Lipid species affect morphology of endoplasmic reticulum: a sea urchin oocyte model of reversible manipulation

The ER is a large multifunctional organelle of eukaryotic cells. Malfunction of the ER in various disease states, such as atherosclerosis, diabetes, cancer, Alzheimer’s and Parkinson’s and amyotrophic lateral sclerosis, often correlates with alterations in its morphology. The ER exhibits regionally variable membrane morphology that includes, at the extremes, large relatively flat surfaces and interconnected tubular structures highly curved in cross-section. ER morphology is controlled by shaping proteins that associate with membrane lipids. To investigate the role of these lipids, we developed a sea urchin oocyte model, a relatively quiescent cell in which the ER consists mostly of tubules. We altered levels of endogenous diacylglycerol (DAG), phosphatidylethanolamine (PtdEth), and phosphatidylcholine by microinjection of enzymes or lipid delivery by liposomes and evaluated shape changes with 2D and 3D confocal imaging and 3D electron microscopy. Decreases and increases in the levels of lipids such as DAG or PtdEth characterized by negative spontaneous curvature correlated with conversion to sheet structures or tubules, respectively. The effects of endogenous alterations of DAG were reversible upon exogenous delivery of lipids of negative spontaneous curvature. These data suggest that proteins require threshold amounts of such lipids and that localized deficiencies of the lipids could contribute to alterations of ER morphology. The oocyte modeling system should be beneficial to studies directed at understanding requirements of lipid species in interactions leading to alterations of organelle shaping.

Vesicular PtdIns(3,4,5)P3 and Rab7 are key effectors of sea urchin zygote nuclear membrane fusion

Regulation of nuclear envelope dynamics is an important example of the universal phenomena of membrane fusion. The signalling molecules involved in nuclear membrane fusion might also be conserved during the formation of both pronuclear and zygote nuclear envelopes in the fertilised egg. Here, we determine that class-I phosphoinositide 3-kinases (PI3Ks) are needed for in vitro nuclear envelope formation. We show that, in vivo, PtdIns(3,4,5)P₃ is transiently located in vesicles around the male pronucleus at the time of nuclear envelope formation, and around male and female pronuclei before membrane fusion. We illustrate that class-I PI3K activity is also necessary for fusion of the female and male pronuclear membranes. We demonstrate, using coincidence amplified Förster resonance energy transfer (FRET) monitored using fluorescence lifetime imaging microscopy (FLIM), a protein-lipid interaction of Rab7 GTPase and PtdIns(3,4,5)P₃ that occurs during pronuclear membrane fusion to create the zygote nuclear envelope. We present a working model, which includes several molecular steps in the pathways controlling fusion of nuclear envelope membranes.

A sea urchin cell-free system to study male pronuclear assembly and activation

Typically sperm nuclei are genetically inert and contain extremely compacted chromatin. Following fertilization, the first steps in their conversion to somatic nuclei (male pronuclei) which will support further development involve chromatin decondensation and the formation of a new nuclear envelope. We have studied the reactivation of sea urchin sperm nuclei in a cell-free system derived from homogenates of activated sea urchin egg cytoplasm. The cell-free system has provided several novel insights including requirements for sperm-specific histone phosphorylation on N- and C-terminal extensions and disassembly of the sperm nuclear lamina for decondensation, the utilization of remnant regions of the sperm nuclear envelope to direct polarized binding and fusion of egg membranes to form the new nuclear envelope, and a role for phosphoinositide metabolism in initiation of membrane fusion through binding of a minor membrane fraction enriched in PtdIns (4,5)P₂, PLCγ and SFK1 which locally produces a fusigenic lipid, diacylglycerol.

