The biologically active form of the sea urchin egg receptor for sperm is a disulfide-bonded homo-multimer

The Effect of Synthetic Polyamine BPA-C8 on the Fertilization Process of Intact and Denuded Sea Urchin Eggs

Sea urchin eggs are covered with layers of extracellular matrix, namely, the vitelline layer (VL) and jelly coat (JC). It has been shown that sea urchin eggs' JC components serve as chemoattractants or ligands for the receptor on the fertilizing sperm to promote the acrosome reaction. Moreover, the egg's VL provides receptors for conspecific sperm to bind, and, to date, at least two sperm receptors have been identified on the surface of sea urchin eggs. Interestingly, however, according to our previous work, denuded sea urchin eggs devoid of the JC and VL do not fail to become fertilized by sperm. Instead, they are bound and penetratedby multiple sperm, raising the possibility that an alternative pathway independent of the VL-residing sperm receptor may be at work. In this research, we studied the roles of the JC and VL using intact and denuded eggs and the synthetic polyamine BPA-C8. BPA-C8 is known to bind to the negatively charged macromolecular complexes in the cells, such as the JC, VL, and the plasma membrane of echinoderm eggs, as well as to the actin filaments in fibroblasts. Our results showed that, when added to seawater, BPA-C8 significantly repressed the Ca2+ wave in the intact P. lividus eggs at fertilization. In eggs deprived of the VL and JC, BPA-C8 binds to the plasma membrane and increases fibrous structures connecting microvilli, thereby allowing the denuded eggs to revert towards monospermy at fertilization. However, the reduced Ca2+ signal in denuded eggs was nullified compared to the intact eggs because removing the JC and VL already decreased the Ca2+ wave. BPA-C8 does not cross the VL and the cell membrane of unfertilized sea urchin eggs to diffuse into the cytoplasm at variance with the fibroblasts. Indeed, the jasplakinolide-induced polymerization of subplasmalemmal actin filaments was inhibited in the eggs microinjected with BPA-C8, but not in the ones bath-incubated with the same dose of BPA-C8.

Effects of Dithiothreitol on Fertilization and Early Development in Sea Urchin

The vitelline layer (VL) of a sea urchin egg is an intricate meshwork of glycoproteins that intimately ensheathes the plasma membrane. During fertilization, the VL plays important roles. Firstly, the receptors for sperm reside on the VL. Secondly, following cortical granule exocytosis, the VL is elevated and transformed into the fertilization envelope (FE), owing to the assembly and crosslinking of the extruded materials. As these two crucial stages involve the VL, its alteration was expected to affect the fertilization process. In the present study, we addressed this question by mildly treating the eggs with a reducing agent, dithiothreitol (DTT). A brief pretreatment with DTT resulted in partial disruption of the VL, as judged by electron microscopy and by a novel fluorescent polyamine probe that selectively labelled the VL. The DTT-pretreated eggs did not elevate the FE but were mostly monospermic at fertilization. These eggs also manifested certain anomalies at fertilization: (i) compromised Ca2+ signaling, (ii) blocked translocation of cortical actin filaments, and (iii) impaired cleavage. Some of these phenotypic changes were reversed by restoring the DTT-exposed eggs in normal seawater prior to fertilization. Our findings suggest that the FE is not the decisive factor preventing polyspermy and that the integrity of the VL is nonetheless crucial to the egg's fertilization response.

Species-specific mechanisms during fertilization

The perpetuation and preservation of distinct species rely on mechanisms that ensure that only interactions between gametes of the same species can give rise to viable and fertile offspring. Species-specificity can act at various stages, ranging from physical/behavioral pre-copulatory mechanisms, to pre-zygotic incompatibility during fertilization, to post-zygotic hybrid incompatibility. Herein, we focus on our current knowledge of the molecular mechanisms responsible for species-specificity during fertilization. While still poorly understood, decades of research have led to the discovery of molecules implicated in species-specific gamete interactions, starting from initial sperm-egg attraction to the binding of sperm and egg. While many of these molecules have been described as species-specific in their mode of action, relatively few have been demonstrated as such with definitive evidence. Thus, we also raise remaining questions that need to be addressed in order to characterize gamete interaction molecules as species-specific.

