Sea urchin egg receptor for sperm: sequence similarity of binding domain and hsp70

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.

Bindin is essential for fertilization in the sea urchin

Species-specific sperm−egg interactions are essential for sexual reproduction. Broadcast spawning of marine organisms is under particularly stringent conditions, since eggs released into the water column can be exposed to multiple different sperm. Bindin isolated from the sperm acrosome results in insoluble particles that cause homospecific eggs to aggregate, whereas no aggregation occurs with heterospecific eggs. Therefore, Bindin is concluded to play a critical role in fertilization, yet its function has never been tested. Here we report that Cas9-mediated inactivation of the bindin gene in a sea urchin results in perfectly normal-looking embryos, larvae, adults, and gametes in both males and females. What differed between the genotypes was that the bindin^−/− sperm never fertilized an egg, functionally validating Bindin as an essential gamete interaction protein at the level of sperm–egg cell surface binding.

Molecular mechanisms and evolution of fertilization proteins

Sexual reproduction involves a cascade of molecular interactions between the sperm and the egg culminating in cell-cell fusion. Vital steps mediating fertilization include chemoattraction of the sperm to the egg, induction of the sperm acrosome reaction, dissolution of the egg coat, and sperm-egg plasma membrane binding and fusion. Despite decades of research, only a handful of interacting gamete recognition proteins (GRPs) have been identified across taxa mediating each of these steps, most notably in abalone, sea urchins, and mammals. This review outlines and compares notable GRP pairs mediating sperm-egg recognition in these three significant model systems and discusses the molecular basis of species-specific fertilization driven by GRP function. In addition, we explore the evolutionary theory behind the rapid diversification of GRPs between species. In particular, we focus on how the coevolution between interacting sperm and egg proteins may contribute to the formation of boundaries to hybridization. Finally, we discuss how pairing structural information with evolutionary insights can improve our understanding of mechanisms of fertilization and their origins.

Nonspecific expression of fertilization genes in the crown-of-thorns Acanthaster cf. solaris: Unexpected evidence of hermaphroditism in a coral reef predator

The characterization of gene expression in gametes has advanced our understanding of the molecular basis for ecological variation in reproductive success and the evolution of reproductive isolation. These advances are especially significant for ecologically important keystone predators such as the coral-eating crown-of-thorns sea stars (COTS, Acanthaster) which are the most influential predator species in Indo-Pacific coral reef ecosystems and the focus of intensive management efforts. We used RNA-seq and transcriptome assemblies to characterize the expression of genes in mature COTS gonads. We described the sequence and domain organization of eight genes with sex-specific expression and well known functions in fertilization in other echinoderms. We found unexpected expression of genes in one ovary transcriptome that are characteristic of males and sperm, including genes that encode the sperm-specific guanylate cyclase receptor for an egg pheromone, and the sperm acrosomal protein bindin. In a reassembly of previously published RNA-seq data from COTS testes, we found a complementary pattern: strong expression of four genes that are otherwise well known to encode egg-specific fertilization proteins, including the egg receptor for bindin (EBR1) and the acrosome reaction-inducing substance in the egg coat (ARIS1, ARIS2, ARIS3). We also found histological evidence of both eggs and sperm developing in the same gonad in several COTS individuals from a parallel study. These results suggest the occurrence of hermaphrodites, and the potential for reproductive assurance via self-fertilization. Our findings have implications for management of COTS populations, especially in consideration of the large size and massive fecundity of these sea stars.

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.

The significance of Hsp70 subnetwork for Dengue virus lifecycle

Viruses hijack host machineries for replicating themselves efficiently. Host protein quality control machineries (QC) not only assist protein folding to form bona fide proteins with active functions but also get rid of un/misfolded proteins via degradation to maintain the protein homeostasis. Previous studies have reported that viruses utilize QC at various steps for their lifecycles. Recently we defined Hsp70s and their cochaperones, DnaJs functions on Dengue lifecycle. Here we summarize the significance of QC on Dengue virus.

Models of selection, isolation, and gene flow in speciation

Many marine ecologists aspire to use genetic data to understand how selection and demographic history shape the evolution of diverging populations as they become reproductively isolated species. I propose combining two types of genetic analysis focused on this key early stage of the speciation process to identify the selective agents directly responsible for population divergence. Isolation-with-migration (IM) models can be used to characterize reproductive isolation between populations (low gene flow), while codon models can be used to characterize selection for population differences at the molecular level (especially positive selection for high rates of amino acid substitution). Accessible transcriptome sequencing methods can generate the large quantities of data needed for both types of analysis. I highlight recent examples (including our work on fertilization genes in sea stars) in which this confluence of interest, models, and data has led to taxonomically broad advances in understanding marine speciation at the molecular level. I also highlight new models that incorporate both demography and selection: simulations based on these theoretical advances suggest that polymorphisms shared among individuals (a key source of information in IM models) may lead to false-positive evidence of selection (in codon models), especially during the early stages of population divergence and speciation that are most in need of study. The false-positive problem may be resolved through a combination of model improvements plus experiments that document the phenotypic and fitness effects of specific polymorphisms for which codon models and IM models indicate selection and reproductive isolation (such as genes that mediate sperm-egg compatibility at fertilization).

Incipient speciation of sea star populations by adaptive gamete recognition coevolution

Reproductive isolation--the key event in speciation--can evolve when sexual conflict causes selection favoring different combinations of male and female adaptations in different populations. Likely targets of such selection include genes that encode proteins on the surfaces of sperm and eggs, but no previous study has demonstrated intraspecific coevolution of interacting gamete recognition genes under selection. Here, we show that selection drives coevolution between an egg receptor for sperm (OBi1) and a sperm acrosomal protein (bindin) in diverging populations of a sea star (Patiria miniata). We found positive selection on OBi1 in an exon encoding part of its predicted substrate-binding protein domain, the ligand for which is found in bindin. Gene flow was zero for the parts of bindin and OBi1 in which selection for high rates of amino acid substitution was detected; higher gene flow for other parts of the genome indicated selection against immigrant alleles at bindin and OBi1. Populations differed in allele frequencies at two key positively selected sites (one in each gene), and differences at those sites predicted fertilization rate variation among male-female pairs. These patterns suggest adaptively evolving loci that influence reproductive isolation between populations.

