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PubMed1985

pH and protease control of acrosomal content stasis and release during the guinea pig sperm acrosome reaction.

The purpose of this study was to examine how trypsin inhibitors affect the guinea pig sperm acrosome reaction in vitro. Using spermatozoa pretreated with lysophosphatidyl choline, we found that both naturally occurring high molecular weight and the smaller synthetic trypsin inhibitor p-aminobenzamidine (PAB) delayed the onset of the acrosome reaction as monitored by light microscopy. Examination with electron microscopy revealed that acrosomal matrix dispersal rather than membrane fusion was affected. Despite the morphologic delay in acrosomal content release, PAB unexpectedly permitted 96% of soluble acrosomal antigen to be released into the supernatant. In addition, total acrosin release in the presence of PAB was 74% of control, with the vast majority as latent rather than active enzyme. A morphologically intact but membrane-free target of acrosomal matrix (AM), which is sensitive to trypsin inhibitor, was partially purified using Triton-x-100 at pH 5.2. AM remained morphologically stable at pH 5.2; however, shift up to pH 7 resulted in rapid dissolution within several minutes as monitored by light and electron microscopy and light scattering. Trypsin inhibitor prevented dispersion of AM at pH 7. The results suggest that, during the acrosome reaction, one distinct region of the acrosomal contents disperses after membrane vesiculation in a pH and trypsin inhibitor-insensitive fashion while a pH sensitive trypsin-like activity (acrosin?) disperses another discrete region of acrosomal matrix.

Acrosin↗
PubMedDate not supplied

Immunocytochemical localization of acrosin on both acrosomal membranes and in the acrosomal matrix of porcine spermatozoa.

Immunocytochemical techniques were employed to determine, at the ultrastructural level, the location of acrosin in porcine spermatozoa. Antisera to highly purified porcine acrosin was produced in rabbits. The (Fab')2 fragments of the immunoglobulins were purified and conjugated with horseradish peroxidase (HRP). Washed, formaldehyde-fixed spermatozoa were reacted with the labeled antiacrosin immunoglobulins, utilizing a direct staining technique. Electron microscopy revealed that the peroxidase reaction product of HRP-antiporcine acrosin was distributed evenly over the outer acrosomal membrane of spermatozoa with intact acrosomes. The labeled antibody was also distributed evenly over the inner acrosomal membrane of cells when the overlying acrosomal structures were absent. In some spermatozoa, labeling was noted throughout the acrosomal matrix. No significant labeling was observed in control specimens when spermatozoa were exposed to HRP-antiporcine acrosin immunoglobulins that had been adsorbed previously with excess purified acrosin or exposed to HRP-conjugated rabbit antiporcine immunoglobulins. This pattern of labeling is consistent with the hypothesis that acrosin may function as a zona lysin. The observation that the outer acrosomal membrane and acrosomal matrix are labeled suggests that acrosin is not exclusively located on the inner acrosomal membrane and, thus, could participate in physiologic events other than zona penetration.

Acrosin↗
PubMed1989

Endpoint of first stage of zona pellucida-induced acrosome reaction in mouse spermatozoa characterized by acrosomal H+ and Ca2+ permeability: population and single cell kinetics.

The acrosome reaction induced by the mouse egg's zona pellucida in mouse sperm has been shown to proceed in two stages as characterized empirically by sequential changes in patterns of chlortetracycline fluorescence on the sperm plasma membrane surfaces. The chlortetracycline fluorescence pattern characteristic of fully intact sperm is designated B; in sperm bound to structurally intact zonae that induce the acrosome reaction, the B pattern changes first to an intermediate pattern S and then to a terminal pattern AR characteristic of the completed acrosome reaction. In the same study, it was shown, using a 9-amino acridine fluorescent pH probe, that completion of the first stage was characterized by increase in H+ permeability such that the H+ gradient between sperm head and medium was dissipated. In this study, we show that the fluorescent pH probe 9-N-dodecylamino acridine and the intracellular Ca2+ fluorescent probe fura-2 are both localized to the anterior part of the sperm head encompassing the acrosomal compartment in intact sperm, and the fluorescence associated with each probe is lost as the first stage of the acrosome reaction is completed. Loss of the pH probe fluorescence, pattern N, corresponds to onset of H+ permeability, and loss of fura-2 fluorescence, pattern F, corresponds to onset of Ca2+ permeability. Localization of intracellular fura-2 fluorescence to the acrosomal compartment required extracellular Mn2+ to quench surface-bound fura-2 AM, the tetra-acetoxymethyl ester of fura-2 used to load the cells. Loss of acrosomal fura-2 fluorescence is due to quenching by tracer Mn2+ accompanying Ca2+. Onset of membrane permeability to both H+ and Ca2+, as seen by loss of patterns N and F, occurred in synchrony in populations of sperm bound to isolated, structurally intact zonae, with an overall time course of 210 min postbinding. The loss of pattern N in individual sperm cells bound to zonae was rapid, with a half time of 2.1 min. Concomitant with this rapid loss of pattern N was a shift in the amplitude of flagellar motion from large to small. The lag times to pattern N loss in 50 individual cells ranged from 30 to 140 min. The variable lag times determine the population kinetics; the rate of the endpoint reaction seen in the individual cells is rapid and constant. Dissipation of the H+ gradient with immediate loss of pattern N was readily achieved by addition of nigericin with no change in the time course of the onset of Ca2+ permeability of the membranes enclosing the acrosome.(ABSTRACT TRUNCATED AT 400 WORDS)

