Nature and role of proteasomes in maturation of fish oocytes

Histological and Transcriptomic Insights into the Ovary Development of Hemibarbus labeo Injected with Spawn-Inducing Hormones

Naturally, the ovaries of many farmed fish can only develop to stage IV (mainly including stage IV oocytes, known as full-grown postvitellogenic oocytes). Therefore, spawn-inducing hormone injections are used to promote ovary development and oocyte maturation, facilitating reproduction in the aquaculture industry. The study of spawn-inducing hormones and their underlying neuroendocrine mechanisms has been a recent focus in fish reproductive biology. However, the intra-ovarian regulatory mechanisms of ovary development and oocyte maturation after hormone injection require further investigation. In this study, we explored the histological and transcriptomic map of the ovary of Hemibarbus labeo after hormone injection to reveal changes in the ovary. The gonad index significantly increased after hormone injection for 5.5 h, after which no significant change was observed. Histological analysis showed that the nuclei had moved to one side of the oocytes at 5.5 h after hormone injection. Moreover, the volume of the oocytes increased and their yolk membranes thickened. Oocytes then underwent their first meiotic division at 5.5-11 h after hormone injection. Subsequently, the follicular membrane was ruptured, and ovulation was completed at 11-16.5 h after hormone injection. In addition, we identified 3189 differentially expressed genes (DEGs) on comparing the transcriptomes at different time points after hormone injection. These DEGs were significantly enriched in the GO terms of nervous system process, molecular transducer activity, and extracellular region, and the KEGG pathways of TNF signaling and cytokine-cytokine receptor interaction; these may play important roles in ovary development and oocyte maturation. Within these pathways, genes such as apoe, creb3, jun, junb, il11, and il8 may play important roles in steroid hormone synthesis and ovulation. Conclusively, our results show detailed sequential dynamics of oocyte development and provide new insights into the intra-ovarian regulatory mechanisms of ovarian development and oocyte maturation in H. labeo. These findings may be important for research on improving egg quality and reproduction in aquaculture.

Purification and Identification of the 20S Proteasome Complex from Zebrafish

The proteasome is a large polymeric protease complex responsible for degradation of intracellular proteins and generation of peptides. In this study, we purified a native 20S proteasome protein complex from zebrafish (Danio rerio) from the whole body. The cytosolic fraction of zebrafish hydrolyzed Suc-Leu-Leu-Val-Tyr-MCA (Suc-LLVY-MCA), a well-known substrate for the proteasome, in the presence of sodium dodecyl sulfate. From the cytosolic fraction, the 20S proteasome was purified using five column chromatography steps: DEAE cellulose, Q-Sepharose, Sephacryl S-300 gel, hydroxylapatite, and phenyl Sepharose. Electrophoresis and Western blot analyses showed that zebrafish 20S proteasome subunits have molecular masses ranging from 22 to 33 kDa. The subunit composition of the purified 20S proteasome was identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF/MS) analysis after two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) separation. Fourteen kinds of 20S subunits were found. As a special characteristic of zebrafish, two proteins of the α1 subunit were identified. In addition, the results suggested that the α8 subunit is in the 20S complex instead of the α4 subunit. In this study, we demonstrated the subunit composition of the 20S proteasome complex present in zebrafish cells.

Influence of ovarian stage on transcript profiles in fathead minnow (Pimephales promelas) ovary tissue