Structure and evolution of the sea star egg receptor for sperm bindin

Selection on coevolving sperm- and egg-recognition molecules is a potent engine of population divergence leading to reproductive isolation and speciation. The study of receptor-ligand pairs can reveal co-evolution of male- and female-expressed genes or differences between their evolution in response to selective factors such as sperm competition and sexual conflict. Phylogeographical studies of these patterns have been limited by targeted gene methods that favour short protein-coding sequences amplifiable by PCR. Here, I use high-throughput transcriptomic methods to characterize the structure and divergence of full-length coding sequences for the gene encoding the protein component of a large complex egg surface glycopeptide receptor for the sperm acrosomal protein bindin from the sea star Patiria miniata. I used a simple but effective method for resolving nucleotide polymorphisms into haplotypes for phylogeny-based analyses of selection. The protein domain organization of sea star egg bindin receptor (EBR1) was similar to sea urchins and included a pair of protein-recognition domains plus a series of tandem repeat domains of two types. Two populations separated by a well-characterized phylogeographical break included lineages of EBR1 alleles under positive selection at several codons (similar to selection on sperm bindin in the same populations). However, these populations shared the same alleles that were under selection for amino acid differences at multiple codons (unlike the pattern of selection for population divergence in sperm bindin). The significance of positively selected EBR1 domains and alleles could be tested in functional analyses of fertilization rates associated with EBR1 (and bindin) polymorphisms.

Role of vitelline envelope during fertilization in the black tiger shrimp, Penaeus monodon

Animal eggs possess investments through which sperm must penetrate. The aim of the present study was to investigate the role of the egg coating, the vitelline envelope, during sperm-egg interactions in the black tiger shrimp, Penaeus monodon. The site(s) of primary binding between sperm and egg and the possible binding molecule(s) for sperm were identified. In vitro adsorption of the vitelline envelope protein onto the sperm surface showed that primary binding occurred between the sperm anterior spike of acrosome intact sperm and the vitelline envelope. Results from streptavidin blotting revealed that the component of the vitelline envelope that interacts with the sperm integral membrane protein is a 370kDa protein. In addition, it was shown that the vitelline envelope protein had no ability to induce acrosome reaction. These results suggest that the function of the vitelline envelope is as a primary binding site for sperm in shrimp, but not a sole trigger for the acrosome reaction.

Amphibian oocytes release heat shock protein A during spawning: evidence for a role in fertilization

Heat shock proteins A (HSPAs, previously known as HSP70s) are widely distributed proteins originally linked with heat shock but now associated with several normal cellular functions. We recently found indirect evidence suggesting a role for HSPAs in sperm-oocyte interaction in the amphibian Bufo arenarum. In the present study our aim was to study its expression, subcellular distribution, and role during fertilization. By Western blot analysis using two different antibodies we detected HSPAs present in B. arenarum oocytes in the absence of any stress. We performed two-dimensional electrophoresis and detected two isoforms with isoelectric points of 5.25 and 5.45. We studied its subcellular distribution isolating total membranes, cytosol, and plasma membranes. HSPAs were present in all of these fractions. We confirmed these results by immunofluorescence microscopy and also found that the HSPA signal was present in the vitelline envelope. To further test this, we performed Western blot analysis in isolated vitelline envelopes and in egg water (diffusible material from deposited oocytes). HSPAs were present in these two fractions. Moreover, human recombinant his-tagged HSPA (HSPA1A) was able to specifically bind to sperm in vitro (midpiece) and enhance sperm membrane integrity. In vitro fertilization assays in the presence of anti-HSPA polyclonal antibodies showed diminished fertilization scores at low sperm concentrations (10(5) cells per milliliter). Our results suggest that HSPAs are present in intracellular and extracellular structures of nonstressed B. arenarum oocytes and participates in fertilization by and that their release during spawning plays a role in sperm membrane integrity.

Primitive vertebrate immunity: what is the evolutionary derivative of molecules that define the adaptive immune system?

The adaptive immune system is capable of responding to an infinite number of antigens with the antigen-specific receptors immunoglobulin (Ig) and the T cell receptor (TCR). Ig binds soluble antigens while TCR recognizes antigen bound in clefts of polymorphic self-encoded major histocompatibility complex (MHC) class I and class II molecules. All of these molecules are wholly or partially composed of Ig superfamily domains. TCR and Ig use V-set Ig superfamily domains, always in heterodimeric forms, in antigen recognition. Although the ways in which TCR and Ig bind antigen are fundamentally different, the structure of the heterodimeric V domains is probably identical. The antigen-binding cleft of MHC proteins has a structure unlike Ig superfamily domains, although several investigators have proposed that this cleft is evolutionarily derived from Ig domains. We believe the MHC cleft is a primitive structure, perhaps related to the peptide-binding domains of intracellular chaperone proteins. A model is proposed whereby chaperone proteins were the primordial MHC molecules, presenting peptides derived from invariant proteins residing inside cells for recognition by lymphocytes with minimally diverse receptors. Such a system may be reflected today by the epithelial immune system, apparently governed by monomorphic MHC molecules and lymphocytes with unconventional antigen-specific receptors.

Complex formation of 70-kDa heat shock protein with acidic glycolipids and phospholipids

A new property of a heat-inducible heat shock protein (Hsp) 70.1 that it forms a complex with acidic lipids was first demonstrated. Based on the behaviors of the complexes on the native PAGE, the acidic lipid/Hsp70.1 complexes are categorized into two groups. The first group is the sulfatide-induced large-sized complex, which stays on the gel top on the native PAGE. Only the N-terminal ATPase domain is responsible for the complex formation. The second group is the ganglioside-induced complex, which is diffused in the resolution gel on the native PAGE. Both the N-terminal ATPase and the C-terminal peptide-binding domains are involved in the complex formation. No complex is formed by neutral glyco- and phospholipids. The complex formation with the acidic glyco- and phospholipids implicates the various functions of Hsp70 on the membrane surfaces.