Quantification of exocytosis kinetics by DIC image analysis of cortical lawns

Cortical lawns prepared from sea urchin eggs have offered a robust in vitro system for study of regulated exocytosis and membrane fusion events since their introduction by Vacquier almost 40 years ago (Vacquier in Dev Biol 43:62-74, 1975). Lawns have been imaged by phase contrast, darkfield, differential interference contrast, and electron microscopy. Quantification of exocytosis kinetics has been achieved primarily by light scattering assays. We present simple differential interference contrast image analysis procedures for quantifying the kinetics and extent of exocytosis in cortical lawns using an open vessel that allows rapid solvent equilibration and modification. These preparations maintain the architecture of the original cortices, allow for cytological and immunocytochemical analyses, and permit quantification of variation within and between lawns. When combined, these methods can shed light on factors controlling the rate of secretion in a spatially relevant cellular context. We additionally provide a subroutine for IGOR Pro® that converts raw data from line scans of cortical lawns into kinetic profiles of exocytosis. Rapid image acquisition reveals spatial variations in time of initiation of individual granule fusion events with the plasma membrane not previously reported.

A structural role for lipids in organelle shaping

The importance of proteins in shaping the membranes that define the perimeters of organelles is well documented. By forming cross-links, motors, or scaffolds or by inserting into membranes, proteins can harness energy to deform membranes, particularly when high degrees of curvature are necessitated-as in small membrane vesicles, tubules of the endoplasmic reticulum, the edges of endoplasmic reticulum sheets or Golgi apparatus cisternae, and membrane fusion intermediates (stalks). Here we propose that membrane lipids displaying negative curvature act in concert with membrane proteins to contribute to the alteration and maintenance of bending in biological membranes. We emphasize recent data from studies of sea urchin eggs and embryos and suggest how novel approaches can lead to future directions for investigating the roles of such lipids in vivo.

Lipid quantification and structure determination of nuclear envelope precursor membranes in the sea urchin

Nuclear envelope assembly is a fundamental cellular process normally taking place once in every cell cycle in eukaryotes. The timing of fusion of nuclear membrane precursors to form the complete double membrane surrounding the chromosomes is tightly controlled, but much remains unclear concerning its regulation. Small amounts of material available and the high background of irrelevant cellular membranes have limited detailed analysis. We have employed several sensitive and high-resolution techniques to analyze the nuclear membrane structure, composition, and dynamics using purified membrane fractions and a cell-free system that results in nuclear envelope formation. We discuss the application of cholesterol and phospholipid colorimetric assays, fluorescent filipin labeling, electrospray ionization tandem mass spectrometry coupled to HPLC (HPLC-ESI/MS/MS), electron microscopy (EM), and solid-state nuclear magnetic resonance (NMR) spectroscopy. Colorimetric assays determine the amounts of inorganic phosphates from phospholipids and cholesterol/ cholesteryl esters present in membrane-containing fractions. Filipin staining of natural membranes allows the localization and relative quantification of cholesterol. HPLC-ESI/MS/MS determines the quantitative composition of membrane phospholipid species from small amounts of membranes. Cryosectioning of cryoprotected sperm cells facilitates EM verification of membrane domains existing in vivo. Deuterium solid-state NMR provides information about membrane rigidity and lipid-phase behavior. The sensitivity, quantification, and structural determinations provided by these techniques should prove useful in studying membrane dynamics in a variety of systems exhibiting membrane fusion.

Two kinase activities are sufficient for sea urchin sperm chromatin decondensation in vitro

Decondensation of compact and inactive sperm chromatin by egg cytoplasm at fertilization is necessary to convert the male germ cell chromatin to an active somatic form. We studied decondensation of sea urchin sperm nuclei in a cell-free extract of sea urchin eggs to define conditions promoting decondensation. We find that egg cytosol specifically phosphorylates two sperm-specific (Sp) histones in vitro in the same regions as in vivo. This activity is blocked by olomoucine, an inhibitor of cdc2-like kinases, but not by chelerythrine, an inhibitor of protein kinase C (PKC). PKC phosphorylates and solubilizes the sperm nuclear lamina, one requirement for decondensation. Olomoucine, which does not inhibit lamina removal, blocks sperm nuclear decondensation in the same concentration range over which it is effective in blocking Sp histone phosphorylation. In a system free of other soluble proteins, neither PKC nor cdc2 alone elicit sperm chromatin decondensation, but the two act synergistically to decondense sperm nuclei. We conclude that two kinases activities are sufficient for sea urchin male pronuclear decondensation in vitro, a lamin kinase (PKC) and a cdc2-like Sp histone kinase.

