Sexual agglutinins from the Chlamydomonas flagellar membrane. Partial purification and characterization

Primary cilia utilize glycoprotein-dependent adhesion mechanisms to stabilize long-lasting cilia-cilia contacts

Background

The central tenet of cilia function is sensing and transmitting information. The capacity to directly contact extracellular surfaces would empower primary cilia to probe the environment for information about the nature and location of nearby surfaces. It has been well established that flagella and other motile cilia perform diverse cellular functions through adhesion. We hypothesized that mammalian primary cilia also interact with the extracellular environment through direct physical contact.

Methods

We identified cilia in rod photoreceptors and cholangiocytes in fixed mouse tissues and examined the structures that these cilia contact in vivo. We then utilized an MDCK cell culture model to characterize the nature of the contacts we observed.

Results

In retina and liver tissue, we observed that cilia from nearby cells touch one another. Using MDCK cells, we found compelling evidence that these contacts are stable adhesions that form bridges between two cells, or networks between many cells. We examined the nature and duration of the cilia-cilia contacts and discovered primary cilia movements that facilitate cilia-cilia encounters. Stable adhesions form as the area of contact expands from a single point to a stretch of tightly bound, adjacent cilia membranes. The cilia-cilia contacts persisted for hours and were resistant to several harsh treatments such as proteases and DTT. Unlike many other cell adhesion mechanisms, calcium was not required for the formation or maintenance of cilia adhesion. However, swainsonine, which blocks maturation of N-linked glycoproteins, reduced contact formation. We propose that cellular control of adhesion maintenance is active because cilia adhesion did not prevent cell division; rather, contacts dissolved during mitosis as cilia were resorbed.

Conclusions

The demonstration that mammalian primary cilia formed prolonged, direct, physical contacts supports a novel paradigm: that mammalian primary cilia detect features of the extracellular space, not just as passive antennae, but also through direct physical contact. We present a model for the cycle of glycoprotein-dependent contact formation, maintenance, and termination, and discuss the implications for potential physiological functions of cilia-cilia contacts.

Evidence for rosettes as an unrecognized stage in the life cycle of Leishmania parasites

Leishmania parasites, which afflict 12 million people in 88 countries, exist as promastigotes transmitted by insect vectors and as amastigotes residing in mammalian macrophages. Promastigote cells arranged in rosettes have also been described but universally disregarded as a distinct stage in the life cycle. We present evidence that only rosettes of Leishmania major promastigotes express cell surface poly-alpha2,8 N-acetyl neuraminic acid (PSA) and PSA containing de-N-acetyl neuraminic acid (NeuPSA). Expression of rosette-specific PSA antigens was mosaic, with individual promastigotes expressing PSA, NeuPSA or both. A 50 kDa protein was detected by Western blot analysis of a detergent-insoluble cell fraction with both PSA and NeuPSA-reactive antibodies. Frequencies of rosette formation as well as cell surface PSA/NeuPSA expression were temperature dependent. Rosettes also engaged in an unusual swarming behavior, congregating into extended clusters. Distinct structures resembling cellular fusion bodies were formed in and released from rosettes. The results indicate that rosettes are an unrecognized stage in the life cycle of Leishmania. We hypothesize that rosettes initiate mating in Leishmania during which PSA/NeuPSA expression plays an important role. Recognizing rosettes as a distinct form of the Leishmania life cycle opens new possibilities for treatment or prevention of disease and, possibly, in vitro genetic recombination without passage of cells through insect vectors.

Uniparental inheritance of cpDNA and the genetic control of sexual differentiation in Chlamydomonas reinhardtii

An intriguing feature of most eukaryotes is that chloroplast (cp) and mitochondrial (mt) genomes are inherited almost exclusively from one parent. Uniparental inheritance of cp/mt genomes was long thought to be a passive outcome, based on the fact that eggs contain multiple numbers of organelles, while male gametes contribute,at best, only a few cp/mtDNA. However, the process is likely to be more dynamic because uniparental inheritance occurs in organisms that produce gametes of identical sizes (isogamous). In Chlamydomonas reinhardtii,the uniparental inheritance of cp/mt genomes is achieved by a series of mating type-controlled events that actively eliminate the mating type minus (mt-) cpDNA.The method by which Chlamydomonas selectively degrades mt- cpDNA has long fascinated researchers, and is the subject of this review.