Examination of the expression of the heat shock protein gene, hsp110, in Xenopus laevis cultured cells and embryos

Eukaryotic organisms respond to various stresses with the synthesis of heat shock proteins (HSPs). HSP110 is a large molecular mass HSP that is part of the HSP70/DnaK superfamily. In this study, we have examined, for the first time, the expression of the hsp110 gene in Xenopus laevis cultured cells and embryos. Sequence analysis revealed that the protein encoded by the hsp110 cDNA exhibited 74% identity with its counterparts in mammals and only 27-29% with members of the Xenopus HSP70 family. Hsp110 mRNA and/or protein was detected constitutively in A6 kidney epithelial cells and was inducible by heat shock, sodium arsenite, and cadmium chloride. However, treatment with ethanol or copper sulfate had no detectable effect on hsp110 mRNA levels. Similar results were obtained for hsp70 mRNA except that it was inducible with ethanol. In Xenopus embryos, hsp110 mRNA was present constitutively during development. Heat shock-inducible accumulation of hsp110 mRNA occurred only after the midblastula stage. Whole mount in situ hybridization analysis revealed that hsp110 mRNA accumulation in control and heat shocked embryos was enriched in selected tissues. These studies demonstrate that Xenopus hsp110 gene expression is constitutive and stress inducible in cultured cells and developmentally- and tissue specifically-regulated during early embryogenesis.

Hspa4l-deficient mice display increased incidence of male infertility and hydronephrosis development

The Hspa4l gene, also known as Apg1 or Osp94, belongs to the HSP110 heat shock gene family, which includes three genes encoding highly conserved proteins. This study shows that Hspa4l is expressed ubiquitously and predominantly in the testis. The protein is highly expressed in spermatogenic cells, from late pachytene spermatocytes to postmeiotic spermatids. In the kidney, the protein is restricted to cortical segments of distal tubules. To study the physiological role of this gene in vivo, we generated mice deficient in Hspa4l by gene targeting. Hspa4l-deficient mice were born at expected ratios and appeared healthy. However, approximately 42% of Hspa4l(-/-) male mice suffered from fertility defects. Whereas the seminiferous tubules of Hspa4l(-/-) testes contained all stages of germ cells, the number of mature sperm in the epididymis and sperm motility were drastically reduced. The reduction of the sperm count was due to the elimination of a significant number of developing germ cells via apoptosis. No defects in fertility were observed in female mutants. In addition, 12% of null mutant mice developed hydronephrosis. Concentrations of plasma and urine electrolytes in Hspa4l(-/-) mice were similar to wild-type values, suggesting that the renal function was not impaired. However, Hspa4l(-/-) animals were preferentially susceptible to osmotic stress. These results provide evidence that Hspa4l is required for normal spermatogenesis and suggest that Hspa4l plays a role in osmotolerance.

Characterization of Bufo arenarum oocyte plasma membrane proteins that interact with sperm

Sperm-oocyte plasma membrane interaction is an essential step in fertilization. In amphibians, the molecules involved have not been identified. Our aim was to detect and characterize oocyte molecules with binding affinity for sperm. We isolated plasma membranes free from vitelline envelope and yolk proteins from surface-biotinylated Bufo arenarum oocytes. Using binding assays we detected a biotinylated 100 kDa plasma membrane protein that consistently bound to sperm. Chromatographic studies confirmed the 100 kDa protein and detected two additional oocyte molecules of 30 and 70 kDa with affinity for sperm. Competition studies with an integrin-interacting peptide and cross-reaction with an anti-HSP70 antibody suggested that the 100 and 70 kDa proteins are members of the integrin family and HSP70, respectively. MS/MS analysis suggested extra candidates for a role in this step of fertilization. In conclusion, we provide evidence for the involvement of several proteins, including integrins and HSP70, in B. arenarum sperm-oocyte plasma membrane interactions.

Defending the zygote: search for the ancestral animal block to polyspermy

Fertilization is the union of a single sperm and an egg, an event that results in a diploid embryo. Animals use many mechanisms to achieve this ratio; the most prevalent involves physically blocking the fusion of subsequent sperm. Selective pressures to maintain monospermy have resulted in an elaboration of diverse egg and sperm structures. The processes employed for monospermy are as diverse as the animals that result from this process. Yet, the fundamental molecular requirements for successful monospermic fertilization are similar, implying that animals may have a common ancestral block to polyspermy. Here, we explore this hypothesis, reviewing biochemical, molecular, and genetic discoveries that lend support to a common ancestral mechanism. We also consider the evolution of alternative or radical techniques, including physiological polyspermy, with respect to our ability to describe a parsimonious guide to fertilization.

Molecular dissection of egg fertilization signaling with the aid of tyrosine kinase-specific inhibitor and activator strategies

Fertilization is triggered by sperm-egg interaction and fusion that initiate a transient rise(s) in the free intracellular calcium ([Ca(2+)](i)) that is responsible for a series of biochemical and cell biological events, so-called "egg activation". Calcium-dependent egg activation leads to the initiation of developmental program that culminates in the birth of individuals. A growing body of knowledge has uncovered the molecular mechanisms underlying sperm-induced transient [Ca(2+)](i) increase(s) to some extent; namely, in most animals so far studied, a second messenger inositol 1,4,5-trisphosphate (IP(3)) seems to play a pivotal role in inducing [Ca(2+)](i) transient(s) at fertilization. However, signaling mechanisms used by sperm to initiate IP(3)-[Ca(2+)](i) transient pathway have not been elucidated. To approach this problem, we have employed African clawed frog, Xenopus laevis, as a model animal and conducted experiments designed specifically to determine the role of the Src family protein-tyrosine kinases (SFKs or Src family PTKs) in the sperm-induced egg activation. This review compiles information about the use of PTK-specific inhibitors and activators for analyzing signal transduction events in egg fertilization. Specifically, we focus on molecular identification of Xenopus Src and the signaling mechanism of the Src-dependent egg activation that has been established recently. We also summarize recent advances in understanding the role of the Src family kinases in egg fertilization of other model organisms, and discuss future directions of the field.

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.