Remodeling the sperm nucleus into a male pronucleus at fertilization

After fertilization, the dormant sperm nucleus undergoes morphological and biochemical transformations leading to the development of a functional nucleus, the male pronucleus. We have investigated the formation of the male pronucleus in a cell-free system consisting of permeabilized sea urchin sperm nuclei incubated in fertilized sea urchin egg extract containing membrane vesicles. The first sperm nuclear alteration in vitro is the disassembly of the sperm nuclear lamina as a result of lamin phosphorylation mediated by egg protein kinase C. The conical sperm nucleus decondenses into a spherical pronucleus in an ATP-dependent manner. The new nuclear envelope (NE) forms by ATP-dependent binding of vesicles to chromatin and GTP-dependent fusion of vesicles to each other. Three cytoplasmic membrane vesicle fractions with distinct biochemical, chromatin-binding and fusion properties, are required for pronuclear envelope assembly. Binding of each fraction to chromatin requires two detergent-resistant lipophilic structures at each pole of the sperm nucleus, which are incorporated into the NE by membrane fusion. Targeting of the bulk of NE vesicles to chromatin is mediated by a lamin B receptor (LBR)-like integral membrane protein. The last step of male pronuclear formation involves nuclear swelling. Nuclear swelling is associated with import of soluble lamin B into the nucleus and growth of the nuclear envelope by fusion of additional vesicles. In the nucleus, lamin B associates with LBR, which apparently tethers the NE to the lamina. Thus male pronuclear envelope assembly in vitro involves a highly ordered series of events. These events are similar to those characterizing the remodeling of somatic and embryonic nuclei transplanted into oocytes. The relationship between sperm nuclear remodeling at fertilization and nuclear remodeling after nuclear transplantation is discussed.

Nuclear envelope dynamics during male pronuclear development

Upon fertilization, the sperm nucleus undergoes reactivation. The poreless sperm nuclear envelope is replaced by a functional male pronuclear envelope and the highly compact male chromatin decondenses. Here some recent evidence is examined: that disassembly of the sperm lamina is required for chromatin decondensation, that remnant portions of the sperm nuclear envelope target the binding of egg membrane vesicles that form the male pronuclear envelope, that functional male pronuclear envelopes containing lamin B receptor assemble prior to lamin import and lamina formation, and that lamina assembly drives male pronuclear swelling. Several unresolved issues are discussed.

Targeting of membranes to sea urchin sperm chromatin is mediated by a lamin B receptor-like integral membrane protein

We have identified an integral membrane protein of sea urchin gametes with an apparent molecular mass of 56 kD that cross-reacts with an antibody against the nucleoplasmic NH2-terminal domain of human lamin B receptor (LBR). In mature sperm, p56 is located at the tip and base of the nucleus from where it is removed by egg cytosol in vitro. In the egg, p56 is present in a subset of cytoplasmic membranes (MV2 beta) which contributes the bulk of the nuclear envelope during male pronuclear formation. p56-containing vesicles are required for nuclear envelope assembly and have a chromatin-binding capacity that is mediated by p56. Lamin B is not present in these vesicles and is imported into the nucleus from a soluble pool at a later stage of pronuclear formation. Lamin B incorporation and addition of new membranes are necessary for pronuclear swelling and nuclear envelope growth. We suggest that p56 is a sea urchin LBR homologue that targets membranes to chromatin and later anchors the membrane to the lamina.