Gametic cell adhesion and fusion in the unicellular alga Chlamydomonas

Differentiation of vegetative cells of the haploid eukaryote Chlamydomonas is dependent on environmental conditions. Upon depletion of nitrogen and exposure to light, vegetative cells undergo a mitotic division, generating gametes that are either mating-type plus (mt[+]) or mating-type minus (mt[-]). As gametes of opposite mating type encounter one another, an initial adhesive interaction mediated by flagella induces a signal transduction pathway that results in activation of gametes. Gametic activation results in the exposure of previously cryptic regions of the plasma membrane (mating structures) that contain the molecules required for gametic cell adhesion and fusion. Recent studies have identified new steps in this signal transduction pathway, including the tyrosine phosphorylation of a cyclic guanosine monophosphate-dependent protein kinase, a requirement for a novel microtubular motility known as intraflagellar transport, and a mt(+)-specific molecule that mediates adhesion between mating structures.

The transcriptional program of synchronous gametogenesis in Chlamydomonas reinhardtii

Cells of Chlamydomonas reinhardtii undergo gametogenesis to produce sexually competent gametes under nitrogen-starved conditions. By using a synchronized system for gametogenesis of early G1 cells, several previously identified marker genes and 18 novel nitrogen-starved gametogenesis (NSG) genes isolated by macroarray analysis were placed into at least three temporal classes of expression. Early genes are induced transiently in the first 2 h after transfer to nitrogen-free medium. Middle genes are strongly induced between 3 h and 4 h after nitrogen removal, a time corresponding to the acquisition of mating competency, suggesting their involvement in the gamete program. Late genes are induced between 5 h and 8 h after nitrogen removal, a time after the completion of gametic differentiation, suggesting that they are not directly involved in the formation of sexually competent gametes. All of the 18 NSG genes examined are induced in both mating-type plus and minus gametes and about two-thirds of the genes are also expressed in the mitotic cell cycle, especially at S/M phases.

Plus and minus sexual agglutinins from Chlamydomonas reinhardtii

Gametes of the unicellular green alga Chlamydomonas reinhardtii undergo sexual adhesion via enormous chimeric Hyp-rich glycoproteins (HRGPs), the plus and minus sexual agglutinins, that are displayed on their flagellar membrane surfaces. We have previously purified the agglutinins and analyzed their structural organization using electron microscopy. We report here the cloning and sequencing of the Sag1 and Sad1 genes that encode the two agglutinins and relate their derived amino acid sequences and predicted secondary structure to the morphology of the purified proteins. Both agglutinin proteins are organized into three distinct domains: a head, a shaft in a polyproline II configuration, and an N-terminal domain. The plus and minus heads are related in overall organization but poorly conserved in sequence except for two regions of predicted hydrophobic alpha-helix. The shafts contain numerous repeats of the PPSPX motif previously identified in Gp1, a cell wall HRGP. We propose that the head domains engage in autolectin associations with the distal termini of their own shafts and suggest ways that adhesion may involve head-head interactions, exolectin interactions between the heads and shafts of opposite type, and antiparallel shaft-shaft interactions mediated by carbohydrates displayed in polyproline II configurations.

The regulatory networks of gene expression during the sexual differentiation of Chlamydomonas reinhardtii, as analyzed by mutants for gametogenesis

Cells of Chlamydomonas reinhardtii differentiate into gametes under conditions of nitrogen (N) starvation, expressing the genes for the N-adaptation program and the gamete program. To investigate the regulatory networks of transcription among the N-starvation-inducible genes, we examined the gene expression in dif mutants, affecting gametic differentiation. In a conditional mutant, dif2, the cells remained 'vegetative' at the restrictive temperature, and the induction of 20 out of 21 genes related to the two programs was impaired. They were expressed soon after transfer of the cells to the permissive temperature, in parallel with the acquisition of mating ability. In an unconditional mutant, dif3, the cells could not differentiate into gametes at all, but the induction of only four genes (FUS1, NSG3, NSG6 and NSG7) related to the gamete program was impaired. The results suggest that Dif3 regulates putative N-starvation signal transduction pathways downstream of a master regulator, Dif2. We also examined a light-dependent laboratory strain that was unable to become gametes in the dark. The 'pre-gametes' placed in the dark, however, could induce normally all of the 21 genes, suggesting that light is required for the gametic differentiation at the translational and/or post-translational levels.

Signal transduction during fertilization in the unicellular green alga, Chlamydomonas

Sexual reproduction in the green alga, Chlamydomonas, is regulated by environmental conditions and by cell-cell interactions. After gametogenesis, flagellar adhesion between gametes triggers gamete activation, leading to cell fusion and zygote formation. Recent studies have identified new molecular events that underlie signal transduction during Chlamydomonas fertilization, including expression of a sex-determining protein, phosphorylation of a homeodomain protein, activity of a kinesin II and regulated translocation of an aurora/Ip11-like protein kinase from the cell body to the flagella.