Oolemmal proteomics--identification of highly abundant heat shock proteins and molecular chaperones in the mature mouse egg and their localization on the plasma membrane

BACKGROUND

The mature mouse egg contains the full complement of maternal proteins required for fertilization, the transition to zygotic transcription, and the beginning stages of embryogenesis. Many of these proteins remain to be characterized, therefore in this study we have identified highly abundant egg proteins using a proteomic approach and found that several of these proteins also appear to localize to the egg surface. Characterization of such molecules will provide important insight into the cellular events of fertilization and early development.

METHODS

In order to identify some of the more abundant egg proteins, whole egg extracts were resolved on coomassie-stained two-dimensional (2D) PAGE gels. Several highly abundant protein spots were cored and microsequenced by tandem mass spectrometry (TMS), and determined to be molecular chaperone proteins. Concurrent experiments were performed to identify oolemmal proteins using 2D avidin blotting. Proteins spots that appeared to be surface labeled by biotinylation were correlated with the initial coomassie-stained reference gel. Surprisingly, some of the surface labelled proteins corresponded to those abundant chaperone proteins previously identified. To confirm whether these molecules are accumulating at the oolemmal surface in eggs, we performed immunofluoresence on live, zona-free eggs using antibodies to HSP70, HSP90, GRP94, GRP78, calreticulin and calnexin.

RESULTS

The putative surface-labeled proteins identified by biotinylation included the molecular chaperones HSP70 (MW 70 KDa, pI 5.5), HSP90a (MW 85 KDa, pI 4.9), GRP94 (MW 92 KDa, pI 4.7), GRP78 (MW 72 KDa, pI 5.0), Oxygen regulated protein 150 (ORP150; MW 111 KDa, pI 5.1), Calreticulin (MW 48 KDa, pI 4.3), Calnexin (MW 65 KDa, pI 4.5), and Protein disulfide isomerase (PDI; MW 57 KDa, pI 4.8). Immunofluoresence results showed that antibodies to HSP90, GRP94, GRP78 and calreticulin were reactive with oolemmal proteins. We were unable to confirm surface localization of HSP70 or calnexin by this method.

CONCLUSIONS

We report here the identification of nine highly abundant molecular chaperones in the mouse egg proteome. In addition, we present preliminary data suggesting that these molecules localize to the oolemma of the mature mouse egg.

Surface accessibility of the 70-kilodalton Chlamydia trachomatis heat shock protein following reduction of outer membrane protein disulfide bonds

Numerous investigations have shown that 70-kDa heat shock protein (Hsp70) homologs interact tightly with hydrophobic proteins and functionally assist proteins in membranous organelles and environments. One such protein is the Chlamydia trachomatis Hsp70 that is associated with isolated outer membrane complexes of infectious elementary bodies (EB). Previous observations have indicated that chlamydial Hsp70 plays a role in EB attachment to, or entry into, endometrial epithelial cells. In this study, immunofluorescence microscopy and transmission electron microscopy observations showed that chlamydial Hsp70 is not a surface-displayed ligand on purified EB. However, brief exposure of EB to the thiol reducing agent dithiothreitol (DTT) led to surface accessibility of the Hsp70 substrate-binding domain. Reduction of the highly disulfide-cross-linked EB outer membrane proteins with DTT resulted in a decrease in EB attachment and infectivity. Interestingly, exposure of EB to the membrane-impermeable thiol-alkylating reagent 5,5'-dithiobis(2-nitrobenzoic acid) enhanced attachment but compromised infectivity, suggesting that EB outer membrane proteins must be reduced for entry and productive infection. Together, our data suggest that (i) the structural integrity of the EB outer membrane, maintained by protein disulfide bonds, is important during the initial stages of attachment; (ii) reduction occurs within the localized microenvironment of host cell surfaces once intimate contact is established between EB and host cells; and (iii) subsequent conformational changes in EB ultrastructure allow productive infection in host cells. The accessibility of the Hsp70 substrate-binding domain may support the hypothesis that this protein plays a role in events following the initial stage of attachment instead of serving as a primary, surface-displayed adhesin.

The aglycone of sulfogalactolipids can alter the sulfate ester substitution position required for hsc70 recognition

3'-Sulfogalactolipids(SGLs), sulfogalactosyl ceramide (SGC), and sulfogalactoglycerolipid (SGG) bind to the N-terminal ATPase-containing domain of members of the heat shock protein 70 family. We have probed this binding specificity using a series of synthetic positional sulfated or phosphorylated glycolipid analogues, containing either a long-chain bisalkyl hydrocarbon-2-(tetradecyl)hexadecane (B30) or C(18) ceramide (SGC(18)) backbone. By TLC overlay and receptor ELISA, recombinant hsc70 bound ceramide-based glycoconjugates having 3'- or 4'-sulfogalactose glycone moieties and the 4'-sulfogalactose positional isomer conjugated to B30. Hsc70 binding was significantly decreased to the 3'-sulfogalactose conjugated to the long-chain branched alkane. 3'-Sulfoglucose conjugated to B30 was not bound, nor were similarly conjugated di-, tri-, and tetra-sulfated or phosphorylated galactolipids. These results highlight the importance of the position, rather than the number of sulfate esters within the galactose ring. This binding selectivity was shared by the sea urchin hsp70-related sperm receptor. A 3'-SGC-based soluble inhibitor, in which the acyl chain was replaced with an adamantyl group, inhibited binding of hsc70 to both 3'- and 4'-SGC species with an IC(50) of 50 and 75 microM, respectively, indicating a shared sulfogalactose binding site. These studies demonstrate the highly specific nature of hsc70/SGL binding and show, for the first time, that the lipid aglycone can alter the substitution position requirement for glycolipid recognition.

Genomic analysis of the Hsp70 superfamily in Arabidopsis thaliana

The Arabidopsis genome contains at least 18 genes encoding members of the 70-kilodalton heat shock protein (Hsp70) family, 14 in the DnaK subfamily and 4 in the Hsp110/SSE subfamily. While the Hsp70s are highly conserved, a phylogenetic analysis including all members of this family in Arabidopsis and in yeast indicates the homology of Hsp70s in the subgroups, such as those predicted to localize in the same subcellular compartment and those similar to the mammalian Hsp110 and Grp170. Gene structure and genome organization suggest duplication in the origin of some genes. The Arabidopsis hsp70s exhibit distinct expression profiles; representative genes of the subgroups are expressed at relatively high levels during specific developmental stages and under thermal stress.