Interpretation of toxicogenomic experiments conducted with ovary tissue from asynchronous-spawning small fish species is complicated by background variation in the relative abundance and proportion of follicles at different stages within the ovary tissue sample. This study employed both real-time quantitative polymerase chain reaction and a 15,000 gene oligonucleotide microarray to examine variation in the fathead minnow (Pimephales promelas) ovarian transcriptional profile as a function of quantitative and qualitative differences in ovarian histology. The objectives were to provide data that could potentially aid interpretation of future toxicogenomics experiments, identify putative stage-related transcriptional markers, and generate insights into basic biological regulation of asynchronous oocyte development. Multiple lines of evidence from the present study indicate that variation in the transcriptional profile is primarily dependent on the relative abundance of previtellogenic versus vitellogenic follicles in the ovary. Due to the relatively small proportions of mature ovulated follicles or atretic follicles in the overall follicle population, few potential transcriptional markers of maturation, ovulation, or atresia could be identified. However, among the 460 differentially expressed genes identified in the present study, several targets, including HtrA serine peptidase 3 (htra3), tissue inhibitor of metalloproteinase 3 (timp3), aquaporin 8 (aqp8), transgelin 2 like (tagln2), Nedd4 family interacting protein 2 (ndfip2), chemokine ligand 12a (cxcl12a), midkine-related growth factor (mdka), and jagged 1b (jag 1b) exhibited responses and functional properties that support them as candidate molecular markers of significant shift in gross ovarian stage. Genes associated with a diversity of functions including cellular development, morphogenesis, coated vesicle transport, sexual reproduction, and neuron development, among others, were statistically enriched within the list of 460 genes differentially expressed among different ovarian classes. Overall, results of this study provide insights into background variation in ovary transcript profiles that should aid and enhance the interpretation of toxicogenomic data generated in experiments conducted with small, asynchronous-spawning fish species.

Highly sensitive saturation labeling reveals changes in abundance of cell cycle-associated proteins and redox enzyme variants during oocyte maturation in vitro

Oocyte maturation is a complex process and a critical issue in assisted reproduction techniques (ART) in humans and other mammals. We used a sensitive 2-D DIGE saturation labeling approach including an internal pooled standard for quantitative proteome profiling of immature versus in vitro matured bovine oocytes in six independent samples. The study comprised 48 2D gel images representing 24 DIGE experiments. From 250 ng sample analyzed per gel, quantitative analysis revealed an average of 2244 spots in pH 4-7 images and 1291 spots in pH 6-9 images. Thirty-eight spots with different intensities were detected in total. Spots of a preparative gel from 2200 oocytes were identified by nano-LC-MS/MS analysis. The ten spots which could be unambiguously identified include the Ca2+-binding protein translationally controlled tumor protein, enzymes of the Krebs and pentose phosphate cycles, clusterin, 14-3-3 epsilon, elongation factor-1 gamma, and redox enzymes such as polymorphic forms of GST Mu 5 and peroxiredoxin-3. The cellular distribution of two proteins was determined by confocal laser scanning microscopy. The interesting protein candidates identified by this study may help to improve the in vitro maturation process in order to increase the rate of successful in vitro fertilization and other ART in cattle and other mammals.

Molecular Cloning of cDNA Encoding APC11, a Catalytic Component of Anaphase-promoting-complex (APC/C), from Goldfish (Carassius auratus), and Establishment ofin vitroUbiquitinating System

Destruction of cyclin B is required for exit from mitosis and meiosis. A cyclin-degrading system, including anaphase-promoting-complex/cyclosome (APC/C), has been shown to be responsible for cyclin B destruction. Here we present the cloning, sequencing, and expression analysis of goldfish (Carassius auratus) APC11, which encodes the catalytic component of APC/C from goldfish ovary. The cloned cDNA is 348 bp long and encodes 88 amino acids. The deduced amino acid sequence is highly homologous to APC11 from other species. The expression of mRNA for APC11 was ubiquitous among tissues, as opposed to that of mRNA for E2-C, which occurred at a very high level in the ovary. Recombinant goldfish APC11 possesses ubiquitinating activity against cyclin B. We established an in vitro ubiquitinating system of proteins composed of purified recombinant E1, E2-C, and APC11 from goldfish. The reconstructed system for these ubiquitinating enzymes makes it feasible to elucidate the molecular mechanism of cyclin B degradation.