Nuclear and mitochondrial compartmentation of oxidative stress and redox signaling

New methods to measure thiol oxidation show that redox compartmentation functions as a mechanism for specificity in redox signaling and oxidative stress. Redox Western analysis and redox-sensitive green fluorescent proteins provide means to quantify thiol/disulfide redox changes in specific subcellular compartments. Analyses using these techniques show that the relative redox states from most reducing to most oxidizing are mitochondria > nuclei > cytoplasm > endoplasmic reticulum > extracellular space. Mitochondrial thiols are an important target of oxidant-induced apoptosis and necrosis and are especially vulnerable to oxidation because of the relatively alkaline pH. Maintenance of a relatively reduced nuclear redox state is critical for transcription factor binding in transcriptional activation in response to oxidative stress. The new methods are applicable to a broad range of experimental systems and their use will provide improved understanding of the pharmacologic and toxicologic actions of drugs and toxicants.

Cell signalling during sea urchin development: a model for assessing toxicity of environmental contaminants

The early development of sea urchins has been thoroughly studied since the beginning of the 20th century thanks to the particular features of the model involving cell signalling, making it easy to follow the complex cell-to-cell interactions that lead to development. In this chapter, the prominent role of cell-to-cell communication in developmental events is discussed, as well as the role of intracellular ion changes that are in turn regulated by signal molecules belonging to the cholinergic system. The results seem to indicate that the zygote stage is the most suitable to study the role of the cholinergic system, as at this stage, a calcium spike can be evoked by exposure to acetylcholine (ACh) or to muscarinic drugs, at any time before the nuclear breakdown. The described outcomes also open a path to a new way of considering biomarkers. In fact, most environmental factors have the capacity to interfere with the cholinergic system: stress, wounds, inflammation and pollution in general. In particular, this offers a way to investigate the presence in the environment and the degree of aggressiveness of neurotoxic contaminants, such as organophosphate and carbamate pesticides, largely used in European countries for many purposes, including agricultural pest control and medical treatment. These drugs exert their function by interfering with the regulation of the cholinergic system and the consequent electrical events. Thus, the sea urchin zygote could represent a reliable model to be used in biosensors with the capacity to translate the effect of neurotoxic pesticides, and generally of stress-inducing contaminants, in living cell responses, such as electrical responses.

Identification of the sea urchin 350-kDa sperm-binding protein as a new sialic acid-binding lectin that belongs to the heat shock protein 110 family: implication of its binding to gangliosides in sperm lipid rafts in fertilization

The 350-kDa sperm-binding protein (SBP), a species-specific sperm-binding protein, is localized in the vitelline layer of sea urchin eggs. In this study, we have shown for the first time that sperm gangliosides are ligands for the intact glycosylated SBP. Using recombinant fragments of the SBP, the N-terminal heat shock protein 110-like domain was shown to be responsible for the binding. The intact SBP could bind various gangliosides, and the binding was sialidase-sensitive and inhibited by sialyllactose, thus indicating that it is the sialic acid-binding protein. Calcium and magnesium ions were not required but they did enhance the binding activity of SBP. The observation that bacterially expressed recombinant SBP and the sialidase-treated intact glycosylated SBP lost divalent cation-dependent enhancement of binding activity suggests that the sialylated carbohydrate moieties of the SBP may be involved in this property. Furthermore, the SBP was shown to bind sperm lipid rafts, in which gangliosides are enriched, and this binding was lost upon sialidase treatment of the lipid rafts. Finally, liposomes containing the ganglioside specifically inhibited fertilization. Taken together, these results allow us to identify SBP as a member of a new class of sialic acid-binding lectin belonging to the Hsp110 family, and indicate that SBP may be involved in interaction of sperm with the vitelline layer of the egg.