Role of a vitelline layer-associated 350 kDa glycoprotein in controlling species-specific gamete interaction in the sea urchin

The process of sperm-egg binding is one of the barriers to cross-fertilization between related sea urchin species. A 350 kDa glycoprotein in the egg vitelline layer of Strongylocentrotus purpuratus has been shown to be a sperm-binding protein (SBP). Sulfated O-linked oligosaccharide chains on the 350 kDa glycoprotein, as well as domains of the polypeptide chain, serve as ligands for this binding process. The hypothesis that species-specific sperm-egg binding is attributed to the interaction between the sperm and the 350 kDa glycoprotein was tested using S. purpuratus and S. franciscanus. It was found that both species had a 350 kDa glycoprotein on the egg surface that cross-reacted immunologically using antibodies prepared against a recombinant form of the SBP. Because earlier studies had implicated the carbohydrate chains of the 350 kDa glycoprotein of S purpuratus in sperm binding, differences in carbohydrate chains on the 350 kDa glycoproteins of these species were examined. It was found that among the lectins tested only wheat germ agglutinin and Sambucus nigra agglutinin showed a significant difference in reactivity to the 350 kDa glycoproteins between species. Finally, using a bead-binding assay, it was shown that the isolated 350 kDa glycoproteins exhibited species-specific sperm-binding activity.

The ATPase domain of hsp70 possesses a unique binding specificity for 3'-sulfogalactolipids

The region(s) of hsp70 critical for sulfogalactolipid (SGL) recognition has been defined through deletion analysis and site-directed mutagenesis. Truncated polymerase chain reaction products of hsp70 generated N-terminal fragments of 43, 35, 29, and 22 kDa. The C terminus substrate-binding domain (28 kDa) was also expressed. The N-terminal ATPase domain (rP43) shared the binding specificity of hsp70, because only sulfogalactosyl ceramide and sulfogalactosyl glycerolipid were recognized by both TLC overlay and RELISA. The C-terminal domain showed no binding. SGL binding of rP29 and rP22 was severely reduced. The loss of SGL binding for rP35 by RELISA but not TLC overlay was considered as a function of receptor presentation. The truncation of rP43 to rP35 demonstrates that residues 318-387 (the base of the ATP binding cleft) are critical for high affinity SGL binding. Mutagenesis showed that Arg(342) and Phe(198) are crucial for this process. SGL binding, mediated by these conserved residues within the ATPase domain of hsp70, implies that this binding specificity is evolutionarily conserved.

The hsp110 and Grp1 70 stress proteins: newly recognized relatives of the Hsp70s

Both the Grp170 and Hsp110 families represent relatively conserved and distinct sets of stress proteins, within a more diverse category that also includes the Hsp70s. All of these families are found in a wide variety of organisms from yeasts to humans. Although Hsp110s or Grp170s are not Hsp70s any more than Hsp70s are Hsp110s or Grp170s, it is still reasonable to refer to this combination of related families as the Hsp70 superfamily based on arguments discussed above and since no obvious prokaryotic Hsp110 or Grp170 has yet been identified. These proteins are related to their counterparts in the Hsp70/Grp78 family of eukaryotic stress proteins but are characterized by significantly larger molecular weights. The members of the Grp170 family are characterized by C-terminal ER retention sequences and are ER localized in yeasts and mammals. As a Grp, Grp170 is recognized to be coregulated with other major Grps by a well-known set of stress conditions, sometimes referred to as the unfolded protein response (Kozutsumi et al 1988; Nakaki et al 1989). The Hsp110 family members are localized in the nucleus and cytoplasm and, with other major Hsps, are also coregulated by a specific set of stress conditions, most notably including hyperthermic exposures. Hsp110 is sometimes called Hsp105, although it would be preferable to have a uniform term. The large Hsp70-like proteins are structurally similar to the Hsp70s but differ from them in important ways. In both the Grp170 and Hspl10 families, there is a long loop structure that is interposed between the peptide-binding ,-domain and the alpha-helical lid. In the Hsp110 family and Grp170, there are differing degrees of expansion in the alpha-helical domain and the addition of a C-terminal loop. This gives the appearance of much larger lid domains for Hsp110 and Grp170 compared with Hsp70. Both Hsp110 and Grp170 families have relatively conserved short sequences in the alpha-helical domain in the lid, which are conserved motifs in numerous proteins (we termed these motifs Magic and TedWylee as discussed earlier). The structural differences detailed in this review result in functional differences between the large (Grp170 and Hspl10) members of the Hsp70 superfamily, the most distinctive being an increased ability of these proteins to bind (hold) denatured polypeptides compared with Hsc70, perhaps related to the enlarged C-terminal helical domain. However, there is also a major difference between these large stress proteins; Hsp110 does not bind ATP in vitro, whereas Grp170 binds ATP avidly. The role of the Grp170 and Hsp110 stress proteins in cellular physiology is not well understood. Overexpression of Hsp110 in cultured mammalian cells increases thermal tolerance. Grp170 binds to secreted proteins in the ER and may be cooperatively involved in folding these proteins appropriately. These roles are similar to those of the Hsp70 family members, and, therefore, the question arises as to the differential roles played by the larger members of the superfamily. We have discussed evidence that the large members of the superfamily cooperate with members of the Hsp70 family, and these chaperones probably interact with a large number of chaperones and cochaperones in their functional activities. The fundamental point is that Hsp110 is found in conjunction with Hsp70 in the cytoplasm (and nucleus) and Grp170 is found in conjunction with78 in tha ER in every eucaryotic cell examined from yeast to humans. This would strongly argue that Hsp110 Grp170 exhibit functions in eucaryotes not effectively performed by Hsp70s or Grp78, respectively. Of interest in this respect is the observation that all Hsp110s loss of function or deletion mutants listed in the Drosophila deletion project database are lethal. The important task for the future is to determine the roles these conserved molecular chaperones play in normal and physiologically stressed cells.

