Relationship between rates of synthesis and intracellular distribution of ribosomal proteins during oogenesis in the mouse

Ribosomal proteins are synthesized continuously in nonequimolar amounts during oogenesis in the mouse (M. J. LaMarca and P. M. Wassarman, 1979, Develop. Biol. 73, 103), even though ribosomal proteins are found in equimolar amounts in ribosomes. In this report, the distribution of newly synthesized ribosomal proteins between the cytoplasm and germinal vesicle (nucleus) of fully grown mouse oocytes has been examined. As compared to total newly synthesized protein, ribosomal proteins were found to be highly concentrated in the oocyte's germinal vesicle. Furthermore, an inverse relationship was found between rates of synthesis of individual ribosomal proteins and percentages of newly synthesized ribosomal proteins associated with germinal vesicles. As a result of this relationship, the amounts of newly synthesized ribosomal proteins associated with germinal vesicles approximated an equimolar situation. Even in the presence of actinomycin D, oocytes continued to synthesize ribosomal proteins which were found associated with germinal vesicles in amounts similar to those observed in the absence of the drug. These results suggest that, although synthesis of ribosomal proteins by mouse oocytes is not coordinately regulated, a post-translational mechanism exists for adjusting the stoichiometry of these proteins within the oocyte's germinal vesicle; this mechanism apparently is not dependent upon concomitant ribosomal-RNA synthesis.

Role of asparagine-linked oligosaccharides in secretion of glycoproteins of the mouse egg's extracellular coat

Tunicamycin, an inhibitor of asparagine-linked glycosylation of glycoprotein, has been used here to examine the role of N-linked oligosaccharides in secretion of ZP2 and ZP3, two of the three glycoproteins that constitute the mouse egg's extracellular coat (zona pellucida). In the absence of tunicamycin, growing mouse oocytes cultured in vitro synthesize a 91,000-Mr ZP2 precursor and 53,000 and 56,000 Mr ZP3 precursors. All of these precursors contain high mannose-type oligosaccharides that are processed to complex-type prior to secretion of mature ZP2 (120,000 Mr) and ZP3 (80,000 Mr) (Greve, J. M., Salzmann, G. S., Roller, R. J., and Wassarman, P. M. (1982) Cell 31, 749-759; Salzmann, G. S., Greve, J. M., Roller, R. J., and Wassarman, P. M. (1983) Eur. Mol. Biol. Org. J. 2, 1451-1456). In the presence of 5 micrograms/ml of tunicamycin, growing oocytes cultured in vitro are unable to carry out "core" glycosylation of nascent ZP2 and ZP3. Consequently, under these conditions, ZP2 and ZP3 appear as 81,000 and 44,000 Mr polypeptide chains, respectively. The apparent rates of synthesis of core-glycosylated ZP2 and ZP3 precursors synthesized in the absence of tunicamycin and of precursors synthesized in the presence of the drug are virtually identical. On the other hand, in the presence of tunicamycin, nascent ZP3 is incorporated into the zona pellucida as an extremely heterogeneous species (approximately equal to 51,000 Mr) at about three times the rate observed for mature ZP3 in the absence of tunicamycin. In the presence of tunicamycin, ZP2 is incorporated into the zona pellucida as 81,000 and 76,000 Mr species at about one-sixth the rate observed for mature ZP2 in the absence of the drug. Results of pulse-chase experiments indicate that the low degree of incorporation of ZP2 lacking N-linked oligosaccharides into the zona pellucida is due to a greatly decreased rate of secretion as compared to the core-glycosylated precursor. ZP2 synthesized in the presence of tunicamycin is relatively stable and accumulates intracellularly. These results suggest that N-linked oligosaccharides are necessary for normal secretion of ZP2, but are probably not necessary for ZP3 secretion.