Vitelline envelope of Bufo arenarum: biochemical and biological characterization

Vitelline envelopes (VEs) of Bufo arenarum were isolated in order to study their composition and their role in fertilization. VEs are composed of four glycoproteins, with molecular masses of 120, 75, 41, and 38 kDa. To characterize its biological properties, we quantitatively determined sperm-VE binding and the induction of the acrosome reaction. Heterologous binding of B. arenarum sperm to Xenopus laevis VE components was observed with about one-third the efficiency of homologous binding. Equivalent binding of X. laevis sperm to the B. arenarum VE was observed. When B. arenarum sperm were incubated with fluorescein isothiocyanate-labeled VE, the labeled glycoproteins bound to the anterior end of the sperm head, showing a lateral distribution. Induction of the acrosome reaction was evaluated by incubating sperm in hypotonic saline media with VE glycoproteins. VEs induced the acrosome reaction in a time- and concentration-dependent manner. The acrosome reaction was maximal after 10 min. The half-maximal effect was obtained at a glycoprotein concentration of 1 microg/ml. Specificity was determined using fertilization envelope glycoproteins, which failed to induce the acrosome reaction. The B. arenarum VE is biochemically similar to other egg envelopes. It also seems that its biological properties are similar to other species in regard to sperm binding and induction of the acrosome reaction. However, as far as we are aware, this is the first observation of the VE inducing the sperm acrosome reaction in amphibians. The relatively small differences observed in heterologous sperm-VE binding in X. laevis and B. arenarum are inconsistent with the current paradigm that species specificity in fertilization is regulated at the sperm-VE binding step.

The biology of cortical granules

An egg-that took weeks to months to make in the adult-can be extraordinarily transformed within minutes during its fertilization. This review will focus on the molecular biology of the specialized secretory vesicles of fertilization, the cortical granules. We will discuss their role in the fertilization process, their contents, how they are made, and the molecular mechanisms that regulate their secretion at fertilization. This population of secretory vesicles has inherent interest for our understanding of the fertilization process. In addition, they have import because they enhance our understanding of the basic processes of secretory vesicle construction and regulation, since oocytes across species utilize this vesicle type. Here, we examine diverse animals in a comparative approach to help us understand how these vesicles function throughout phylogeny and to establish conserved themes of function.

Disulfide bond structure of the atrial natriuretic peptide receptor extracellular domain: conserved disulfide bonds among guanylate cyclase-coupled receptors

The disulfide bond structure of the extracellular domain of rat atrial natriuretic peptide (ANP) receptor (NPR-ECD) has been determined by mass spectrometry (MS) and Edman sequencing. Recombinant NPR-ECD expressed in COS-1 cells and purified from the culture medium binds ANP with as high affinity as the natural ANP receptor. Reaction with iodoacetic acid yielded no S-carboxymethylcysteine, indicating that all six Cys residues in NPR-ECD are involved in disulfide bonds. Electrospray ionization MS of NPR-ECD deglycosylated by peptide-N-glycosidase F gave a molecular mass of 48377.5+/-1.6 Da, which was consistent with the presence of three disulfide bonds. Liquid chromatography MS analysis of a lysylendopeptidase digest yielded three cystine-containing fragments with disulfide bonds Cys(60)-Cys(86), Cys(164)-Cys(213) and Cys(423)-Cys(432) based on their observed masses. These bonds were confirmed by Edman sequencing of each of the three fragments. No evidence for an inter-molecular disulfide bond was found. The six Cys residues in NPR-ECD, forming a 1-2, 3-4, 5-6 disulfide pairing pattern, are strictly conserved among A-type natriuretic peptide receptors and are similar in B-type receptors. We found that in other families of guanylate cyclase-coupled receptors, the Cys residues involved in 1-2 and 5-6 disulfide pairs are conserved in nearly all, suggesting an important contribution of these disulfide bonds to the receptor's structure and function.