Fertilization signalling and protein-tyrosine kinases

Fertilization is initiated by species-specific gamete cell recognition, i.e. sperm-egg interaction, followed by a rapid and sustained activation of multiple cellular and biochemical events, collectively called 'egg activation', which is indispensable for successful formation of zygotic nucleus and later embryogenesis. It is well known that sperm-induced egg activation is mediated by a transient release of calcium ions that originates from the sperm entry point and propagates through the entire egg cytoplasm. It is unclear, however, what kind of upstream events prelude to the calcium transient after sperm-egg interaction. Recently, much attention has been paid to the role of protein-tyrosine phosphorylation in egg activation process by a number of studies on some well-established model organisms. These includes marine invertebrates, frogs, and mammals. In this review, we will summarize the recent findings that begin to uncover a 'missing link' between sperm-egg interaction and egg activation with emphasis on the role of egg protein-tyrosine kinases (PTKs) in Xenopus egg fertilization.

Fusomorphogenesis: cell fusion in organ formation

Cell fusion is a universal process that occurs during fertilization and in the formation of organs such as muscles, placenta, and bones. Very little is known about the molecular and cellular mechanisms of cell fusion during pattern formation. Here we review the dynamic anatomy of all cell fusions during embryonic and postembryonic development in an organism. Nearly all the cell fates and cell lineages are invariant in the nematode C. elegans and one third of the cells that are born fuse to form 44 syncytia in a reproducible and stereotyped way. To explain the function of cell fusion in organ formation we propose the fusomorphogenetic model as a simple cellular mechanism to efficiently redistribute membranes using a combination of cell fusion and polarized membrane recycling during morphogenesis. Thus, regulated intercellular and intracellular membrane fusion processes may drive elongation of the embryo as well as postembryonic organ formation in C. elegans. Finally, we use the fusomorphogenetic hypothesis to explain the role of cell fusion in the formation of organs like muscles, bones, and placenta in mammals and other species and to speculate on how the intracellular machinery that drive fusomorphogenesis may have evolved.

Glycolipid receptors for verotoxin and Helicobacter pylori: role in pathology

Eukaryotic cell surface glycolipids can act as both the primary interface between bacteria and their host and secondly as a targeting mechanism for bacterial virulence factors. The former is characterized by redundancy in adhesin-receptor interactions and the latter by a higher affinity, more restrictive glycolipid binding specificity for targeting. Interactions of verotoxin with its glycolipid receptor globotriaosylceramide and Helicobacter pylori binding to a variety of different glycolipids, which can be environmentally regulated, provide examples of these differing modes of glycolipid receptor function. Verotoxins are involved in endothelial targeting in the microangiopathies of hemorrhagic colitis and hemolytic uremic syndrome (HUS). The highly restricted binding specificity and crystal structure of the verotoxin B subunit have allowed theoretical modeling of the Gb3 binding site of the verotoxin B subunit pentamer which provides an approach to intervention. Studies of the role of glycolipid function in verotoxin-induced disease have concentrated on the distribution of Gb3 and its ability to mediate the internalization of the toxin within the target cell. The distribution of Gb3 within the renal glomerulus plays a central role in defining the age-related etiology of HUS following gastrointestinal infection with VT producing Escherichia coli. H. pylori, on the other hand, instigates a less distinct but more complex disseminated gastric inflammation. Studies on the role of glycolipid receptors in H. pylori infection have been bogged down in establishing the importance of each binding specificity defined. In addition, the physiological condition of the organism within the various binding assays has not been extensively considered, such that spurious non-physiological interactions may have been elucidated. The identification and cloning of a Le(b) binding adhesin and the identification of cell surface hsp70 as a mediator of sulfoglycolipid binding under stress conditions may now allow a more molecular approach to define the role of glycolipid recognition in this infection.

Studies on heat shock proteins in sea urchin development

Work on stress proteins in sea urchin embryos carried out over the last 20 years is reviewed and the following major results are described. Entire sea urchin embryos, if subjected to a rise in temperature at any postblastular stage undergo a wave of heat shock protein (hsp) synthesis and survive. If subjected to the same rise between fertilization and blastula formation, they are not yet able to synthesize hsp and die. Four clones coding for the major hsp, hsp70, have been isolated and sequenced; evidence for the existence of a heat shock factor has been provided, and a mechanism for the developmental regulation of hsp synthesis discussed. Intraembryonic and intracellular hsp location has been described; and a mechanism for achievement of thermotolerance proposed. A chaperonine role for a constitutive mitochondrial hsp56 has been suggested, as well as a role for the constitutive hsp70 in cell division. Heat shock, if preceded by 12-O-tetradecanoylphorbol-12-acetate (TPA) treatment causes apoptosis.

The cortical granule serine protease CGSP1 of the sea urchin, Strongylocentrotus purpuratus, is autocatalytic and contains a low-density lipoprotein receptor-like domain

Trypsin-like activity is secreted from eggs of many species at fertilization, and this activity is believed to be critical for the block to polyspermy. Here we show that a cortical granule serine protease of sea urchins is the major and perhaps only protease family member important for fertilization. Zymography assays suggest that the cortical granules contain a single serine protease that can undergo autocatalysis and is secreted upon egg activation. We used this finding to identify a cDNA clone from a Strongylocentrotus purpuratus ovary cDNA library that encodes a 581-amino-acid-residue protein that we refer to as cortical granule serine protease 1 (CGSP1). The catalytic domain of the protein contains the essential residues of the catalytic triad characteristic of a member of the trypsin-like family of serine proteases and the N-terminus of CGSP1 resembles the ligand-binding domain of the low-density lipoprotein (LDL) receptor. Antibodies raised separately to both the protease and LDL receptor-like domains each localize to the cortical granules of unfertilized eggs. Furthermore, the full-length form of CGSP1, as well as intermediate and active forms of the protease, is detected in cortical granules by immunoblot analysis. Our evidence suggests that CGSP1 is activated at fertilization and is responsible for the protease-mediated reactions that follow cortical granule exocytosis and contribute to the block to polyspermy.