Comparative proteome analysis of changes in the 26S proteasome during oocyte maturation in goldfish

Proteasomes are large, multi-subunit particles that act as the proteolytic machinery for most of the regulated intracellular protein degradation in eukaryotic cells. An alteration of proteasome function may be important for the regulation of the meiotic cell cycle. To study the change at the subunit level of the 26S proteasome during meiotic maturation, we purified 26S proteasomes from immature and mature oocytes of goldfish. Two-dimensional polyacrylamide gel electrophoresis was used to separate proteins. For differential analysis, whole spots of the 26S proteasome from goldfish oocytes were identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry and database analysis. Four spots that were different (only detected in mature oocyte 265 proteasomes and not in immature ones) and four protein spots that were up- or down-regulated were identified unambiguously. The mature-specific spots were not 26S proteasome components but rather their interacting proteins, and were identified as chaperonin-containing TCP-1 subunits and myosin light chain. Minor spots of three subunits of the 20S core particle and one of the 19S regulatory particle showed meiotic cell cycle-dependent changes. These results demonstrate that modifications of proteasomal subunits and cell cycle phase-dependent interactions of proteins with proteasomes occur during oocyte maturation in goldfish.

Molecular cloning and expression of cDNA coding for four spliced isoforms of casein kinase Ialpha in goldfish oocytes

Casein kinase I (CKI) is a member of the serine/threonine protein kinases and located in a separate group within the superfamily of eukaryotic protein kinases. CKI isoforms regulate several checkpoints of the cell cycle and meiosis. In higher eukaryotes, CKIalpha has four isoforms produced through the alternative splicing of two short inserts. Here, we report the cloning, sequencing and expression of four alternatively spliced isoforms of CKIalpha from goldfish ovary. The cloned cDNAs were 2099-3002-bp long and classified as CKIalpha, CKIalphaS, CKIalphaL and CKIalphaLS. It was revealed that two major (3.0 and 2.0 kb) messages were strongly expressed in the ovary. Four isoforms are expressed in previtellogenic to vitellogenic oocytes. In the huge nucleus of the oocyte, referred to as the germinal vesicle, CKIalphaS is dominant and CKIalphaL is expressed at a detectable level. Immunoblot analysis revealed that CKIalpha and CKIalphaS are major products in both immature and mature oocytes. These two isoforms were expressed in a tissue-dependent manner.

Identification of alpha-type subunits of the Xenopus 20S proteasome and analysis of their changes during the meiotic cell cycle

Background

The 26S proteasome is the proteolytic machinery of the ubiquitin-dependent proteolytic system responsible for most of the regulated intracellular protein degradation in eukaryotic cells. Previously, we demonstrated meiotic cell cycle dependent phosphorylation of α4 subunit of the 26S proteasome. In this study, we analyzed the changes in the spotting pattern separated by 2-D gel electrophoresis of α subunits during Xenopus oocyte maturation.

Results

We identified cDNA for three α-type subunits (α1, α5 and α6) of Xenopus, then prepared antibodies specific for five subunits (α1, α3, α5, α6, and α7). With these antibodies and previously described monoclonal antibodies for subunits α2 and α4, modifications to all α-type subunits of the 26S proteasome during Xenopus meiotic maturation were examined by 2D-PAGE. More than one spot for all subunits except α7 was identified. Immunoblot analysis of 26S proteasomes purified from immature and mature oocytes showed a difference in the blots of α2 and α4, with an additional spot detected in the 26S proteasome from immature oocytes (in G2-phase).

Conclusions

Six of α-type subunits of the Xenopus 26S proteasome are modified in Xenopus immature oocytes and two subunits (α2 and α4) are modified meiotic cell cycle-dependently.