Oligomerization of the sarcoplasmic reticulum Ca2+-ATPase SERCA2 in cardiac muscle

The slow/cardiac isoform of the sarcoplasmic reticulum Ca2+-ATPase plays an important role in cardiac muscle Ca2+-homeostasis. To determine the native configuration of the SERCA2 ion pump, a chemical cross-linking analysis of heart microsomes was performed. Using one- and two-dimensional immunoblotting following incubation with the hydrophilic probe bis-sulfosuccinimidyl suberate or the hydrophobic crosslinker dithiobis-succinimidyl-propionate, we demonstrate here that SERCA2 forms high-molecular-mass aggregates. In contrast to the Na+/Ca2+-exchanger, Ca2+-ATPase clusters can be stabilized by hydrophilic 1.2 nm crosslinkers and are sensitive to chemical reduction. Hence, the native form of this important Ca2+-regulatory membrane protein involved in cardiac muscle relaxation appears not to exist as a monomeric ion pump unit. Protein-protein interactions might play an important role in the physiological functioning of this Ca2+-ATPase isoform, as has previously been shown for skeletal muscle Ca2+-pumps, Ca2+-binding proteins and Ca2+-channels.

The 350-kDa sea urchin egg receptor for sperm is localized in the vitelline layer

Previous studies have established by several methods that the 350-kDa egg receptor for sperm is localized on the plasma membrane-vitelline layer complex of the egg of the sea urchin Strongylocentrotus purpuratus. In addition, it has been found that molecules which are cross-reactive with anti-receptor antibody are present in the cortical granules located at the inner face of the plasma membrane. The objective of this study was to define more precisely the locale of the cell surface receptor. We have found that following fertilization, the immunoreactive receptor initially found on the egg surface moved to the fertilization envelope (FE) and then disappeared in parallel with the loss of sperm binding activity. A cross-linked, high-molecular-weight derivative of soybean trypsin inhibitor (hMW-SBTI) which was unable to pass through the elevating FE blocked the loss of both immunoreactivity and the sperm binding activity of the FE, but did not inhibit the vitelline delaminase activity that has been implicated in FE formation. Western blot analysis following SDS-PAGE of the proteins of the FE isolated in the presence of hMW-SBTI and benzamidine revealed the presence of the 350/320-kDa proteins which cross-reacted with anti-receptor antibody. Experiments in which molecules on the surface of unfertilized eggs were labeled with biotin and traced after FE formation revealed that a significant portion of the 350/320-kDa glycoproteins that were incorporated into the FE originated from the cell surface, rather than from the cortical granules. These findings provide strong evidence that in unfertilized eggs the egg receptor for sperm exists as part of the protein complex known as the vitelline layer which serves as a precursor of the FE. Evidence is presented indicating that some of the receptor in the vitelline layer is cryptic and a possible function for this cryptic form of the receptor is proposed.

Identification of a new sea urchin vitelline envelope sperm binding glycoprotein

The sea urchin egg vitelline envelope (VE) is composed of eight major glycopolypeptides that are heavily mannosylated and contain fucose and N-acetylglucosamine moieties based on lectin staining. In the present study, the macromolecular composition of the VE and the potential role of a purified VE glycoprotein in initial gamete binding was investigated. The VE components were solubilized from the surface of intact, dejellied eggs with dithiothreitol in divalent cation-free seawater, and analyzed using native, reduced electrophoresis and immunoblotting. Three major VE glycoproteins, VE-A, VE-B and VE-C, and one minor component, VE-D, were identified with antisera against whole VE preparations and against glutaraldehyde-fixed, unfertilized eggs. The electrophoretically purified glycoproteins resolved into a common subunit doublet and one unique subunit each of decreasing size on blots of sodium dodecylsulfate polyacrylamide gels. Lectin affinity chromatography was used for analysis and purification of reduced VE components; a glycoprotein eluted from Con A columns with methyl-mannoside comigrated with VE-B when analyzed by immunoblotting. Whole VE preparations and VE-B obtained from Con A columns were found to inhibit fertilization when preincubated with sperm, thus directly establishing a role for VE-B in gamete binding.