Flagellar adhesion between mt(+) and mt(-) Chlamydomonas gametes regulates phosphorylation of the mt(+)-specific homeodomain protein GSP1

During fertilization in Chlamydomonas, flagellar adhesion between mt(+) and mt(-) gametes induces a cAMP-dependent signal transduction pathway that prepares the gametes for cell fusion and zygote formation. Previously, our laboratory identified a homeodomain protein (GSP1) whose expression was restricted to the cell bodies of mt(+) gametes and whose transcript level was up-regulated during flagellar adhesion. In this report, we describe a new form of GSP1 that appears early during gamete interactions. Immunoblot analysis showed that in addition to the 120-kDa form of GSP1 normally present in mt(+) gametes, a 122-kDa form was detected when the cells were mixed with mt(-) gametes. The more slowly migrating form of GSP1 was detectable within minutes after gametes were mixed together, and its appearance did not require new protein synthesis. Thus, the 122-kDa form represents a post-translational modification of the pre-existing 120-kDa form of GSP1. Moreover, conversion to the 122-kDa form did not require cell fusion. Although the 120-kDa form was expressed 10 h after vegetative cells were transferred to gametic induction medium, the 122-kDa form was detected only after mt(+) gametes were induced to undergo the sexual signaling that accompanies fertilization. Incubation of mt(+) gametes with dibutyryl cAMP led to the appearance of the 122-kDa form of GSP1, and the cyclic nucleotide-dependent protein kinase inhibitor H-8 inhibited the adhesion-induced conversion. Incubation of GSP1 immunoprecipitated from signaling mt(+) gametes with alkaline phosphatase showed that the conversion was due to phosphorylation. The results indicate that flagellar adhesion induces a rapid, cAMP-dependent phosphorylation of the homeodomain protein GSP1 early during fertilization in Chlamydomonas.

The Chlamydomonas mating type plus fertilization tubule, a prototypic cell fusion organelle: isolation, characterization, and in vitro adhesion to mating type minus gametes

In the biflagellated alga Chlamydomonas, adhesion and fusion of the plasma membranes of gametes during fertilization occurs via an actin-filled, microvillus-like cell protrusion. Formation of this approximately 3-microm-long fusion organelle, the Chlamydomonas fertilization tubule, is induced in mating type plus (mt+) gametes during flagellar adhesion with mating type minus (mt-) gametes. Subsequent adhesion between the tip of the mt+ fertilization tubule and the apex of a mating structure on mt- gametes is followed rapidly by fusion of the plasma membranes and zygote formation. In this report, we describe the isolation and characterization of fertilization tubules from mt+ gametes activated for cell fusion. Fertilization tubules were detached by homogenization of activated mt+ gametes in an EGTA-containing buffer and purified by differential centrifugation followed by fractionation on sucrose and Percoll gradients. As determined by fluorescence microscopy of samples stained with a fluorescent probe for filamentous actin, the method yielded 2-3 x 10(6) fertilization tubules/microg protein, representing up to a 360-fold enrichment of these organelles. Examination by negative stain electron microscopy demonstrated that the purified fertilization tubules were morphologically indistinguishable from fertilization tubules on intact, activated mt+ gametes, retaining both the extracellular fringe and the internal array of actin filaments. Several proteins, including actin as well as two surface proteins identified by biotinylation studies, copurified with the fertilization tubules. Most importantly, the isolated mt+ fertilization tubules bound to the apical ends of activated mt- gametes between the two flagella, the site of the mt- mating structure; a single fertilization tubule bound per cell, binding was specific for gametes, and fertilization tubules isolated from trypsin-treated, activated mt+ gametes did not bind to activated mt- gametes.

Signal transduction in the sexual life of Chlamydomonas

Several signal transduction pathways play important roles in the sexual life cycle of Chlamydomonas. Nitrogen deprivation, perhaps sensed as a drop in intracellular [NH4+], triggers a signal transduction pathway that results in altered gene expression and the induction of the gametogenic pathway. Blue light triggers a second signalling cascade which also culminates in gene induction and completion of gametogenesis. New screens have uncovered several mutants in these pathways, but so far we know little about the biochemical events that transduce the environmental signals of nitrogen deprivation and blue light into the changes in gene transcription that produce gametes. Cell-cell contact of mature, complementary gametes elicits a number of responses that prepare the cells for fusion. Contact is sensed by the agglutinin-mediated cross-linking of flagellar membrane proteins. An increase in [cAMP] couples protein cross-linking to the mating responses. In C. reinhardtii the cAMP signal appears to be generated by the sequential stimulation of as many as 3 distinct adenylyl cyclase activities. Although the molecular mechanisms of adenylyl cyclase activations are poorly understood, Ca2+ may play a role. Most of the mating responses appear to be triggered by a cAMP-dependent protein kinase, but here too, Ca2+ may play a role. Numerous mutants are facilitating studies of the signalling pathways that trigger the mating responses. Cell fusion triggers another series of events that culminate in the expression of zygote specific genes. The mature zygote is sensitive to a light signal which stimulates the expression of genes whose products are essential for germination. The signal transduction pathways that trigger zygospore formation and germination are ripe for investigation in this experimentally powerful system.