Distinct egg membrane vesicles differing in binding and fusion properties contribute to sea urchin male pronuclear envelopes formed in vitro

We have identified three distinct membrane vesicle populations from sea urchin egg cytoplasm that cooperate in assembling the male pronuclear envelope in vitro. Membranes from sea urchin egg homogenates were separated by buoyant density into five vesicle fractions, three of which bind to demembranated sperm nuclei. Each requires a membranous element (lipophilic structure) derived from the sperm nuclear envelope at the tip and base (poles) of the nucleus in order to bind. Binding is differentially sensitive to protease, high salt and N-ethyl maleimide treatment of the membrane vesicles. MV1 binds at the poles and is required for fusion of the membrane vesicle fractions to each other and to the lipophilic structures. MV2 beta binds over the entire chromatin surface and is enriched in an endoplasmic reticulum marker enzyme. MV2 alpha binds at the nuclear poles, is enriched in a Golgi enzyme marker and is required for fusion of MV2 beta. All three fractions are required for nuclear envelope formation in vitro. The results suggest a multistep process for nuclear envelope formation involving contributions from both sperm and egg, roles for both endoplasmic reticulum and non-endoplasmic reticulum-derived vesicles, and the localization of a critical element of the fusion machinery in MV1.

Localization and characterization of blastocoelic extracellular matrix antigens in early sea urchin embryos and evidence for their proteolytic modification during gastrulation

Previously, results were presented showing a spatiotemporal expression of matrix metalloproteases consistent with a role in remodeling the blastocoelic extracellular matrix (bECM) of the gastrulating sea urchin embryo [35]. In the present work, we provide evidence suggesting that the bECM is in fact the substrate for developmentally regulated proteolysis. Monoclonal antibody (mAb) LG11C7 was generated against testicular tissue of the sea urchin, Strongylocentrotus purpuratus, and recognizes extracellular matrix antigens overlying the perivisceral epithelium. Indirect immunofluorescence microscopy shows that mAb LG11C7 cross-reacts with components of the basal lamina lining the blastocoeles of early embryos and Western immunoblots of detergent extracts indicate that it recognizes gastrula-stage antigens with M(r)s of 158, 68, and 37 kDa. Glycosidase treatments reveal that the embryonal antigens contain multiple N-linked oligosaccharides. Developmental studies employing immunoprecipitations and Western blot analyses of staged embryonal detergent extracts show that the 68-kDa antigen appears between 18 and 24 h after fertilization and is accompanied by a substantial increase in the 37-kDa antigen. Thus, the appearances of the 68- and 37-kDa antigens occur during the blastula-gastrula transition, and their spatiotemporal expression is similar to that of the matrix metalloproteases reported previously. The appearance of the 68-kDa antigen and the increase in the 37-kDa antigen may be blocked by exposing the embryos to the metalloprotease inhibitor 1,10-phenanthroline, which also blocks gastrulation reversibly. These results suggest (1) that the 68- and 37-kDa antigens are products of developmentally regulated proteolysis of a basal laminar glycoprotein, and (2) that this proteolysis is required for the cell-cell/cell-matrix interactions and morphogenetic movements associated with normal gastrulation in the sea urchin embryo.

Lipophilic organizing structures of sperm nuclei target membrane vesicle binding and are incorporated into the nuclear envelope

We report the existence of lipophilic structures (LS's) associated with demembranated sperm nuclei which together act as a pronuclear envelope organizing center. These structures can be visualized as objects which stain with lipophilic dyes but are resistant to solubilization in 0.1% of the non-ionic detergent Triton X (TX)-100. The structures, located at the acrosomal and centriolar poles of the sea urchin sperm nucleus, initiate ATP-dependent membrane vesicle binding in a fertilized egg extract. The lipophilic material in LS's is incorporated into the nuclear envelope during GTP-induced membrane fusion. Removal of the LS's from sperm nuclei with 1% TX-100 abolishes membrane vesicle binding to the nuclei. LS's recovered from supernatants of extracted nuclei can be reconstituted to their original locations on the stripped nuclei. Rebinding of isolated LS's occurs preferentially at the acrosomal pole. Such reconstituted nuclei direct membrane vesicle binding only to the acrosomal pole region and result in incomplete nuclear envelope assembly following membrane fusion. Binding of LS's and subsequently membrane vesicles to both nuclear poles allows complete nuclear envelope formation.