The chaperoning activity of hsp110. Identification of functional domains by use of targeted deletions

hsp110 is one of major heat shock proteins of eukaryotic cells and is a diverged relative of the hsp70 family. It has been previously shown that hsp110 maintains heat-denatured luciferase in a soluble, folding competent state and also confers cellular heat resistance in vivo. In the present study the functional domains of hsp110 that are responsible for its chaperoning activity are identified by targeted deletion mutagenesis using the DnaK structure as the model. The chaperoning activity of mutants is assessed based on their ability to solubilize heat-denatured luciferase as well as to refold luciferase in the presence of rabbit reticulocyte lysate. It is shown that these functions require only an internal region of hsp110 that includes the predicted peptide binding domain and two immediately adjacent C-terminal domains. It is also shown that although hsp110 binds ATP, binding can be blocked by its C-terminal region.

Specific glycoconjugates are present at the oolemma of the fertilization site in the egg of Discoglossus pictus (Anurans) and bind spermatozoa in an in vitro assay

In the egg of the anuran Discoglossus pictus, the site of fertilization is restricted to the central portion of an animal hemisphere indentation (the dimple). Previous studies showed that the acrosome reaction of D. pictus sperm is triggered in the jelly, and yet sperm arrive at the dimple surface with the plasma membrane at an early stage of vesiculation. Reactivity of the dimple surface with specific lectins suggests that fucose might be utilized as a marker of glycoproteins located at the dimple surface. In this paper, proteins of the egg surface were labeled with the membrane impermeable sulfo-NHS-biotin. Four main bands of 200, 230, 260, and 270 kDa labeled only at the dimple surface, although they were detected in the cortex of the whole egg. The 270-kDa band reacted with Galanthus nivalis agglutinin only in the cortex of the dimple, suggesting that this band is differently glycosylated according to its localization. The alpha-l-fucose-specific lectin Ulex europaeus agglutinin I was utilized both in lectin blotting and in affinity chromatography and cross-reacted with the 200- and 270/260-kDa bands. Furthermore, two polypeptides were obtained by exposure of intact eggs to lysylendoproteinase C. They were also reactive to Ulex europaeus agglutinin I. The 200- and 270/260-kDa bands were eluted from the acrylamide gels and adsorbed to polystyrene beads. An assay for sperm binding to 200-kDa glycoprotein-bound beads was developed. Sperm stuck to the beads before but not after Ca-ionophore treatment. When the beads were coated with the 270/260-kDa glycoproteins, binding occurred after ionophore treatment. In these assays, the 200- and 270/260-kDa glycoproteins competitively inhibited sperm binding to the beads coated with the corresponding glycoprotein. These results indicate that the assayed glycoproteins, located either in the glycocalyx or in the plasma membrane of the fertilization site, are involved in sperm binding.

Voltage-dependent activation of frog eggs by a sperm surface disintegrin peptide

Fertilin, a sperm protein of the metalloprotease/disintegrin/cysteine-rich (MDC) family, plays a critical role in sperm-egg binding in mammals. Peptides corresponding to the disintegrin domain of fertilin and antibodies against fertilin have been shown to inhibit mammalian sperm-egg binding and fusion. A protein from the same family, xMDC16, was recently cloned from frog (Xenopus laevis) testis and was found to be involved in frog sperm-egg binding. Here we report that xMDC16 is localized predominantly on the posterior surface of egg jelly-activated sperm, and peptides from the disintegrin domain of this protein activate eggs when applied near the egg surface. Egg activation was dependent on (1) specific amino acid residues (KTX); (2) the presence of divalent cations, but not external Ca2+ alone; and (3) voltage across the egg plasma membrane. This is the first demonstration of egg activation in vertebrates by the surface application of a peptide derived from a sperm surface protein, supporting a model for egg activation that involves a signal transducing receptor for sperm in the egg's plasma membrane.

Sperm-egg binding in the sea urchin: a high level of intracellular ATP stabilizes sperm attachment to the egg receptor

Previous studies have established that a recombinant protein fragment (45A) of the egg receptor for sperm of the sea urchin Strongylocentrotus purpuratus exhibits several characteristics that are consistent with that expected of a receptor. Using a quantitative sperm binding assay with glutathione S-transferase fused to a recombinant protein containing the C-terminal half of the 45A construct immobilized on glutathione beads, it was found that the interaction between sperm and this protein is a kinetically transient event. Sperm binding to the receptor fragment reached a maximum at 20 s after adding sperm in the presence of egg jelly to beads coated with recombinant receptor. In the next 20-120 s, approximately 50-70% of the sperm detached from the beads. Similar phenomena were observed when the kinetics of sperm binding to dejellied, glutaraldehyde-fixed eggs were studied. Because the acrosome reaction, a prelude to binding, is known to be accompanied by a decrease in the ATP level of sperm, we studied the effect of various inhibitors on both sperm detachment and the level of ATP. It was found that the detachment rate increased slightly when respiration inhibitors that blocked ATP production in mitochondria were added. In contrast, the dynein ATPase inhibitor, erythro-9-[3-hydroxynonyl]adenine, which is known to inhibit flagellum motility by blocking ATP utilization, stabilized the binding of sperm to the receptor and allowed maintenance of a high internal ATP level. Immotile, tailless sperm that physically lacked dynein ATPase, and therefore sustained their internal ATP levels, also exhibited stable binding provided that the sperm and beads were physically mixed. These results suggest that the internal ATP level of the sperm controls the stability of its binding to the receptor. The possible mechanism of the detachment and its significance with respect to the overall process of fertilization are discussed.

Constitutive and heat-inducible expression of HSP105 in neurons and glial cells in culture

The constitutive and heat-inducible expression of HSP105 was investigated in newborn mouse brain cell cultures by Northern blotting, Western blotting and immunocytochemistry. HSP105 was expressed most abundantly in the brain among the various tissues examined. HSP105 mRNA and protein were both present at substantial levels in brain cell cultures under unstressed conditions and up-regulated greatly during 3-48 h following exposure to heat stress (43 degrees C/20 min). HSP105 was expressed in nearly all neurons, oligodendrocytes, microglia and astrocytes with its location of both cytoplasmic and nuclear regions under unstressed and heat-stressed conditions. HSP105 expression was significantly down-regulated in astrocytes following treatment with IL-beta or TNF-alpha (50 ng/ml for 6 days), both of which are known growth-stimulatory cytokines for astrocytes. These results indicate that HSP105 is constitutive and heat-inducible HSP in neurons and glial cells in which its expression is under the control of both stressful stimuli and growth-regulatory factors.