Ubiquitin-proteasome-mediated local protein degradation and synaptic plasticity

A proteolytic pathway in which attachment of a small protein, ubiquitin, marks the substrates for degradation by a multi-subunit complex called the proteasome has been shown to function in synaptic plasticity and in several other physiological processes of the nervous system. Attachment of ubiquitin to protein substrates occurs through a series of highly specific and regulated steps. Degradation by the proteasome is subject to multiple levels of regulation as well. How does the ubiquitin-proteasome pathway contribute to synaptic plasticity? Long-lasting, protein synthesis-dependent, changes in the synaptic strength occur through activation of molecular cascades in the nucleus in coordination with signaling events in specific synapses. Available evidence indicates that ubiquitin-proteasome-mediated degradation has a role in the molecular mechanisms underlying synaptic plasticity that operate in the nucleus as well as at the synapse. Since the ubiquitin-proteasome pathway has been shown to be versatile in having roles in addition to proteolysis in several other cellular processes relevant to synaptic plasticity, such as endocytosis and transcription, this pathway is highly suited for a localized role in the neuron. Because of its numerous roles, malfunctioning of this pathway leads to several diseases and disorders of the nervous system. In this review, I examine the ubiquitin-proteasome pathway in detail and describe the role of regulated proteolysis in long-term synaptic plasticity. Also, using synaptic tagging theory of synapse-specific plasticity, I provide a model on the possible roles and regulation of local protein degradation by the ubiquitin-proteasome pathway.

Regulatory roles of ubiquitin–proteasome pathway in pig oocyte meiotic maturation and fertilization

The ubiquitin-proteasome pathway is involved in the degradation of proteins related to cell cycle progression including cyclins. The present study, using two specific proteasome inhibitors, for the first time investigated the roles of ubiquitin-proteasome in cell cycle progression during pig oocyte meiotic maturation and after fertilization. In contrast to its effect in rodent oocytes, proteasome inhibition strongly prevented germinal vesicle breakdown (GVBD). After GVBD, proteasome inhibition disrupted meiotic apparatus organization, cell cycle progression, and first polar body (PB1) extrusion. Sperm penetration into the oocytes was completely inhibited when proteasome inhibitors were added at the beginning of insemination. However, sperm chromatin decondensation and metaphase-interphase transition were not affected when inhibitors were added once sperm penetrated. The results suggest that ubiquin-proteasome complex is one of the critical regulators of meiotic cell cycle, but proteasome inhibitors do not affect major fertilization events when added after sperm penetration into the oocytes in the pig.

Non-traditional roles of ubiquitin–proteasome system in fertilization and gametogenesis

Fertilization and gametogenesis are key events in sexual reproduction. Our recent studies, together with several reports by other authors, demonstrated that the extracellular ubiquitin-proteasome system plays a role in fertilization and gametogenesis in addition to the traditional intracellular ubiquitin-proteasome system. Here, we summarize our recent results showing the importance of the extracellular ubiquitin-proteasome system in the sperm penetration through the vitelline coat of the egg during ascidian fertilization, together with our recent reports implicating the participation of a novel proteasome-associating complex PC530 in starfish oocyte maturation. We also describe the results by other authors showing the participation of the ubiquitin system both in the elimination of defective sperm in epididymis and in the elimination of paternal mitochondria in fertilized eggs. These are evidence of non-traditional extracellular functions of the ubiquitin system.

Activation and development of porcine oocytes matured in vitro following injection of inositol 1,4,5-trisphosphate