Mapping sperm binding domains on the sea urchin egg receptor for sperm

The receptor on the surface of the egg of the sea urchin Strongylocentrotus purpuratus that mediates species-specific binding of sperm is a 350-kDa cell surface glycoprotein. Earlier studies established that a recombinant protein encompassing a major portion of the N-terminal half of the receptor inhibited fertilization when tested in a competitive fertilization bioassay. To identify in more detail the sites in this domain of the receptor that are involved in binding sperm, a series of deletion constructs were expressed as glutathione S-transferase fusion proteins and tested for inhibitory activity in a fertilization bioassay. In addition, a novel assay for directly testing the sperm binding activity of these proteins was developed. In this assay we quantitated sperm binding to recombinant proteins representing various domains of the receptor immobilized on glutathione agarose beads. Using this new assay, two domains in the N-terminal half of the receptor were found to be involved in sperm binding. One of the peptide domains, composed of 247 amino acids, binds both the sperm of S. purpuratus and the sperm of another genus of sea urchin, Lytechinus pictus. In contrast, binding to the second domain consisting of a 32-amino-acid residue peptide was found to be genus specific; no binding of L. pictus sperm was observed. A working model is proposed incorporating these findings with earlier studies on the function of the oligosaccharide chains of the receptor. In this model it is postulated that the sperm initially interact with the nonspecific binding domain on the polypeptide and the sulfated O-linked oligosaccharide chains of the receptor. This interaction is followed by binding to the more specific polypeptide binding site on the receptor. It is proposed that only subsequent to binding at this second site can gamete fusion occur.

Sperm-egg interaction in the painted frog (Discoglossus pictus): an ultrastructural study

The ultrastructure of sperm changes and penetration in the egg was studied in the anuran Discoglossus pictus, whose sperm have an acrosome cap with a typical tip, the apical rod. The first stage of the sperm apical rod and acrosome reaction (AR) consists in vesiculation between the plasma membrane and the outer acrosome membrane. The two components of the acrosome cap are released in sequence. The innermost component (component B) is dispersed first. The next acrosome change is the dispersal of the outermost acrosome content (component A). At 30 sec postinsemination, when the loss of component B is first observed, holes are seen in the innermost jelly coat (J1), surrounding the penetrating sperm. Therefore, this acrosome constituent might be related to penetration through the innermost egg investments. At 1 min postinsemination, during sperm penetration into the egg, a halo of finely granular material is observed around the inner acrosome membrane of the spermatozoon, suggesting a role for component A at this stage of penetration. Gamete-binding and fusion take place between D1 (the egg-specific site for sperm interaction) and the perpendicularly oriented sperm. Spermatozoa visualized at their initial interaction (15 sec postinsemination) with the oolemma are undergoing vesiculation. The first interaction is likely to occur between the D1 glycocalyx and the plasma membrane of the hybrid vesicles surrounding the apical rod. As fusion is observed between the internal acrosome membrane and the oolemma, it can be postulated that gametic interaction might be followed by fusion of the latter with the apical rod internal membrane that extends posteriorly into the inner acrosome membrane. Insemination of the outermost jelly layer (J3) dissected out of the egg, and observations of the ultrastructural changes of spermatozoa in this coat, indicate that J3 rather than the vitelline coat (VC) induces the AR. Interestingly, at the late postinsemination stage, VC fibrils are seen crosslinking the inner acrosome membrane. The role of this binding is here discussed.

Cross-linking analysis of the ryanodine receptor and alpha1-dihydropyridine receptor in rabbit skeletal muscle triads

In mature skeletal muscle, excitation-contraction (EC) coupling is thought to be mediated by direct physical interactions between the transverse tubular, voltage-sensing dihydropyridine receptor (DHPR) and the ryanodine receptor (RyR) Ca2+ release channel of the sarcoplasmic reticulum (SR). Although previous attempts at demonstrating interactions between purified RyR and alpha1-DHPR have failed, the cross-linking analysis shown here indicates low-level complex formation between the SR RyR and the junctional alpha1-DHPR. After cross-linking of membranes highly enriched in triads with dithiobis-succinimidyl propionate, distinct complexes of more than 3000 kDa were detected. This agrees with numerous physiological and electron-microscopic findings, as well as co-immunoprecipitation experiments with triad receptors and receptor domain-binding studies. However, a distinct overlap of immunoreactivity between receptors was not observed in crude microsomal preparations, indicating that the triad complex is probably of low abundance. Disulphide-bonded, high-molecular-mass clusters of triadin, the junctional protein proposed to mediate interactions in triads, were confirmed to be linked to the RyR. Calsequestrin and the SR Ca2+-ATPase were not found in cross-linked complexes of the RyR and alpha1-DHPR. Thus, whereas recent studies indicate that the two receptors exhibit temporal differences in their developmental inductions and that receptor interactions are not essential for the formation and maintenance of triads, this study supports the signal transduction hypothesis of direct physical interactions between triad receptors in adult skeletal muscle.