Acrosome↗
PubMed1993

Induction of the acrosome reaction in human spermatozoa by human zona pellucida and effect of cervical mucus on zona-induced acrosome reaction.

OBJECTIVES: To investigate the induction of the acrosome reaction in human spermatozoa by the zona pellucida (ZP), cumulus oophorus, and cervical mucus (CM), and to examine the effect of cumulus oophorus and CM on the zona-induced acrosome reaction. DESIGN, PATIENTS, MAIN OUTCOME MEASURES: The acrosome status of spermatozoa from healthy donors that were cultured with salt-stored human ZP, cumulus oophorus, and/or cervical mucus was assessed using fluoresceinated pisum sativum agglutinin. RESULTS: The acrosome reaction rate after sperm attachment to the zona for 6 hours was 35.7% +/- 17.7%, which was higher than controls (2.8% +/- 1.9%). The acrosome reaction was not observed after passage through cumulus oophorus or CM; however, the acrosome reaction rate of spermatozoa passed through CM after the attachment to zona was 51.6% +/- 6.8%, a higher value in comparison with spermatozoa that had not passed through CM (25.6% +/- 9.4%). CONCLUSION: These data indicate that human ZP are capable of inducing the acrosome reaction in human spermatozoa; however, cumulus oophorus and CM are unable to do so. The data further provide evidence that CM promotes the zona-induced acrosome reaction in human spermatozoa.

Acrosome↗
PubMed2002

Tracing sperm acrosome differentiation in the testis and maturation in the epididymis of the tammar wallaby (Macropus eugenii) with a 45-kDa acrosome-membrane-associated protein.

A 45-kDa protein was originally extracted from a depression, where the acrosome is lodged, on the anterior end of the sperm nucleus of ejaculated wallaby spermatozoa. Using immunofluorescent and confocal microscopes, this study demonstrates that the 45-kDa protein is persistently localized to the sperm acrosome throughout the periods of spermiogenesis, spermiation, epididymal maturation and ejaculation in the tammar wallaby. The distribution of the 45-kDa protein is always on the perimeter of the acrosome and associated with the acrosomal membrane, so that changes in the shape of the 45-kDa immunofluorescent labelling mirror changes in the shape of the acrosome during its differentiation in the testis and epididymis. Thus, the 45-kDa protein may be used as a molecular marker to study the marsupial acrosome differentiation and to chart the events of testicular and epididymal maturation of the spermatozoa. Furthermore, the behaviour of the 45-kDa protein during the immunostaining process suggests that this protein is a largely insoluble and detergent-resistant protein and may play an important role in attachment of the acrosome to the nucleus during sperm formation, similar to those inner acrosomal-membrane-associated proteins that have been reported in eutherian spermatozoa.

Acrosome↗
PubMed1992

Interactions of human sperm acrosomal protein SP-10 with the acrosomal membranes.