Localization of bindin expression during sea urchin spermatogenesis

Expression of the bindin gene was examined in testicular cells of the sea urchin Strongylocentrotus purpuratus. In situ hybridization studies, using an 35S-labeled antisense RNA probe transcribed from a bindin cDNA, reveal that bindin mRNAs are localized in spermatogenic cells displaced towards the lumens of maturing testicular acini. Little or no hybridization is observed in spermatogenic cells displaced towards the perivisceral epithelium or in somatic cells of the testis. A similar localization of the bindin protein itself is observed using a rhodamine-conjugated polyclonal antibody against bindin, which shows a punctate immunofluorescence pattern in late spermatogenic cells. Immunogold labeling of ultrathin sections and electron microscopy reveal that this punctate immunofluorescence is an apparent result of localized deposits of bindin in intracellular vesicles. Through the terminal stages of spermatogenesis, these bindin-containing vesicles apparently fuse to form the single acrosomal vesicle of the mature spermatozoon. These results indicate 1) that bindin mRNAs are transcribed relatively late in spermatogenesis, 2) that bindin is translated soon after production of its mRNA, 3) that bindin quickly associates with intracellular vesicles during or soon after its synthesis, and 4) that these vesicles fuse to form the single acrosomal vesicle during the terminal stage of spermatogenesis.

6DMAP inhibits chromatin decondensation but not sperm histone kinase in sea urchin male pronuclei

Treatment of sea urchin eggs for 10 min prior to fertilization with the kinase inhibitor 6DMAP (6-dimethylaminopurine) reversibly inhibits swelling and loss of conical morphology of the male pronucleus. Male pronuclei inhibited with 1 mM 6DMAP for 25 min undergo phosphorylation of Sp H1 and Sp H2B histones as fully as do control nuclei. Therefore, Sp histone kinase, whose target sequences resemble those of the M-phase histone kinase, is not inhibited by 6DMAP, and Sp histone phosphorylation, although it may be necessary, is not sufficient for chromatin decondensation.

Histone gene expression during sea urchin spermatogenesis: an in situ hybridization study

The expression of testis-specific and adult somatic histone genes in sea urchin testis was investigated by in situ hybridization. The testis-specific histone genes (Sp H2B-1 of Strongylocentrotus purpuratus and Sp H2B-2 of Lytechinus pictus) were expressed exclusively in a subset of male germ line cells. These cells are morphologically identical to replicating cells pulse-labelled with 3H-thymidine. Genes coding for histones expressed in adult somatic and late embryo cells (H2A-beta for S. purpuratus and H3-1 for L. pictus) were expressed in the same germ line cells, as well as in the supportive cells (nutritive phagocytes) of the gonad. All histone mRNAs detected in the male germ lineage declined precipitously by the early spermatid stage, before cytoplasmic reduction. The data suggest that both testis-specific and adult somatic histone genes are expressed in proliferating male germ line cells. Testis-specific gene expression is restricted to spermatogonia and premeiotic spermatids, but somatic histone expression is not. The decline of histone mRNA in nondividing spermatids is not merely a consequence of cytoplasmic shedding, but probably reflects mRNA turnover.

In vitro differentiation of male germ line cells from sea urchin testis

Fragments of sea urchin testicular tissue were cultured at 15 degrees C in serum-supplemented seawater for 5 weeks. Cells whose DNA had been pulse-labelled with 3H-thymidine at the beginning of the experiment were followed by autoradiography, and counts were made of the proportions of each male germ-line cell type from sections of tissue at several time points. Differentiation of spermatogonia to mid-spermatids occurred within the first 10 days, with a decline of about 40% in the total number of spermatogonia and an increase of 250% in the number of mid-spermatids. Thereafter, no changes occurred in the proportions of germ-line cells, although tissue integrity was maintained throughout. The results indicate that sea urchin male germ-line cells can complete meiosis and the first stages of spermiogenesis including nuclear condensation and cytoplasmic reduction in culture with kinetics similar to those in vivo. The system should permit analysis of factors responsible for male germ-cell differentiation.

