated from Ht cells, while the major cytoplasmic product exhibits structural anomalies in the region of the N-terminal helix. These anomalies are shown to result from the retention of the N-terminal methionine in the cytoplasmic product, and not from heme attachment errors. The (1)H NMR chemical shifts of the heme methyls of the oxidized ( S=1/2) expression products display a unique pattern not previously reported for a cytochrome c with histidine-methionine axial ligation, although they are consistent with native-like heme ligation. These results support the hypothesis that proper heme attachment can occur spontaneously in the E. coli cytoplasm for Ht cyt c(552).

Bacteria, Aerobic↗
PubMed1997

Mechanisms of cytoplasmic pH regulation in alkaliphilic strains of Bacillus.

The central challenge for extremely alkaliphilic Bacillus species is the need to establish and sustain a cytoplasmic pH that is over two units lower than the highly alkaline medium. Its centrality is suggested by the strong correlation between the growth rate in the upper range of pH for growth, i.e., at values above pH 10.5, and the cytoplasmic pH. The diminishing growth rate at extremely high pH values correlates better with the rise in cytoplasmic pH than with other energetic parameters. There are also general adaptations of alkaliphiles that are crucial prerequisites for pH homeostasis as well as other cell functions, i.e., the reduced basic amino acid content of proteins or segments thereof that are exposed to the medium, and there are other challenges of alkaliphily that emerge from solution of the cytoplasmic pH problem, i.e., reduction of the chemiosmotic driving force. For cells growing on glucose, strong evidence exists for the importance of acidic cell wall components, teichuronic acid and teichuronopeptides, in alkaliphily. These wall macromolecules may provide a passive barrier to ion flux. For cells growing on fermentable carbon sources, this and other passive mechanisms may have a particularly substantial role, but for cells growing on both fermentable and nonfermentable substrates, an active Na+-dependent cycle is apparently required for alkaliphily and the alkaliphile's remarkable capacity for pH homeostasis. The active cycle involves primary establishment of an electrochemical gradient via proton extrusion, a secondary electrogenic Na+/H+ antiport to achieve net acidification of the cytoplasm relative to the outside pH, and mechanisms for Na+ re-entry. Recent work in several laboratories on the critical antiporters involved in this cycle has begun to clarify the number and characteristics of the porters that support active mechanisms of pH homeostasis.

Bacillus↗
PubMed1975

Characterization of a low density cytoplasmic membrane subfraction isolated from Escherichia coli.

We have used freeze fracture electron microscopy to study the distribution of membrane proteins in the cytoplasmic membrane of Escherichia coli W3110. While these proteins were distributed randomly at the growth temperature (37 degrees C), there was extensive protein lipid segregation when the temperature was lowered, resulting in bare patches containing no visible particles (protein), and areas of tightly packed or aggregated particles. To understand the segregation process, we have separated the bare patches from the particle rich membrane areas. Lysis of spheroplasts at 0 degrees C leads to cytoplasmic membrane fragments with different amounts of membrane particles per unit area; such fragments have been separated on isopycnic sucrose gradients. The bare patches occurred as low density membranes which were completely devoid of particles. They were compared to normal density cytoplasmic membranes with respect to fatty acid composition, protein distribution as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and their content of several cytoplasmic membrane marker enzymes. The phospholipid to protein ratio of low density membranes was five times greater than that of normal membranes; unsaturated fatty acids were more abundant in the low density membranes. Most proteins had disappeared from the low density membranes. One protein, which had an apparent molecular weight of 26000 on sodium dodecyl sulfate gels appeared to be concentrated in the low density membranes; it accounted for about 50% of the total protein found in this membrane fraction. Of the cytoplasmic membrane markers tested, NADH oxidase and succinate dehydrogenase were excluded, while D-lactate dehydrogenase remained, and even appeared to be concentrated in the low density membranes. These results indicate that while most membrane proteins are associated with the fluid portion of the bilayer, some proteins evidently associate preferentially with phospholipids in the gel or frozen state.

Bacterial Proteins↗
PubMed1975

The outer membrane of Proteus mirabilis. I. Isolation and characterization of the outer and cytoplasmic membrane fractions.