The interaction of the human acrosomal protein SP-10 with the acrosomal membranes was analyzed by the ability of Triton X-114 (TX-114) and other agents to release SP-10 from the acrosome. Treatment of human sperm with TX-114 revealed a pool of SP-10 that was released by TX-114 and a pool of SP-10 that was TX-114-resistant. TX-114-resistant SP-10 was associated with the equatorial segment and with TX-114-resistant portions of the acrosomal matrix and the inner acrosomal membrane. Phase partitioning of TX-114-released and TX-114-resistant SP-10 pools showed that both were hydrophilic, indicating that these pools consist of proteins that are peripherally associated with, rather than integral to, the acrosomal membranes. Sequential treatments of human sperm with various agents showed that repeated washes with TX-114 or 1.5 M NaCl had little or no effect on TX-114-resistant SP-10, whereas treatment with a chaotropic salt (150 mM sodium thiocyanate) and buffers at pH extremes (pH 2.0 and 10.0) completely released this pool of SP-10 from the acrosome. Together the results suggest that SP-10 is a hydrophilic peripheral acrosomal membrane protein that may be associated with a TX-114-resistant "anchor."

Acrosome↗
PubMed2000

Kinetics of onset of mouse sperm acrosome reaction induced by solubilized zona pellucida: fluorimetric determination of loss of pH gradient between acrosomal lumen and medium monitored by dapoxyl (2-aminoethyl) sulfonamide and of intracellular Ca(2+) changes monitored by fluo-3.

The onset of the zona pellucida-induced acrosome reaction in mouse sperm is marked by loss of the pH gradient existing in acrosome-intact sperm between the acidic acrosomal lumen and the suspending medium, due to pore formation between outer acrosomal and plasma membranes. In earlier work, it was shown that this pH gradient loss occurred in single sperm bound to structurally intact zonae pellucidae with a half-time of 2.1 min; the extended kinetics of this loss determined in a sperm population bound to intact zonae was due to a 180-min range of variable lag times. We hypothesized that this lag time range was due to steric constraints imposed by the three-dimensional structure of the structurally intact zona pellucida, and that this constraint should be removed in solubilized zonae. The fluorescent probe, Dapoxyl(TM) (2-aminoethyl)sulfonamide (DAES) allowed a test of this hypothesis in a population of sperm cells. It is a weak base that is non-fluorescent in aqueous solution, but which accumulates in the acidic acrosomal compartment due to the pH gradient with highly enhanced fluorescence; loss of the pH gradient leads to a decrease in fluorescence. The half-time for DAES fluorescence loss in a population of capacitated, acrosome-intact sperm in response to solubilized zona pellucida protein was 2.13 +/- 0.10 min (SEM, n = 9). The agreement between single cell and cell population kinetics validates the hypothesis of steric constraint in the structurally intact zona pellucida. The change in intracellular Ca(2+) concentration in response to solubilized zona pellucida, as monitored with fluo-3, was a rapid increase, followed by a decrease, with a half-time of 0.85 +/- 0.09 min (SEM, n = 6) to a steady state level higher than the initial level, indicating this Ca(2+) transient as the precursor reaction to onset of the zona-induced acrosome reaction.

Acrosome↗
PubMed2001

Calcium-binding proteins from the outer acrosomal membrane of ram spermatozoa: potential candidates for involvement in the acrosome reaction.

After an intracellular calcium influx, fusion of the sperm plasma membrane and outer acrosomal membrane (the acrosome reaction) precedes mammalian fertilization in vivo. This study describes the isolation of outer acrosomal membrane from ram spermatozoa and the subsequent characterization of calcium-binding proteins. Pooled ejaculates were diluted, cooled slowly and washed. Incubation with Hyamine 1622 (benzethenium chloride) and subsequent slow centrifugation gently dislodged and concentrated acrosomal membranes, the fragments of which were isolated on a two-step discontinuous sucrose gradient. The acrosomal membrane material stained with Giemsa, whereas spermatozoa from the gradient pellet stained intensely only in the equatorial segment. The acrosomal fraction showed a limited number of polypeptides by SDS-PAGE. Incubation with 45Ca2+ revealed two radioactive bands at 34 and 39 kDa. Extraction in the presence of EGTA implied that these proteins are not peripheral proteins associated with the membrane only in the presence of calcium ions, but are integral membrane proteins. Polyclonal antisera raised to the two bands showed specific binding to the anterior acrosomal region and demonstrated the intracellular location of the proteins. Sequence data of protein A revealed 83% homology with calnexin homologue precursor and 70% homology with annexin XI. Protein B showed 68% homology with protein SP-10 precursor and 64-72% homology with various annexins. However, crossreactivity with a range of commercial annexin antibodies and a specific antibody to a synthetic motif encompassing the annexin calcium-binding site was not demonstrable. It is concluded that the isolated proteins are unlikely to be annexins, but are possibly novel calcium-binding proteins.