Changing localizations of site-specific surface antigens during sea urchin spermiogenesis

Whole mount preparations of dissociated testicular cells from the sea urchin, Strongylocentrotus purpuratus, were exposed to monoclonal antibodies (mAbs) directed against sperm surface proteins. Indirect immunofluorescence microscopy and Western immunoblot analysis show that mAb J18/29 binds to the entire surface of the mature spermatozoon and membrane proteins ranging in relative molecular masses from 25 to 340 kDa. MAb J18/2 binds to the acrosomal and tail regions of the mature spermatozoon and mainly to a 210-kDa membrane protein. MAb J17/30 binds to the midpiece and tail regions and monospecifically to a 60-kDa membrane protein. MAb J16/33 binds specifically to the sperm midpiece but does not bind to Western immunoblots of sperm membrane proteins. With the exception of J16/33, which shows a punctate binding pattern, all of these mAbs show uniform binding over the entire surface of the early spermatid. This uniform and complete surface binding is observed through all stages of spermiogenesis for mAb J18/29. By the midspermatid stage, when tail formation first begins, but before the nucleus condenses and the cytoplasm decreases in volume, localized binding patterns of mAbs J17/30 and J16/33 become evident. Localized binding of mAb J18/2 is not observed until the late spermatid stage. These results show that the sea urchin sperm surface is composed of at least four different domains and provide the first insight into differentiation of the cell surface during sea urchin spermatogenesis.

Sea urchin testicular cells evaluated by fluorescence microscopy of unfixed tissue

A procedure is presented for rapid, quantitative evaluation of cell and nuclear types present in the male gonad of the sea urchin. Vitally stained whole mounts of tissue fragments or dissociated cells are prepared, which reveal detailed 3-dimensional chromatin patterns and enough cytoplasmic features to provide reliable markers for most of the somatic and germ line cell types. Representative cellular morphologies are described. Nuclear volume changes during spermatogenesis are quantified. Spermatid nuclei contain an apparently interconnected network of heterochromatin. Regions relatively devoid of chromatin decrease in size as nuclear condensation proceeds and spherical nuclear shape is maintained. The major decrease in nuclear volume occurs prior to the late spermatid stage. The volume of the spermatozoan nucleus is achieved by the smallest late spermatid nucleus before the change from spherical to conoid morphology. The relationship of this morphological transition to sperm histone dephosphorylation is discussed.

RNA synthesis in male pronuclei of the sea urchin

Transcription in male pronuclei of fertilized sea urchin eggs was measured by comparison of [3H]uridine incorporation into RNA in polyspermic, monospermic and activated eggs under conditions where uptake of the isotope and conversion to UTP were equivalent. RNA accumulation from male pronuclei begins by S phase of the first cell cycle. Initiation of this RNA synthesis does not require DNA synthesis. A major fraction of the newly synthesized transcripts are mRNAs coding for early embryo (alpha-) histones. In addition, several other unidentified transcripts are detected by gel electrophoresis. The pattern of RNA transcription remains constant for at least 4 h post-fertilization. These results demonstrate that specific transcription of male pronuclear sequences is activated in the first cell cycle following fertilization.

Remodeling of sperm chromatin following fertilization: nucleosome repeat length and histone variant transitions in the absence of DNA synthesis