Biosynthesis of the sperm receptor during oogenesis in the mouse

During their growth phase, mouse oocytes synthesize and secrete three different glycoproteins, called ZP1, 2 and 3, that constitute the extracellular coat, or zona pellucida, of the oocyte. One of these glycoproteins, ZP3, exhibits properties expected for a sperm receptor. We have now used rabbit antisera that recognize ZP3 to immunoprecipitate [35S]methionine-labeled, intracellular precursors of this glycoprotein from growing oocytes cultured in vitro in the presence or absence of tunicamycin, a drug that prevents addition of N-linked oligosaccharides to nascent polypeptide chains. Electrophoretic analyses of these immunoprecipitates, as well as of immunoprecipitates digested with endo-beta-N-acetylglucosaminidase H (Endo H), indicate that ZP3 is synthesized as a 44,000 mol. wt. polypeptide chain to which either three or four high-mannose-type oligosaccharides are added, resulting in 53,000 and 56,000 mol. wt. ZP3 precursors, respectively. The latter species are converted to mature ZP3 (mol. wt. approximately 80,000) by processing of the high-mannose-type oligosaccharides (Endo H-sensitive) to complex-type oligosaccharides (Endo H-insensitive) prior to ZP3 secretion. The evidence presented reveals that the extreme heterogeneity of mature ZP3, with respect to both mol. wt. and isoelectric point, is partly a consequence of the N-linked oligosaccharides and not the polypeptide chain itself.

Structure of chromatin at deoxyribonucleic acid replication forks: nuclease hypersensitivity results from both prenucleosomal deoxyribonucleic acid and an immature chromatin structure

Relative to nonreplicating DNA in mature simian virus 40 (SV40) chromosomes, newly synthesized DNA in replicating SV40 chromosomes was found to be hypersensitive to the nonspecific endonucleases, micrococcal nuclease (MNase), DNase I, and DNase II. Nascent DNA, pulse labeled in either intact cells or nuclear extracts supplemented with cytosol, was digested about 5-fold faster and about 25% more extensively than uniformly labeled DNA in mature viral chromosomes. Pulse-chase experiments in vitro revealed a time-dependent chromatin maturation process that involved two distinct steps: (i) conversion of prenucleosomal DNA (PN-DNA) into immature nucleosomal oligomers and (ii) maturation of newly assembled chromatin into a structure with increased nuclease resistance. PN-DNA was hypersensitive to MNase, releasing short DNA fragments which were subsequently solubilized by the nuclease. However, when the nascent PN-DNA was specifically removed by digestion of replicating viral chromosomes with Escherichia coli exonuclease III (3'-5') and phage T7 exonuclease (5'-3'), subsequent digestion of the remaining chromatin with MNase revealed the same degree of hypersensitivity observed prior to exonuclease treatment. Furthermore, newly assembled nucleosomal oligomers, isolated after a brief MNase digestion of replicating viral chromosomes, were also hypersensitive to MNase relative to oligomers isolated from mature chromosomes. Hybridization analysis of the DNA in these immature oligomers revealed that it originated from both sides of replication forks. Inhibition of DNA polymerase alpha by aphidicolin inhibited conversion of PN-DNA into nucleosomes but did not inhibit loss of nucleosomal hypersensitivity to MNase. In contrast, components in the soluble fraction of the subcellular system ("cytosol") were required for both DNA replication and chromatin maturation. Analysis of the nucleoprotein products from a MNase digestion of replicating and mature SV40 chromosomes failed to detect a change in nucleosome structure that corresponded to the loss of nuclease hypersensitivity. However, the results presented demonstrate that both PN-DNA and newly assembled immature chromatin, present on both arms of SV40 replication forks, contribute to the commonly observed hypersensitivity of newly replicated chromatin to endonucleases.

Sperm-egg interactions in the mouse: sequence of events and induction of the acrosome reaction by a zona pellucida glycoprotein