1. The crude envelope preparation obtained by sonication of Proteus mirabilis cells in the presence of lysozyme was separated into outer and cytoplasmic membrane fractions by sucrose density gradient centrifugation. The outer membrane fraction accounted for about two thirds of the dry weight of the envelope preparation. 2. In thin sections, the outer and cytoplasmic membrane fractions were shown to consist of vesicles bounded by a single trilaminar membrane, but those of the outer membrane were considerably smaller and were frequently open, forming C-shaped structures. The cytoplasmic membrane vesicles were cleaved by freeze fracturing to expose fracture faces studded with particles, while the outer membrane fragments resisted cleavage. 3. The outer membrane fraction consisted of protein (similar to 40%), lipopolysaccharide (similar to 36%) and lipid (similar to 18%) and had a density of about 1.22 g/cm3. The cytoplasmic membrane fraction consisted mostly of protein (similar to 56%) and lipid (similar to 38%), had a density of about 1.16 g/cm3, and contained almost all the NADH oxidase, succinate and D-lactate dehydrogenase activities of the crude envelope preparation. 4. Electrophoresis in polyacrylamide gels containing sodium dodecylsulfate revealed over 20 polypeptide bands in the cytoplasmic membrane fraction and only 6-7 in the outer membrane fraction. The outer membrane electrophorogram was dominated by a major band (mol. wt 40 000) which was resolved into two bands when electrophoresed in an acidic gel system. Amino acid analysis revealed a higher content of polar amino acids in the protein moiety of the outer membrane.

Amino Acids↗
PubMed1975

Cytoplasmic origin of the so-called nuclear neutral histone protease.

1. We have investigated the origin of proteolytic activity which causes degradation of histones in chromatin isolated from Xenopus liver and the rat liver at neutral pH. Polyacrylamide disc gel electrophoresis was used for detection of proteolytic products of histones. 2. No proteolytic degradation of histones occurs in chromatin isolated from Xenopus erythrocytes and rat liver according to our procedure even after prolonged incubation at pH 8.0 and pH 5.0. However with chromatin isolated from Xenopus liver a high level of histone degradation is observed under similar conditions. 3. Mixing isolated nuclei from Xenopus erythrocytes with a crude cytoplasmic fraction from Xenopus liver causes histone proteolysis in isolated chromatin at pH 8.0. In similar experiments with corresponding fractions from rat liver histone proteolysis can be introduced only after repeated freezing and thawing of the cytoplasmic fraction. 4. A purified lysosomal preparation from rat liver causes a similar type of histone degradation upon incubation with chromatin from Xenopus erythrocytes and rat liver. 5. The neutral proteolytic activity that can be introduced in isolated chromatin by a crude cytoplasmic fraction and by a purified lysosomal erythrocytes and rat liver. 5. The neutral proteolytic activity that can be introduced in isolated chromatin by a crude cytoplasmic fraction and by a purified lysosomal fraction from rat liver is inhibited by sodium bisulphite. 6. We conclude that the neutral proteolytic activity which causes degradation of histones in isolated chromatin is due to a contamination with neutral protease(s) originating from cytoplasmic organelles.

Animals↗
PubMed1986

Galanin inhibits glucose-stimulated insulin release by a mechanism involving hyperpolarization and lowering of cytoplasmic free Ca2+ concentration.

The intrapancreatic neuropeptide galanin has been demonstrated to lower plasma insulin levels in vivo. The effects of this peptide on insulin secretion, cytoplasmic free Ca2+ concentration and membrane potential have now been studied in vitro. Glucose-stimulated insulin secretion was inhibited by galanin under these conditions, indicating a direct effect of the peptide on the beta-cells. The neuropeptide reversed both the increase in membrane potential and cytoplasmic free Ca2+ in response to glucose stimulation. At a non-stimulatory concentration of the sugar, galanin induced a slight hyperpolarization without any effect on cytoplasmic free Ca2+. Galanin did not affect K+-induced increase in cytoplasmic free Ca2+, excluding a direct inhibitory effect on the voltage-activated Ca2+ channels. The results indicate that galanin inhibition of glucose-stimulated insulin release involves hyperpolarization with a subsequent decrease in cytoplasmic free Ca2+.

Animals↗
PubMed1990

The difference in murine CDC2 kinase activity between cytoplasmic and nuclear fractions during the cell cycle.

The mouse analog of yeast CDC2+ kinase was detected in the cytoplasmic and nuclear fractions of cultured mouse FM3A cells. Its activity in the nuclear fraction increased in the G2/M phase became seven times higher than that in the G1/S phase, while the activity in the cytoplasmic fraction remained was almost constant from the G1/S to G1 phases. The activity in the cytoplasmic fraction was similar to that in the nuclear fraction in the G2/M phase. The amount of the enzyme remained almost constant during the cell cycle in both the nuclear and cytoplasmic fractions. These findings suggest that the cytoplasmic enzyme might play an independent role in the cell cycle.