Within the first cell cycle following fertilization the average nucleosomal repeat length of sea urchin male pronuclear chromatin declines by 30-40 base pairs to a value typical of that found in the embryo. This decline occurs after a lag of about 30 min postfertilization, and is accompanied by replication of the male chromatin and accumulation of cleavage-stage (CS) core histone variants. When replication is inhibited by greater than 95% with aphidicolin, the decline in repeat length still occurs, although it is slightly retarded. The decline in repeat length also occurs when protein synthesis is blocked by greater than 98% and DNA synthesis by 60-70% with emetine. The adjustment of nucleosome repeat length therefore can occur in vivo without extensive movement of replication forks across the length of the chromatin, or normal progression of the cell cycle, and appears to require no proteins synthesized postfertilization. Blocking of DNA synthesis or protein synthesis also does not prevent the normal histone variant transitions involved in male pronuclear chromatin remodeling. Although their accumulation is slowed, CS core variants eventually become the predominant male pronuclear histones in their classes when replication is inhibited. Since a shortening of the average nucleosomal repeat length of approximately 10-20% is not sufficient to account for this large acquisition of CS variants, some of the sperm (Sp) core histones are probably displaced from the replication-blocked pronucleus. Therefore, accumulation of CS H2A and CS H2B are temporally correlated with the repeat length transition, whereas replication, normal progression of the cell cycle, and the early histone transitions involving SpH1 and SpH2B are not.

Translational regulation of histone synthesis in the sea urchin strongylocentrotus purpuratus

The pattern and schedule of histone synthesis in unfertilized eggs and early embryos of the sea urchin Strongylocentrotus purpuratus were studied using two-dimensional gel electrophoresis. After fertilization there is an abrupt change in the pattern of histone variant synthesis. Although both cleavage-stage (CS) variants. However, after fertilization, both CS and alpha messages are translated. Since alpha histone mRNA isolated from unfertilized eggs can be translated in vitro, the synthesis of alpha histone subtypes appears to be under translational control. Although the synthesis of alpha subtypes is shown here to occur before the second S phase after fertilization, little or no alpha histone is incorporated into chromatin at this time. Thus, early chromatin is composed predominantly of CS variants probably recruited for the most part from the large pool of CS histones stored in the unfertilized egg.

Electrophoretic analysis of the stored histone pool in unfertilized sea urchin eggs: quantification and identification by antibody binding

A maternal store of histones in unfertilized sea urchin eggs is demonstrated by two independent criteria. Stored histones are identified by their ability to assemble into chromatin of male pronuclei of fertilized sea urchin eggs in the absence of protein synthesis, suggesting a minimum of at least 25 haploid equivalents for each histone present and functional in the unfertilized egg. In addition, electrophoretic analysis of proteins from acid extracts of unfertilized whole eggs and enucleated merogons reveals protein spots comigrating with cleavage stage histone standards, though not with other histone variants found in later sea urchin development or in sperm. Quantification of the amount of protein per histone spot yields an estimate of several hundred haploid DNA equivalents per egg of stored histone. The identity of some of the putative histones was verified by a highly sensitive immunological technique, involving electrophoretic transfer of proteins from the two-dimensional polyacrylamide gels to nitrocellulose filters. Proteins in amounts less than 2 x 10(-4) micrograms can be detected by this method.

Alterations in chromatin structure during early sea urchin embryogenesis

Sea urchin sperm before fertilization possess the longest nucleosome repeat length yet determined for any chromatin. By the time the fertilized egg gives rise to a blastula or gastrula embryo, the chromatin has a considerably shorter repeat length and, in addition, a sequence of different histone variants of H1, H2A, and H2B has appeared. We have investigated the relationship between these variations in histone composition and concomitant alterations in chromatin structure during the earliest stages of embryogenesis in two species of sea urchin. In contrast to the long repeat distance in sperm, chromatin loaded with cleavage stage histones has a much smaller repeat. Later stages containing predominantly alpha histones display an intermediate spacing. More detailed analysis of the events in the first cell cycle was carried out with polyspermically fertilized eggs. During the first 30 min after fertilization, in which sperm-specific H1 is completely replaced by cleavage-stage H1, the male pronuclear repeat remains unchanged. The decrease toward the repeat length of cleavage stages begins at about the time of DNA synthesis. Higher degrees of polyspermy extend the length of the cell cycle, including the duration of S phase and the length of time to reach the first chromosome condensation. At these higher degrees of polyspermy, the decrease in repeat length is also slowed. We conclude that the adjustment of the arrangement of nucleosomes in embryonic chromatin from that found in sperm can occur within the first cell cycle and that its timing is cell-cycle dependent. The adjustment is separable from a corresponding change in H1 composition.