Inositol 1,4,5-trisphosphate (IP3) is considered to be important for activation of mammalian oocytes at the time of fertilization, and activation induces a rise in intracellular Ca2+ concentration ([Ca2+]i) by release from the Ca2+ stores in the oocytes. Therefore, IP3 could act as an artificial activator of porcine oocytes. Activation and development, and rise in [Ca2+]i in matured oocytes injected with various concentrations of IP3 were investigated in this study. Porcine oocytes were recovered from the ovaries of prepubertal gilts, matured for 46-48 h and cultured in vitro for 7 days in following treatments as non-injected oocytes (NI), injected with carrier buffer, 2.5, 5 and 500 microM of IP3. The result showed that IP3 activated porcine oocytes matured in vitro (NI 3.8%, buffer 7.1%, 2.5 microM IP3 73.5%, 5 microM IP3 76.2%, 500 microM IP3 85.2%). There was a slight but not significant increase in the proportion of oocytes activated as the level of IP3 increased. The rate of development to the cleavage stage increased remarkably when the concentration of IP3 increased (NI 4.9%, buffer 5.7%, 2.5 microM IP3 30.3%, 5 microM IP3 47.1%, 500 microM IP3 78.1%). Blastocyst development was only observed in oocytes that had been injected with a higher concentration of IP3 (5 microM IP3 6.1% and 500 microM IP3 5.3%). Both the peak value and duration of [Ca2+]i rise also increased as the concentration of IP3 increased. Baseline values (ratio value, R) for [Ca2+]i ranged from 1.51 to 1.57 and was not affected by the buffer treatment. The peak value of [Ca2+]i rose significantly with increasing level of IP3 treatment (2.5 microM IP3, 3.54 +/- 0.32; 5 microM IP3, 7.50 +/- 0.37; 500 microM IP3, 8.54 +/- 0.33). Similarly, the duration of the [Ca2+]i rise increased as the level of IP3 increased (2.5 microM IP3, 43.7+/- 7.00 s; 5 microM IP3, 93.5 +/- 9.17 s; 500 microM IP3, 160.6 +/- 18.9 s). It was concluded that injected IP3 promotes the development of porcine matured oocytes and that their developmental ability is positively correlated with the rise in [Ca2+]i induced by IP3.

Regulated interaction between polypeptide chain elongation factor-1 complex with the 26S proteasome during Xenopus oocyte maturation

BACKGROUND

During Xenopus oocyte maturation, the amount of a 48 kDa protein detected in the 26S proteasome fraction (p48) decreased markedly during oocyte maturation to the low levels seen in unfertilized eggs. The results indicate that the interaction of at least one protein with the 26S proteasome changes during oocyte maturation and early development. An alteration in proteasome function may be important for the regulation of developmental events, such as the rapid cell cycle, in the early embryo. In this study, we identified p48.

RESULTS

p48 was purified by conventional column chromatography. The resulting purified fraction contained two other proteins with molecular masses of 30 (p30) and 37 (p37) kDa. cDNAs encode elongation factor-1gamma and delta were obtained by an immuno-screening method using polyclonal antibodies against purified p48 complex, which recognized p48 and p37. N-terminal amino acid sequence analysis of p30 revealed that it was identical to EF-1beta. To identify the p48 complex bound to the 26S proteasome as EF-1betagammadelta, antibodies were raised against the components of purified p48 complex. Recombinant EF-1 beta,gamma and delta were expressed in Escherichia coli, and an antibody was raised against purified recombinant EF-1gamma. Cross-reactivity of the antibodies toward the p48 complex and recombinant proteins showed it to be specific for each component. These results indicate that the p48 complex bound to the 26S proteasome is the EF-1 complex. MPF phosphorylated EF-1gamma was shown to bind to the 26S proteasome. When EF-1gamma is phosphorylated by MPF, the association is stabilized.

CONCLUSION

p48 bound to the 26S proteasome is identified as the EF-1gamma. EF-1 complex is associated with the 26S proteasome in Xenopus oocytes and the interaction is stabilized by MPF-mediated phosphorylation.

Molecular cloning of cDNA encoding polypeptide chain elongation factor 1alpha from goldfish (Carassius auratus)

The goldfish homologue of polypeptide chain elongation factor-1alpha (EF-1alpha) isolated from the ovary of the goldfish is described. The deduced amino acid sequence is highly homologous to EF-1alpha from other species. Analysis of its tissue distribution revealed a single 1.7 kilobase message ubiquitous among various tissues.