Reexamination of the sequence of the sea urchin egg receptor for sperm: implications with respect to its properties

A reexamination of the cDNA clones of the Strongylocentrotus purpuratus sea urchin egg receptor for sperm resulted in several important changes to the sequence. By using both rapid amplification of cDNA ends (RACE) and Northern blot analysis for confirmation, the corrected deduced amino acid sequence was shown to lack a classical signal peptide. In addition, a frame shift resulted in a stop codon terminating the deduced sequence prior to a putative transmembrane domain, thereby truncating the protein to 889 amino acids. The deduced amino acid sequence of the receptor has high sequence similarity to the hsp 110 subfamily of proteins. These findings on the primary structure of the egg receptor for sperm raise important questions concerning the mechanism by which the heavily glycosylated receptor is localized to the extracellular surface of the egg and to the cortical granules.

Surface localization of the sea urchin egg receptor for sperm

The sea urchin egg receptor for sperm is thought to be involved in species-specific sperm-egg interactions at the egg surface. Recent revisions in the deduced amino acid sequence of the cloned cDNAs indicate that the protein encoded does not possess the common structural hallmarks of a membrane protein. Thus, investigation of the localization and association of the protein with the egg surface is crucial. We describe and characterize a new monoclonal antibody raised against recombinant sperm receptor protein. This antibody, in conjunction with several polyclonal antibodies, was used to study the receptor protein in eggs. Immunoprecipitation studies indicated that the antibodies recognize the high Mr (ca. 350 K) sperm receptor protein which copurified with egg plasma membrane-vitelline layer complexes. The sperm receptor protein was solubilized only by detergents and not by treatments designed to solubilize peripherally associated or lipid-anchored membrane proteins, suggesting a tight association with the membrane fraction. Confocal immunofluorescence microscopy of live eggs indicated surface staining. Finally, lysylendoproteinase C treatment of live eggs resulted in a loss of the high Mr receptor protein epitopes, and the concomitant release of a 70-kDa proteolytic fragment, which correlated with a reduced ability of the eggs to be fertilized. Taken together, these data indicate that at least some fraction of the sperm receptor protein is present on the egg surface, a requisite locale for a sperm binding protein.

Characterization of the vitelline envelope of the sea urchin Strongylocentrotus purpuratus

The vitelline envelope (VE) is an extremely thin, acellular, proteinaceous coat that surrounds the extracellular surface of sea urchin eggs. Despite previous studies on VE composition, structure and function, our understanding to the envelope is still incomplete at the molecular level. We have isolated VE components from intact, unfertilized Strongylocentrotus purpuratus eggs by reduction with alkaline dithiothreitol-see water solutions and have characterized the macromolecules by SDS-PAGE. There were eight major glycoprotein bands, including two high molecular weight components at 265 and 300 kDa, and several minor components. We have revealed, by lectin blot analysis, that most components contain mannose, while a subset of glycoproteins contain fucose and N-acetylglucosamine; galactose and sialic acid were also detected. The components in the VE preparations were compared with cell surface complex preparations by immunoblot analysis, using antisera against a VE preparation, a 305 kDa electrophoretically purified VE glycoprotein and an extracellular portion of the sea urchin egg recombinant 350 kDa sperm receptor. Serum against the recombinant sperm receptor reacted with a component of approximately 350 kDa on blots, but did not react with the 300 kDa component found in VE preparations. Therefore, we suggest these two glycoproteins are not the same.