Characterization of an acidic-pH-inducible stress protein (hsp70), a putative sulfatide binding adhesin, from Helicobacter pylori

The in vitro glycolipid binding specificity of the gastric pathogen Helicobacter pylori is altered to include sulfated glycolipids (sulfatides) following brief exposure of the organism to acid pH typical of the stomach. This change is prevented by anti-hsp70 antibodies, suggesting that hsp70 may be a stress-induced surface adhesin, mediating sulfatide recognition. To facilitate investigation of the role of hsp70 in attachment, we have cloned and sequenced the H. pylori hsp70 gene (dnaK). The hsp70 gene was identified by probing a cosmid DNA library made from H. pylori 439 with a PCR amplicon generated with oligonucleotides synthesized to highly conserved regions of dnaK. The 1.9-kb H. pylori hsp70 gene encodes a product of 616 amino acids. Primer extension analysis revealed a single transcription start site, while Northern blot analysis established that hsp70 was preferentially induced by low pH rather than by heat shock. The ability of H. pylori to alter its glycolipid binding specificity following exposure to low pH by upregulating hsp70 and by expressing hsp70 on the bacterial surface may provide a survival advantage during periods of high acid stress.

Characterization of the Hsp110/SSE gene family response to hyperosmolality and other stresses

Hsp110, Osp94, and Hsp70RY are members of the recently described Hsp110/SSE subfamily of (heat and osmotic) stress proteins whose members are structurally related to the Hsp70/BiP gene superfamily. To date, little is known about the response of this gene family to stresses in vitro or in vivo. In this study, an analysis of mRNA expression showed that Hsp110 and Osp94, like Hsp70, are induced in renal murine inner medullary collecting duct (mIMCD3) epithelial cells by heat shock, hyperosmotic NaCl, and cadmium, whereas low pH had a suppressive effect on Osp94. H2O2 decreased expression of Osp94 while inducing levels of Hsp110 and Hsp70 message. Tunicamycin, hypertonic urea, and tumor necrosis factor- had no effects. Hsp70RY was responsive exclusively to cadmium chloride. Moreover, enhanced expression of Hsp110 and Osp94 was subsequent to induction of Hsp70 and was suppressed by inhibition of protein synthesis by cycloheximide. RT-PCR analysis showed Hsp110, Osp94, and Hsp70RY are ubiquitously expressed in mouse tissues. In murine kidney, there was a corticomedullary gradient of expression of Hsp110, Osp94, Hsp70RY, and Hsp70 but not Hsc70 or BiP. Furthermore, dehydration increased inner medullary expression of Hsp110 and Osp94. An analysis of stress tolerance in mIMCD3 cells showed that heat shock and hyperosmotic NaCl stress are cross-tolerant stresses, suggesting hyperosmolality is a physiological correlate of heat shock in mammalian kidney. Thus Hsp110 and Osp94 behave as heat shock proteins, although they are regulated differently than Hsp70.

Link between fertilization-induced Ca2+ oscillations and relief from metaphase II arrest in mammalian eggs: a model based on calmodulin-dependent kinase II activation

Mammalian eggs are ovulated in metaphase II of meiosis, in a state characterized by high levels of cyclin B and of active maturation promoting factor (MPF). This arrest is mediated by an activity referred to as cytostatic factor (CSF) which prevents the degradation of cyclin. Fertilization triggers a train of Ca2+ spikes which is responsible for the decrease in activity of both MPF and CSF. The decline in MPF however much precedes that in CSF. Experimental observations on mammalian eggs indicate that the kinetics of cell cycle resumption much depends on the temporal pattern of the repetitive Ca2+ spikes. Here, we propose a theoretical model which accounts for Ca(2+)-induced relief from metaphase II arrest in mammalian eggs. The model is based on the fact that Ca2+/calmodulin kinase II (CaMKII) activation is the primary event leading to inactivation of both CSF and MPF. To account for experimental observations, it has to be assumed that CaMKII activation affects the level of the active form of the anaphase promoting complex (APC), which initiates the degradation of cyclin, through two pathways characterized by different time scales. Thus, we hypothesize that CaMKII activation by Ca2+ leads to the transformation of a mediator protein from a form which stimulates the inactivation of the APC into a form which gradually and indirectly induces the deactivation of CSF. In consequence, a sufficient number of Ca2+ spikes first triggers the decrease of MPF, thus allowing the egg to enter in interphase, and later that of CSF. Finally, when CSF is low and when Ca2+ oscillations have stopped, the level of MPF can increase again, a phenomenon that would correspond to the first mitosis. This model also accounts for the observed dependence of the time of entry in interphase (marked by the appearance of the pronuclei) on the frequency of Ca2+ spikes, as well as for the possible entry in metaphase III arrest, a pathological state of the egg which results from an insufficient activation by Ca2+. This study provides some theoretical prediction as to the time of the first mitosis as a function of the temporal pattern of Ca2+ oscillations.

Immune responses to stress proteins: applications to infectious disease and cancer

Heat shock proteins, or stress proteins have been identified as part of a highly conserved cellular defence mechanism mediated by multiple, distinct gene families and corresponding gene products. As intracellular chaperones, stress proteins participate in many essential biochemical pathways of protein maturation and function active during times of stress and during normal cellular homeostasis. In addition to their well-characterized role as protein chaperones, stress proteins are now realized to possess another important biological property: immunogenicity. Stress proteins are now understood to play a fundamental role in immune surveillance of infection and malignancy and this body of basic research has provided a framework for their clinical application. As key targets of both humoral and cellular immunity during infection, stress proteins have accordingly received considerable research interest as prophylactic vaccines for infectious disease applications. The unique and potent immunostimulatory properties of stress proteins have similarly been applied to the development of new approaches to cancer therapy, including both protein and gene-based modalities.