Acrosome↗
PubMed1998

The biological and functional significance of the sperm acrosome and acrosomal enzymes in mammalian fertilization.

The mammalian spermatozoon undergoes continuous modifications during spermatogenesis, maturation in the epididymis, and capacitation in the female reproductive tract. Only the capacitated spermatozoa are capable of binding the zona-intact egg and undergoing the acrosome reaction. The fertilization process is a net result of multiple molecular events which enable ejaculated spermatozoa to recognize and bind to the egg's extracellular coat, the zona pellucida (ZP). Sperm-egg interaction is a species-specific event which is initiated by the recognition and binding of complementary molecule(s) present on sperm plasma membrane (receptor) and the surface of the ZP (ligand). This is a carbohydrate-mediated event which initiates a signal transduction cascade resulting in the exocytosis of acrosomal contents. This step is believed to be a prerequisite which enables the acrosome reacted spermatozoa to penetrate the ZP and fertilize the egg. This review focuses on the formation and contents of the sperm acrosome as well as the mechanisms underlying the induction of the acrosome reaction. Special emphasis has been laid on the synthesis, processing, substrate specificity, and mechanism of action of the acid glycohydrolases present within the acrosome. The hydrolytic action of glycohydrolases and proteases released at the site of sperm-zona binding, along with the enhanced thrust generated by the hyperactivated beat pattern of the bound spermatozoon, are important factors regulating the penetration of ZP. We have discussed the most recent studies which have attempted to explain signal transduction pathways leading to the acrosomal exocytosis.

Acrosome↗
PubMed1994

Calmodulin binding proteins in the membrane vesicles released during the acrosome reaction and in the perinuclear material in isolated acrosome reacted sperm heads.

Calmodulin has been suggested as the Ca(2+)-mediator in diverse cellular functions via its interaction with a number of proteins in a calcium-dependent manner. Its participation in the acrosome reaction has been suggested based on its localization in the acrosome region, on the effects produced by calmodulin antagonists, and by the changes in calmodulin compartmentation observed to occur throughout guinea pig acrosome reaction. To define the role of calmodulin in the membrane fusion events that occur during the acrosome reaction, the identification of calmodulin-binding proteins, by the overlay technique with biotinylated or unmodified calmodulin, was made in the following sperm fractions: in the membrane vesicles released during the acrosome reaction, in the remaining perinuclear material of acrosome reacted sperm heads and in a total membrane fraction from intact spermatozoa. The membrane vesicles released after the acrosome reaction showed four major calmodulin-binding proteins, M(r)s 66, 95, 97 and 110 kDa. The perinuclear material showed a 31-34, 43 and 97 kDa calmodulin-binding polypeptides. The membrane fraction from intact sperm showed eleven calmodulin-binding proteins, M(r)s between 14-110 kDa. Most of the binding proteins detected by this method corresponded to the class of calcium-independent calmodulin-binding proteins but proteins which only interacted with calmodulin in a calcium-inhibited mode were also observed. No calcium-dependent calmodulin-binding proteins were detected in any of the fractions studied. A possible role of these binding proteins in calmodulin compartmentation is discussed. The potential role of these binding proteins in membrane fusion and in membrane receptor localization in the postacrosomal region remain to be defined.

Acrosome↗
PubMed1998

Acrosin accelerates the dispersal of sperm acrosomal proteins during acrosome reaction.

Using homologous recombination, we have previously produced male mice carrying a disruptive mutation (Acr-/-) in the acrosin gene. Although Acr-/- mouse sperm lacking the acrosin protease activity still penetrated the zona pellucida and fertilized the egg, the mutant sperm exhibited a delay in penetration of the zona pellucida solely at the early stages after insemination. To further elucidate the role of acrosin in fertilization, we have examined the involvement of acrosin in the acrosome reaction of sperm using the Acr-/- mutant mice. When the ability of sperm to adhere (attach) and bind to the zona pellucida of cumulus-free eggs was assessed in vitro, no significant difference was observed among Acr+/+, Acr+/-, and Acr-/- mouse sperm. Immunocytochemical analysis demonstrated that the release of several acrosomal proteins from the acrosome of Acr-/- mouse sperm was significantly delayed during the calcium ionophore- and solubilized zona pellucida-induced acrosome reaction, despite normal membrane vesiculation. These data indicate that the delayed sperm penetration of the zona pellucida in the Acr-/- mouse results from the altered rate of protein dispersal from the acrosome and provide the first evidence that the major role of acrosin is to accelerate the dispersal of acrosomal components during acrosome reaction.