Amino Acid Sequence↗
PubMed1987

Role of microtubules in the cytoplasmic compartmentation of neurons.

We have examined the role of microtubules (MTs) in the development and maintenance of the cytoplasmic compartmentation of ribosomes in cultured embryonic chick sensory neurons. Control neurons show a sharp cytoplasmic demarcation between the ribosome-rich/MT-deficient 'translational' cytoplasm of the perikaryon and the ribosome-deficient/MT-rich 'expressional' cytoplasm of the axon-like neurites. Neurites treated with 1.0 microgram/ml nocodazole for 15-20 min show in addition to the depolymerization of MTs a clear loss of the cytoplasmic demarcation between the soma and neurite. We found a decreasing density of ribosomes with increasing distance from the soma in the nocodazole treated neurites. Ribosomes, as confirmed by their diameter and sensitivity to ribonuclease treatment, populated the first 100 microns of neurites at roughly the same density as found in the soma (150-300 ribosomes per microns2 in 50 mm longitudinal thin sections). The second 100 microns contained a lower density of ribosomes than the first, 20-60 per microns2, but still higher than controls. Distances past 200 microns were devoid of unambiguous ribosomes. Neurites amputated between 50 and 150 microns from the soma then treated with nocodazole contained a dense population of ribosomes in the proximal segment still attached to the soma, but few or none in the distal segment. We interpret these findings as evidence that ribosomes are able to move from the soma down the neurite after MT disassembly.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

Regulation of cytoplasmic pH in phagocytic cell function and dysfunction.

To ensure effective antimicrobial or tumouricidal function, phagocytic cells must maintain their cytoplasmic pH (pHi) at a level conductive to optimal intracellular enzyme activity. The mechanisms by which neutrophils and macrophages regulate their cytoplasmic pH include bicarbonate-independent ion transport systems, most notably the Na+/H+ exchanger, and bicarbonate-dependent ion transport systems, which can be subdivided into the cation-independent and Na(+)-dependent forms of chloride/bicarbonate exchange. In addition, macrophages have been shown to recover from intracellular acid loading by means of an ATP-dependent proton extrusion mechanism, which has the characteristics of a vacuolar-type H+ ATPase. In the microenvironment typically associated with abscesses, the low extracellular pH and the presence of short chain fatty acid by-products of bacterial metabolism tend to induce cytoplasmic acid loading. In this setting, the ability of the various pHi regulatory mechanisms to protect pHi may be overcome, leading to cytoplasmic acidification. Several investigators have shown that cytoplasmic acidification impairs the ability of neutrophils to migrate in response to chemotactic stimuli, and also impairs their ability to generate a respiratory burst, thus inhibiting the release of toxic oxygen radicals. This may result in the inability of phagocytes to effect complete abscess resolution.

Abscess↗
PubMed1985

Germinal vesicle contents are required for the cytoplasmic cycle during meiotic division of starfish oocytes.

Enucleated oocytes of starfish still show cyclic changes in cortical tension with a temporal pattern similar to that exhibited by intact oocytes during meiotic division, provided that the enucleation is performed a certain time after the breakdown of the germinal vesicle (K. Yamamoto and M. Yoneda, Dev. Biol. 96, 166-172, 1983). If an oocyte is bisected immediately after germinal vesicle breakdown, the resulting nonnucleate fragment shows some change in tension, but the pattern of change is much less regular than that seen in intact oocytes, suggesting that the dispersion of germinal vesicle (GV) contents into cytoplasm is required for the establishment of the cytoplasmic cycle. In order to demonstrate the role of GV contents directly, nonnucleate fragments derived from immature oocytes were injected with GV contents taken from other immature oocytes. On treatment with 1-methyladenine (1-MA) these fragments showed two rounds of increase in tension as is characteristic of intact maturing oocytes. The first rise in tension was always observed 50-70 min after the treatment with 1-MA, similar to the time of first polar body formation in intact oocytes, regardless of the time of injection of GV contents. Even when GV contents were injected into nonnucleate fragments which had been already treated with 1-MA, these fragments showed two rounds of change in tension. The timing of the first rise in tension was found to be 38 +/- 7 min after injection, irrespective of the time of the foregoing treatment with 1-MA. These results prove the indispensability of GV contents for inducing the cytoplasm of the maturing starfish oocyte to initiate its own cyclic activity, and suggest that the normal process of cytoplasmic maturation may consist of two phases, i.e., (1) a GV-independent phase initiated by 1-MA treatment, and (2) a second phase initiated by mixing of GV contents with cytoplasm.

Adenine↗