Molecular cloning of ubiquitin-activating enzyme (E1) from Xenopus laevis

The Xenopus homologue to the ubiquitin-activating enzyme (E1) from a Xenopus ovary is described. The deduced amino acid sequence is highly homologous to E1 from other species.

Molecular cloning of cDNA encoding a ubiquitin-activating enzyme (E1) from goldfish (Carassius auratus) and expression analysis of the cloned gene

Destruction of cyclin B is required to the mitotic and meiotic cycles. A cyclin-specific ubiquitinating system, including ubiquitin-activating enzyme (E1), is thought to be responsible for cyclin B destruction. Here we present the cloning, sequencing and expression analysis of goldfish, Carassius auratus, E1 from goldfish ovary. The cloned cDNA is 4069 bp long and encodes 1059 amino acids. The deduced amino acid sequence is highly homologous to E1 from other species. Recombinant goldfish E1 could transfer ubiquitin to cyclin-selective ubiquitin-conjugating enzyme. Tissue distribution revealed a single 4.0-kb message ubiquitous among tissues.

Identification of the goldfish 20S proteasome beta6 subunit bound to nuclear matrix

Proteasomes are large, multisubunit particles that act as the proteolytic machinery for most of the regulated intracellular protein breakdown in eukaryotic cells. Proteasomes are present in both the nucleus and cytoplasm. When we analyzed the molecular composition of protein constituents of the nuclear matrix preparation of goldfish oocytes by two-dimensional polyacrylamide gel electrophoresis followed by sequence analysis, we found a 26 kDa spot identical in amino acid sequence to the beta6 subunits of the 20S proteasome. No spot of other subunits of 20S proteasome was detected. Here we describe the cloning, sequencing and expression analysis of Carassius auratus, beta6_ca, which encodes one of the proteasome beta subunits from goldfish ovary. From the screening of an ovarian cDNA library, two types of cDNA were obtained, one 941 bp and the other 884 bp long. The deduced amino acid sequences comprise 239 and 238 residues, respectively. These deduced amino acid sequences are highly homologous to those of beta6 subunits of other vertebrates. Immunoblot analysis of nuclear matrix using anti-proteasome antibodies showed only a spot of beta6_ca. These results suggest that the beta6 subunit of the goldfish 20S proteasome, beta6_ca, is responsible for anchoring proteasomes in the nucleus.

Two proteins, a goldfish 20S proteasome subunit and the protein interacting with 26S proteasome, change in the meiotic cell cycle

To investigate the regulatory mechanism for the proteasome in the meiotic cell cycle, we purified the 26S proteasome from immature (in G2-phase) and mature (in M-phase) oocytes, and compared its subunits by immunoblotting. At least two protein bands, at 30 kDa (detected by GC3beta antibody) and 62 kDa (detected by 1-4D5 antibody), differed between 26S proteasomes. A monoclonal antibody, GC3beta cross-reacted with two bands in the 26S proteasome from immature oocytes, however, the upper band was absent in the 26S proteasome from mature oocytes. The 62-kDa protein band detected by 1-4D5 antibody was not detected in the immature oocyte 26S proteasome; however, a band was detected in mature oocyte 26S proteasome. The cDNAs encoding these proteins were isolated by an immunoscreening method using the monoclonal antibodies. The 30-kDa protein was an alpha4 subunit, which is one of the alpha-subunit group of the 20S proteasome, and the 62-kDa protein was a homologue of CCTepsilon, one of the components of eukaryotic molecular chaperones. Phosphatase treatment of the 26S proteasome revealed that a part of the alpha4 subunit of goldfish 20S proteasome, alpha4_ca, is phosphorylated in G2-phase and dephosphorylated in M-phase. A binding assay using a recombinant goldfish CCTepsilon revealed that unmodified CCTepsilon interacts with the 26S proteasome. Fertilization triggers a transition from meiotic metaphase to mitotic interphase. During fertilization, a GC3beta cross-reacting upper band reappeared. The 62-kDa band dissociated from the 26S proteasome. As a result, the 26S proteasome changed to an immature type from a mature type during fertilization. These results suggest that the 26S proteasome is changed reversibly during the meiotic cell cycle by modification of its subunits and interactions between regulators.