Dissection of eukaryotic transmembrane signalling using Chlamydomonas

Novel insights and surprises are often generated when investigators choose an organism that permits a new approach to a problem. For example, secretory and cell-cycle mutants in yeast have provided quantum leaps in elucidating these processes. Similarly, genetic systems are providing exciting new insights into signal transduction. The 'green yeast' Chlamydomonas has the potential to be a particularly rich organism for genetic analysis of signal transduction because, although unicellular, it has several interesting behaviours, which are discussed in this article by Lynne Quarmby and Criss Hartzell. Phototaxis results from the transduction of a light signal received by the eyespot to changes in flagellar beat. The mating reactions, which culminate in the fusion of gametes, are initiated in response to adhesion of flagellar proteins. Deflagellation, or flagellar shedding, is an acute response to a variety of stimuli. Molecular genetic analysis of behavioural mutants is providing new directions for understanding signal integration and segregation.

Tipping of flagellar agglutinins by gametes of Chlamydomonas reinhardtii

The migration of cross-linked agglutinins to the gametic flagellar tips (tipping) is a hallmark of the Chlamydomonas mating reaction. In this study, an assay was developed to analyze the kinetics and biological requirements for the tipping response: isolated flagella from mt- gametes of C. reinhardtii were allowed to agglutinate to the immotile flagella of pf-18 mt+ gametes, and their migration to the tips was monitored by phase microscopy. The tipping process is shown to require both adhesion and elevated levels of cAMP. The cAMP may activate tipping motors directly. In addition, cAMP stimulates the recruitment of agglutinins to the flagellar surface to replace those inactivated by adhesion. These results are compared with previous studies on the tipping of flagellar surface proteins cross-linked by soluble ligands, and an integrated model is presented.

Isolation and purification of N-formylmethionine aminopeptidase from rat intestine

The intestinal mucosal epithelium is exposed to products of intestinal bacteria including potent inflammatory N-formylmethionyl oligopeptides. An N-formylmethionine aminopeptidase has been purified 2300-fold from rat intestine and was shown to degrade natural fMet oligopeptides from Escherichia coli culture supernatants with loss of bioactivity (release of specific granule constituents from human polymorphonuclear leucocytes) and immuno-reactivity (assessed using a polyclonal anti-fMet-Leu-Phe antiserum). The enzyme which was specific for N-terminal acyl-methionine residues had a native Mr of 340,000 and comprised four sub-units of Mr 82,000. The presence of this enzyme in intestinal mucosa could prevent absorption of intact bioactive fMet peptides produced by commensal bacteria in the gut lumen.

Rapid and slow mechanisms for loss of cell adhesiveness during fertilization in Chlamydomonas

Although vegetative cells, gametes, and zygotes of the biflagellated alga Chlamydomonas bear flagella, only the flagella of mt+ and mt- gametes are adhesive. The molecules responsible for adhesiveness, mt+ and mt- agglutinins, are long rod-shaped glycoproteins displayed on the flagellar membrane. These flagellar agglutinins, which gametes use both as adhesion and signaling molecules during the early events of fertilization, are lost from the flagella during adhesion. Flagellar adhesiveness can be maintained, however, by recruitment and activation of preexisting, inactive agglutinins from the plasma membrane of the cell body (Hunnicutt et al, 1990, J. Cell Biol. 111, 1605-1616) unless the gametes of opposite mating types fuse to form zygotes. Upon cell fusion, flagellar adhesiveness is lost. In the studies presented here, we have employed an in vitro bioassay to measure agglutinins in both cell bodies and flagella at various times during gametogenesis, during fertilization, and after zygote-formation. By use of the bioassay, which can detect agglutinins that are functionally inactive in vivo, we found that vegetative cells are devoid of agglutinins. These adhesion molecules appear only after gametogenesis is underway with the cell body agglutinins appearing first and then the flagellar agglutinins. Surprisingly, 30 min after zygote formation, when the zygotes' flagella are no longer adhesive, the flagellar agglutinin activity detectable with the bioassay remains high. One interpretation of these results is that zygotes continue to recruit agglutinins from the cell body to the flagella, but cell fusion abrogates activation of the agglutinins. Within 45-90 min after fusion both the cell body and flagellar agglutinins are lost and can be detected in the medium. These mechanisms, which render the zygotes nonadhesive to other zygotes and unmated gametes, contribute to the Chlamydomonas equivalent of a block to polyspermy.