Acrosin↗
PubMed1985

Acrosomal reaction of the Thyone sperm. III. The relationship between actin assembly and water influx during the extension of the acrosomal process.

In an attempt to investigate the role of water influx in the extension of the acrosomal process of Thyone sperm, we induced the acrosomal reaction in sea water whose osmolarity varied from 50 to 150% of that of sea water. (a) Video sequences of the elongation of the acrosomal processes were made; plots of the length of the acrosomal process as a function of (time)1/2 produced a straight line except at the beginning of elongation and at the end in both hypotonic and hypertonic sea water (up to 1.33 times the osmolarity of sea water), although the rate of elongation was fastest in hypotonic sea water and was progressively slower as the tonicity was raised. (b) Close examination of the video sequences revealed that regardless of the tonicity of the sea water, the morphology of the acrosomal processes were similar. (c) From thin sections of fixed sperm, the amount of actin polymerization that takes place is roughly coupled to the length of the acrosomal process formed so that sperm with short processes only polymerize a portion of the actin that must be present in those sperm. From these facts we conclude that the influx of water and the release of actin monomers from their storage form in the profilactin (so that these monomers can polymerize) are coupled. The exact role of water influx, why it occurs, and whether it could contribute to the extension of the acrosomal process by a hydrostatic pressure mechanism is discussed.

Acrosome↗
PubMed1998

The effects of brefeldin A on acrosome formation and protein transport to the acrosome in organ cultures of rat seminiferous tubules.

To investigate the contribution of vesicular transport to acrosome formation, we examined the effect of brefeldin A (BFA), a potent inhibitor of membrane traffic, on the ultrastructural changes of the organella in early spermatids. The experiments were done in an organ culture system of rat seminiferous tubules. After exposure to BFA, the fuzzy-coated projections of the endo-plasmic reticulum (ER) cisternae directing toward the Golgi stack disappeared. The Golgi stacks were also disrupted, forming numerous small vesicles and tubular networks. The head-cap was fragmented in step 4-7 spermatids. We also investigated the effect of BFA on protein transport to the acrosome by using a monoclonal antibody to the acrosomal protein MN7 (mMN7) as a model. After a 5-h exposure, the mMN7 immunoreactivity was absent from the developing acrosomic system in step 2-3 spermatids. These results indicate that BFA blocks the vesicular transport, causing an insufficient supply of acrosomal materials to the acrosome, which eventually leads to abnormal acrosome formation in early spermiogenesis.

Acrosome↗
PubMed1994

A domain-specific 50-kilodalton structural protein of the acrosomal matrix is processed and released during the acrosome reaction in the guinea pig.

The sperm acrosome contains a variety of hydrolytic enzymes that exhibit differential release during the acrosome reaction. The interaction between hydrolases and the acrosomal matrix, and their compartmentalization into chemically unique matrix domains may represent mechanisms by which this ordered release is achieved. This study characterizes an acrosomal matrix protein that is restricted to the ventral-most region of the apical segment of guinea pig cauda epididymal spermatozoa. Polyclonal antiserum, prepared against an SDS-PAGE-purified apical segment protein, recognized a 50-kDa band, termed AM50, on Western blots. Native AM50 is resistant to solubilization. However, during the acrosome reaction, AM50 is converted into a 42-43-kDa doublet protein (AM50AR) and released into the incubation medium. AM50 and AM50AR are not recognized by antiserum to guinea pig proacrosin and do not exhibit protease activity in a gelatinolytic SDS-PAGE assay. However, AM50AR does bind to p-aminobenzamidine affinity columns, suggesting that it may remain associated with proteases after the acrosome reaction. Light and electron microscopic immunocytochemistry established that AM50 was exclusively localized to the ventral aspect of the apical segment of the acrosome.

Acrosin↗
PubMed1995

Fertilization and pregnancy with acrosome intact sperm by intracytoplasmic sperm injection in patients with disordered zona pellucida-induced acrosome reaction.