Identification of the Xenopus 20S proteasome alpha4 subunit which is modified in the meiotic cell cycle

The proteasomes are large, multi-subunit particles that act as the proteolytic machinery for most of the regulated intracellular protein degradation in eukaryotic cells. To investigate the regulatory mechanism for the 26S proteasome in cell-cycle events, we purified this proteasome from immature and mature oocytes, and compared its subunits. Immunoblot analysis of 26S proteasomes showed a difference in the subunit of the 20S proteasome. A monoclonal antibody, GC3beta, cross-reacted with two bands in the 26S proteasome from immature oocytes (in G2-phase); however, the upper band was absent in the 26S proteasome from mature oocytes (in M-phase). These results suggest that changes in the subunits of 26S proteasomes are involved in the regulation of the meiotic cell cycle. Here we describe the molecular cloning of one of the alpha subunits of the 20S proteasome from a Xenopus ovarian cDNA library using an anti-GC3beta monoclonal antibody. From the screening, two types of cDNA are obtained, one 856bp, the other 984bp long. The deduced amino-acid sequences comprise 247 and 248 residues, respectively. These deduced amino-acid sequences are highly homologous to those of alpha4 subunits of other vertebrates. Phosphatase treatment of 26S proteasome revealed the upper band to be a phosphorylated form of the lower band. These results suggest that a part of the alpha4 subunit of the Xenopus 20S proteasome, alpha4_xl, is phosphorylated in G2-phase and dephosphorylated in M-phase.

Molecular cloning of cDNA encoding a cyclin-selective ubiquitin carrier protein (E2-C) from Carassius auratus (goldfish) and expression analysis of the cloned gene

Destruction of cyclin B is required for exit from mitosis and meiosis. A cyclin-specific ubiquitinating system, including cyclin-selective ubiquitin carrier protein (E2-C), is thought to be responsible for cyclin B destruction. Here we present the cloning, sequencing and expression analysis of goldfish, Carassius auratus, E2-C which encodes the cyclin-selective ubiquitin carrier protein from goldfish ovary. The cloned cDNA is 677 bp long and encodes 172 amino acids. The deduced amino acid sequence is highly homologous to E2-C from other species. Recombinant goldfish E2-C possesses ubiquitinating activity against cyclin B. The expression of mRNA for E2-C was similar to that of mRNA for cyclin B, occurring at very high level in the ovary. The similarity of the expression pattern of E2-C and cyclin B suggests that E2-C mediates a cyclin-specific ubiquitination.

The ubiquitin system in gametogenesis

Ubiquitin is a ubiquitous and highly conserved protein of 76 amino acid residues, that can be covalently attached to cellular acceptor proteins. The attachment of ubiquitin to target proteins is achieved through a multi-step enzymatic pathway, which involves activities of ubiquitin-activating E1 enzymes, ubiquitin-conjugating E2 enzymes, and ligating E3 enzymes. Mono- or poly-ubiquitination of proteins can lead to protein degradation or modification of protein activity. Many components of the complex ubiquitin system show remarkable evolutionary conservation, from yeast to mammalian species. The ubiquitin system is essential to all eukaryotic cells. Among others, several signal transduction cascades show involvement of the ubiquitin system, but there are currently little data supporting a specific role of the ubiquitin system in hormonal control of reproduction. Interestingly, during gametogenesis, many specialized and important aspects of the ubiquitin system become apparent. Components of the ubiquitin system appear to be involved in different steps and processes during gametogenesis, including control of meiosis, and reorganization of chromatin structure.