In this investigation, the interaction of mouse sperm with unfertilized eggs and embryos, solubilized zonae pellucidae isolated from eggs and embryos, and purified zona pellucida glycoproteins ZP1, 2, and 3 (J. D. Bleil, and P. M. Wassarman, (1980b) Dev. Biol. 76, 185-202) has been examined in vitro by light and electron microscopy. The experiments described were carried out in order to determine the temporal sequence of events during sperm-egg interaction in vitro and to identify the component(s) of zonae pellucidae responsible for inducing mouse sperm to undergo the acrosome reaction. "Pulse-chase" analysis of the sequence of sperm-egg interactions revealed that mouse sperm first "attach" loosely and then "bind" tightly to the unfertilized egg's zona pellucida. Binding of sperm to egg zonae pellucidae is followed by induction of the acrosome reaction. Induction of the acrosome reaction can be mediated by the zona pellucida, since solubilized zonae pellucidae isolated from unfertilized eggs were found to be just as effective as the calcium ionophore A23187 in inducing the reaction in vitro. Furthermore, ZP3 purified from zonae pellucidae isolated from unfertilized eggs, but not from two-cell embryos, was also just as effective as either solubilized zonae pellucidae from eggs or ionophore A23187 in inducing the acrosome reaction. ZP1 and 2 from both eggs and embryos, and ZP3 from embryos, had little effect on the extent of the acrosome reaction as compared to control samples. The results of these and other experiments (J. D. Bleil, and P. M. Wassarman, (1980b) Cell 20, 873-882) strongly suggest that, at least in vitro, mouse sperm recognize and bind to ZP3 of egg zonae pellucidae, and that such binding leads to the induction of the acrosome reaction. Modification of ZP3 following fertilization eliminates sperm binding to zonae pellucidae and, consequently, induction of the acrosome reaction is precluded.

Biosynthesis of the major zona pellucida glycoprotein secreted by oocytes during mammalian oogenesis

An antiserum directed specifically against ZP2, the major glycoprotein of the mouse egg's extracellular coat (zona pellucida), has been used to immunoprecipitate intracellular precursors of ZP2 that were synthesized by growing mouse oocytes cultured in vitro. Pulse-chase experiments revealed that the immediate precursor of mature, 120 kilodalton (kd) ZP2 is a 91 kd species that unlike mature ZP2, is sensitive to digestion by endo-beta-N-acetylglucosaminidase H (Endo H) and is converted by the endoglycosidase into an 81 kd species. An 81 kd species is only found intracellularly when growing oocytes are cultured in the presence of tunicamycin. These results suggest that ZP2 is synthesized as an 81 kd polypeptide chain that is first "core"-glycosylated at asparagine residues with high-mannose-type oligosaccharides, giving rise to a 91 kd intermediate (Endo-H-sensitive), and then processed to complex-type oligosaccharides prior to secretion as mature, 120 kd ZP2 (Endo H-insensitive). Furthermore, electrophoretic analyses of mature ZP2, ZP2 precursor (91 kd) and Endo H-treated ZP2 precursor (81 kd) suggest that there are six N-linked oligosaccharides per molecule and that the extreme heterogeneity of mature ZP2 is a consequence of the oligosaccharides and not the polypeptide chain itself.

Structure of chromatin at deoxyribonucleic acid replication forks: prenucleosomal deoxyribonucleic acid is rapidly excised from replicating simian virus 40 chromosomes by micrococcal nuclease

Replicating simian virus 40 (SV40) chromosomes were found to be similar to other eukaryotic chromosomes in that the rate and extent of micrococcal nuclease (MNase) digestion were greater with replicating than with nonreplicating mature SV40 chromatin. MNase digestion of replicating SV40 chromosomes, pulse labeled in either intact cells or nuclear extracts, resulted in the rapid release of nascent DNA as essentially bare fragments of duplex DNA (3-7S) that had an average length of 120 base pairs and were degraded during the course of the reaction. In addition, nucleosomal monomers, equivalent in size to those from mature chromosomes, were released. On the other hand, MNase digestion of uniformly labeled mature SV40 chromosomes resulted in the release of only nucleosomal monomers and oligomers. The small nascent DNA fragments released from replicating chromosomes represented prenucleosomal DNA (PN-DNA) from the region of replication forks that encompasses the actual sites of DNA synthesis and includes Okazaki fragments. Predigestion of replicating SV40 chromosomes with both Escherichia coli exonuclease III (3'-5') and bacteriophage T7 gene 6 exonuclease (5'-3') resulted in complete degradation of PN-DNA. This result, together with the observation that isolated PN-DNA annealed equally well to both strands of SV40 restriction fragments, demonstrated that PN-DNA originates from both sides of replication forks. Over 90% of isolated Okazaki fragments annealed only to the retrograde DNA template. The characteristics of isolated PN-DNA were assessed by examining its sensitivity to MNase and single strand specific S1 endonuclease, sedimentation behavior before and after deproteinization, buoyant density in CsCl after formaldehyde treatment, and size on agarose gels. In addition, it was observed that MNase digestion of purified SV40 DNA also resulted in the release of a transient intermediate similar in size to PN-DNA, indicating that a DNA-protein complex is not required to account for the appearance of PN-DNA. These and other data provide a model of replicating chromosomes in which DNA synthesis occurs on a region of replication forks that is free of nucleosomes and is designated as prenucleosomal DNA.