Oligomerization of sarcoplasmic reticulum Ca2+-ATPase from rabbit skeletal muscle

Although the primary structure and catalytic cycle of the sarcoplasmic reticulum Ca2+-ATPase has been revealed, it is not well understood whether functional Ca2+ pump proteins exist in a monomeric or an oligomeric state in native skeletal muscle membranes. Here, we show that the Ca2+-ATPase tends to form high molecular weight complexes, estimated to be dimers and tetramers using immunoblotting of two-dimensionally separated microsomal membranes following crosslinking. This agrees with both electron microscopical and biochemical findings which demonstrate that Ca2+-ATPase clusters are the predominant molecular species in native membranes and that oligomerization may play a role in cooperative kinetics and enzyme stabilization.

The 170 kDa glucose regulated stress protein is a large HSP70-, HSP110-like protein of the endoplasmic reticulum

The existence of a family of unusually large and highly diverged hsp70-like proteins (the hsp110/SSE family) has recently been described. The 170 kDa glucose regulated stress protein (grp170) is a retained endoplasmic reticulum glycoprotein that may be involved in immunoglobulin folding and/or assembly. We describe here the cloning of the cDNA for grp170 and show that it, like hsp110, is a large and highly diverged hsp70-like polypeptide which shares specific features with hsp70 (the dnaK family) and the hsp110/SSE family, while also differing from both. Grp170 contains an ATP binding domain and binds ATP, it possesses a carboxyl terminal NDEL sequence, and its mRNA is anoxia inducible.

Identification of a major subfamily of large hsp70-like proteins through the cloning of the mammalian 110-kDa heat shock protein

A major mammalian heat shock protein of 110 kDa (hsp110) has long been observed, but has not been cloned. We have cloned the hamster cDNA for hsp110 and show that it hybridizes on a Northern blot to a 3.5-kilobase heat-inducible message in hamster and mouse. The hsp110 sequence was found to share an approximately 30-33% amino acid identity with members of the hsp70 family, most of which occurs in the conserved ATP-binding domain of these molecules. In addition, five sequences were found to be highly similar to hsp110. These are the sea urchin egg receptor for sperm (Foltz, K.R., Partin, J. S., and Lennarz, W.J. (1993) Science 259, 1421-1425) and additional sequences from human and Caenorhaditis elegans and two from yeast. The carboxyl-terminal two-thirds of hsp110 and these five related proteins contain a pattern of highly conserved regions of sequence unique to this group. A probe containing these conserved sequences was found to strongly cross-react on a Southern blot with genomic sequences from yeast to man. A Western blot analysis of several murine tissues indicates that hsp110 is constitutively expressed in all mouse tissues and is highly expressed in brain. Therefore, hsp110 belongs to a new category of large and structurally unique stress proteins that are the most distantly related known members of the hsp70 family.

Sperm-binding proteins

Gamete recognition and binding are mediated by specific proteins on the surface of the sperm and egg. Identification and characterization of some of these proteins from several model systems, particularly mouse and sea urchin, have focused interest on the general properties and functions of gamete recognition proteins. Sperm-binding proteins located in egg extracellular coats as well as sperm-binding proteins that are localized to the egg plasma membrane are presented in the context of their structure and function in gamete binding. Unifying and disparate characteristics are discussed in light of the diverse biology of fertilization among species. Outstanding questions, alternative mechanisms and models, and strategies for future work are presented.

Role of the sea urchin egg receptor for sperm in gamete interactions

Embryonic development is initiated by a multi-step fertilization process involving induction of the acrosome reaction in sperm, sperm-egg binding, gamete membrane fusion and egg activation. In sea urchins, acrosome-reacted sperm interact, presumably via the sperm protein bindin, with a highly glycosylated receptor on the egg surface. This article highlights the recent advances in the molecular structure of the sea urchin sperm receptor and discusses its possible role in egg activation.

Fertilisation in sea urchins: how many different molecules are involved in gamete interaction and fusion?

It has been established that fertilisation in the sea urchin involves binding of acrosome-reacted sperm to an egg cell surface receptor. The structure and function of this receptor, as well as the possible involvement of other cell surface molecules in the binding, fusion and activation events, is discussed.