OBJECTIVE: To determine whether fertilization and pregnancy could be obtained by injection of acrosome-intact sperm into the cytoplasm in patients with persistent failure of fertilization in IVF-ET associated with disordered zona pellucida (ZP)-induced acrosome reaction (AR). DESIGN: Sperm-ZP binding and penetration and ZP-induced AR were compared between patients and fertile donors. Acrosome-intact sperm removed from the ZP were injected into the cytoplasm of the oocytes. SETTING: Reproductive Biology Unit, Royal Women's Hospital. MAIN OUTCOME MEASURES: The proportion of ZP penetrated and ZP-induced AR and fertilization and pregnancy after intracytoplasmic sperm injection were analyzed. RESULTS: Most (7/8) of the patients had consistently normal sperm characteristics including concentration, motility and velocity, acrosomes, and morphology. Mean number of sperm bound to the ZP was not significantly different between patients (97 sperm/ZP) and fertile donor controls (100 sperm/ZP). However, AR of sperm bound to the ZP was significantly lower in the patients (4%) than in controls (61%). None of the ZP (n = 32) were penetrated by patient sperm whereas all (n = 32) ZP were penetrated by control sperm. Acrosome intact sperm removed from the ZP were used for intracytoplasmic sperm injection. All patients had > or = 50% normal fertilization and embryo development. Three pregnancies (one early aborted, two ongoing) were achieved after the transfer of fresh or frozen embryos. CONCLUSION: Acrosome-intact human sperm can produce a high fertilization rate and pregnancies after intracytoplasmic sperm injection in patients with disordered ZP-induced AR. The acrosome reaction is unlikely to be important for fertilization with intracytoplasmic sperm injection.

Acrosome↗
PubMed1998

Antibody against 28-kDa intra-acrosomal sperm protein as a tool for evaluation of acrosomal integrity in bull spermatozoa.

Monoclonal antibody ACR.4 recognizing specifically the 28-kDa intra-acrosomal protein was prepared by immunization of mice with acetic acid extract of boar spermatozoa, but cross-reacted also with bull intra-acrosomal protein. This monoclonal antibody was used for immunostaining analysis of bull spermatozoa before and during capacitation and ionophore-induced acrosome reaction. Immunostaining analysis showed changes of 28-kDa protein in the acrosome during capacitation and loss of this protein after induced acrosome reaction by ionophore A23187. Therefore, this monoclonal antibody can be used in the bull spermatozoa as an immunological test for detection of the acrosome state after manipulation with spermatozoa or after freezing/thawing. This test could be useful (apart from morphology and motility) for the selection of suitable spermatozoa for insemination or in vitro fertilization.

Acrosome↗
PubMed1996

Lectin histochemistry during in vitro capacitation and acrosome reaction in boar spermatozoa: new lectins for evaluating acrosomal status of boar spermatozoa.

Six peroxidase (HRP)-conjugated lectins have been used to analyze the distribution of carbohydrates in ejaculated boar spermatozoa during in vitro capacitation and acrosome reaction in air-dried preparations. The membranes of boar spermatozoa were positive to WGA and Con-A in the acrosome, middle piece and principal piece. PNA labelled the acrosomal region. DBA, UEA-I and LTA bound weakly or not to the spermatozoa. The lectin labelling pattern did not change during the in vitro capacitation. A diminution in the proportion of spermatozoa with acrosome positive to lectins PNA, WGA and Con-A was observed after coincubation with homologous immature oocytes. A positive correlation existed between the percentages of lectins bound to spermatozoa and the triple stain technique. The results show that peroxidase-conjugated lectins may be useful in determining acrosomal integrity of boar spermatozoa during in vitro capacitation and acrosome reaction.

Acrosome↗
PubMed1995

Liposome fusion induced by a M(r) 18,000 protein localized to the acrosomal region of acrosome-reacted abalone spermatozoa.

A M(r) 18,000 protein is secreted by abalone spermatozoa during the acrosome reaction. Immunofluorescence of acrosome-reacted sperm localizes the protein as a coating on the spent acrosomal granule hull and on the surface of the acrosomal process. The membrane of the acrosomal process fuses with the egg plasma membrane at fertilization. The M(r) 18,000 acrosomal protein aggregates negatively charged (but not neutral) large unilamellar liposomes and renders them permeable to internal probe. The M(r) 18,000 proteins from two abalone species are potent inducers of intervesicular lipid mixing in the resonance energy transfer assay, suggesting that they mediate the fusion of lipid bilayers. Predicted secondary structures of these proteins show the presence of strongly amphipathic alpha-helices that may be active in the perturbation of phospholipid bilayers. The M(r) 18,000 protein may mediate sperm-egg fusion during fertilization.

Acrosome↗