Chromatin assembly. Relationship of chromatin structure to DNA sequence during simian virus 40 replication

The accessibility of five specific DNA sequences to six different single site restriction endonucleases was evaluated in replicating and mature simian virus 40 chromosomes isolated by three different methods. Electron microscopic and gel electrophoretic analysis of the DNA digestion products demonstrated that DNA accessibility in chromatin was established within 400 base pairs of replication forks and remained essentially unchanged during production of mature chromosomes and their subsequent re-entry into the replication pool. Saturating amounts of each enzyme reproducibly cut a fraction of the chromosomes, ranging from 13 to 49%. This is consistent with a nearly random phasing of chromatin structure. Examples in which all chromosomes were either cleaved or intact were never observed. Although variation in the accessibility of DNA sites near the origin of replication could be interpreted as preferred phasing in about 25% of the chromosomes, the finding that two isoschizomers, Hpa II and Msp I, did not cut chromosomes to the same extent precludes an unambiguous interpretation of the extents of cleavage of individual restriction enzymes. Since the extent of DNA cleavage observed at each restriction site was essentially indistinguishable in replicating as compared to mature chromosomes, the accessibility of DNA sequences near the origin is not obviously related to replication. Furthermore, the accessibility of DNA sites on one arm of a single replication fork was the same as the homologous sites on the other arm, consistent with a nearly random phasing of chromatin structure on both arms. This suggests that chromatin assembly occurs independently on the 2 sibling molecules of a single replicating chromosome.

Structure of chromatin at deoxyribonucleic acid replication forks: location of the first nucleosomes on newly synthesized simian virus 40 deoxyribonucleic acid

Exonucleases specific for either 3' ends (Escherichia coli exonuclease III) or 5' ends (bacteriophage T7 gene 6 exonuclease) of nascent DNA chains have been used to determine the number of nucleotides from the actual sites of DNA synthesis to the first nucleosome on each arm of replication forks in simian virus 40 (SV40) chromosomes labeled with [3H]thymidine in whole cells. Whereas each enzyme excised all of the nascent [3H]DNA from purified replicating SV40 DNA, only a fraction of the [3H]DNA was excised from purified replicating SV40 chromosomes. The latter result was attributable to the inability of either exonuclease to digest nucleosomal DNA in native replicating SV40 chromosomes, as demonstrated by the following observations: (i) digestion with either exonuclease did not reduce the amount of newly synthesized nucleosomal DNA released by micrococcal nuclease during a subsequent digestion period; (ii) in briefly labeled molecules, as much as 40% of the [3H]DNA was excised from long nascent DNA chains; (iii) the fraction of [3H]DNA excised by exonuclease III was reduced in proportion to the actual length of the radiolabeled DNA; (iv) the effects of the two exonucleases were additive, consistent with each enzyme trimming only the 3' or 5' ends of nascent DNA chains without continued excision through to the opposite end. When the fraction of nascent [3H]DNA excised from replicating SV40 DNA by exonuclease III was compared with the fraction of [32P]DNA simultaneously excised from an SV40 DNA restriction fragment, the actual length of nascent [3H]DNA was calculated. From this number, the fraction of [3H]DNA excised from replicating SV40 chromosomes was converted into the number of nucleotides. Accordingly, the average distance from either 3' or 5' ends of long nascent DNA chains to the first nucleosome on either arm of replication forks was found to be 125 nucleotides. Furthermore, each exonuclease excised about 80% of the radiolabel in Okazaki fragments, suggesting that less than one-fifth of the Okazaki fragments were contained in nucleosomes. On the basis of these and other results, a model for eukaryotic replication forks is presented in which nucleosomes appear rapidly on both the forward and retrograde arms, about 125 and 300 nucleotides, respectively, from the actual site of DNA synthesis. In addition, it is proposed that Okazaki fragments are initiated on nonnucleosomal DNA and then assembled into nucleosomes, generally after ligation to the 5' ends of long nascent DNA chains is completed.

Synthesis of zona pellucida proteins by denuded and follicle-enclosed mouse oocytes during culture in vitro

During growth of the ovarian follicle, the mammalian oocyte becomes surrounded by an acellular coat called the zona pellucida. Whether the zona pellucida originates from the oocyte, surrounding follicle cells, or both has remained an unresolved issue. In experiments described here, denuded and follicle-enclosed mouse oocytes at various stages of growth were isolated and cultured in vitro in the presence of either [(35)S]methionine or [(3)H]fucose in order to determine the site of synthesis of the three, recently identified, zona pellucida proteins, ZP1, ZP2, and ZP3 [Bleil, J. D. & Wassarman, P. M. (1980) Dev. Biol., in press]. Approximately 1.5% of the [(35)S]methionine, and as much as 45% of the [(3)H]fucose, that was incorporated into trichloroacetic acid-insoluble material by denuded or follicle-enclosed oocytes during a 12-hr culture period was found associated with zonae pellucidae removed from the cultured oocytes. Incorporation of [(35)S]methionine into zona pellucida proteins was depressed to less than 1/50th when denuded oocytes were cultured in the presence of puromycin, and secretion of zona pellucida proteins by denuded oocytes was demonstrated by pulse-chase experiments. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis of [(35)S]methionine- and [(3)H]fucose-labeled proteins present in oocytes, zonae pellucidae, and follicle cells revealed that denuded oocytes synthesize and secrete zona pellucida proteins, whereas no evidence was obtained to suggest that follicle cells synthesize these proteins. Denuded oocytes, ranging in diameter from 48 to 68 mum, incorporated both [(35)S]methionine and [(3)H]fucose into zona pellucida proteins during culture in vitro, whereas zonae pellucidae removed from fully-grown oocytes (85 mum) were not radiolabeled to a significant extent. After culture of denuded or follicle-enclosed oocytes for 12 hr, more than 95% of the [(3)H]fucose incorporated into oocyte proteins was found in ZP1, ZP2, and ZP3, indicating that the zona pellucida proteins are the major class of proteins glycosylated during oocyte growth. These results provide biochemical evidence supporting the idea that the zona pellucida originates from the mammalian oocyte itself, rather than from the surrounding follicle cells.

Structure, spacing, and phasing of nucleosomes on isolated forms of mature simian virus 40 chromosomes

Mature Simian virus 40 (SV40) chromosomes were isolated from infected CV-1 monkey cells by a hypotonic extraction method that not only allowed replicating viral chromosomes to faithfully continue DNA replication in vitro, but also was found to assemble nascent DNA into nucleosomes with a structure and arrangement typical of native viral chromosomes. Detailed analysis of the DNA and nucleoprotein products from micrococcal nuclease and DNase I digestion of mature viral chromosomes assembled in intact cells showed that the structure of SV40 and CV-1 cellular nucleosomes was the same. Furthermore, the histone composition of viral chromosome was indistinguishable from that of its host. In contrast to the identity in nucleosome structure, nucleosome spacing on isolated SV40 chromosomes was not as regular as on cellular chromatin. When 1% of the DNA was solubilized by micrococcal nuclease, as many as 20 cellular DNA bands were clearly resolved by gel electrophoresis, but only 6 to 7 viral DNA bands were observed and they were broader and less well resolved. In addition, micrococcal nuclease digested SV40 chromosomes at a faster initial rate and to a greater extent than CV-1 chromatin present in the same tube. Either BglI or EcoRI restriction endonuclease cut a single site in 30% of the SV40 chromosomes which suggested that viral nucleosomes were not located in a unique phase with respect to DNA sequence, but appeared to be randomly spaced around the genome. Viral chromosome structure was basically unaltered in hypotonically extracted chromosomes exposed to 200 or 600 mM NaCl and in isotonically extracted chromosomes prepared in the presence of Triton X-100 and EDTA. These results confirm and extend our previous data on the arrangement of SV40 nucleosomes inside isolated nuclei and demonstrate that the structure of viral chromosomes was not altered by the isolation procedures employed. The data are consistent with a model in which an average of 22 nucleosomes, randomly distributed around the SV40 genome, are separated by non-nucleosomal spacer regions which account for about 20% of the total DNA.