Isolation and characterization of Chlamydomonas temperature-sensitive mutants affecting gametic differentiation under nitrogen-starved conditions

Two conditional mutants of Chlamydomonas reinhardtii, dif-1 and dif-2, affecting gametic differentiation under conditions of nitrogen (N)-starvation, have been isolated. These mutant cell remain "vegetative" at the restrictive temperature (35 degrees C) in -N medium, as defined by assays of cell body-agglutinin and cell wall lytic enzyme activities in the soluble fractions of cell homogenates. Moreover, the mutants fail to form mating structures at the restrictive temperature, but do so at the permissive temperature (25 degrees C). Temperature-shift experiments show that mutant cells which have differentiated into gametes at 25 degrees C dedifferentiate into "vegetative" cells under N-starvation conditions after transfer to 35 degrees C, but differentiate again into gametes at 25 degrees C. Genetic analyses indicate that the dif-1 and dif-2 genes are recessive and unlinked to each other or to the mating-type locus; the dif-1 phenotype cosegregates with a conditional flagellaless phenotype expressed in both +N and -N medium at the restrictive temperature.

The cell wall of Chlamydomonas reinhardtii gametes: Composition, structure and autolysin-mediated shedding and dissolution

The cell walls of Chlamydomonas gametes are multilayered structures supported on frameworks of polypeptides extending from the plasma membrane. The wall-polypeptide catalogue reported by Monk et al. (1983, Planta 158, 517-533) and extended by U.W. Goodenough et al. (1986, J. Cell Biol. 103, 405-417) was re-evaluated by comparative analysis of mechanically isolated cell walls purified from several strains. The extracellular locus of wall polypeptides was verified by in vivo iodogen-catalysed iodination and by autolysin-mediated elimination of the bulk of these polypeptides from the cell surface. Three (w15, w16, w17) and possibly four (w14) polypeptides were located to the most exterior aspect of the wall because of their susceptibility to Enzymobeadcatalysed iodination and their retention by a cell-wall-less mutant. The composition of shed walls stabilised with ethylenediaminetetraacetic acid during natural mating and kinetic analysis of the dissolution of walls purified from a bald-2 mutant demonstrated the rapid and specific destruction of polypeptide w3. Differential solubilisation of wall polypeptides occurred after loss of w3. Wall dissolution, characterised by the generation of fishbone structures from the W2 layer, gave as many as four additional polypeptides. Charged detergents and sodium perchlorate extracted a comparable range of polypeptides at room temperature from mechanically isolated walls, i.e. components of the W4-W6 layers, hot sodium dodecyl sulphate solubilised framework polypeptides, while reducing agent was required to solubilise the W2 layer. A model of wall structure is presented.

High-pressure liquid chromatography fractionation of Chlamydomonas dynein extracts and characterization of inner-arm dynein subunits

A rapid procedure for fractionating salt-stable dynein subunits from high-salt extracts of Chlamydomonas axonemes has been developed using a high-pressure liquid chromatography system with an anion exchange column and gradient salt elution. Five distinct fractions are shown to be highly enriched for five distinct subunits or subunit complexes by SDS/polyacrylamide gel electrophoresis. ATPase activity and electron microscopy. Peaks 1 and 4 contain, respectively, the single-headed gamma-subunit and the two-headed alpha/beta-heteropolymer that form the outer arm in situ and are dissociated by salt exposure; both peaks are absent from the outer arm-less mutant pf-28. Peaks 2, 3 and 5 contain, respectively, two distinct single-headed species and a double-headed species that derive from inner arms; all three peaks are missing from the inner arm-less mutant pf-23. Sucrose-gradient sedimentation analysis confirms these assignments and provides additional information on the intermediate-chain and light-chain composition of the inner-arm species. Electron microscopy of the purified inner-arm species visualized by the quick-freeze deep-etch technique complements a previous analysis of outer-arm species. Each protein is shown to have a unique morphology, and both the inner- and outer-arm proteins clearly belong to a common family whose structural divergence presumably reflects functional specialization.

Composition and properties of the sexual agglutinins of the flagellated green alga Chlamydomonas eugametos

Sexual interaction between gametes of opposite mating type (mt) of the unicellular green alga Chlamydomonas eugametos starts with agglutination of the cells via particular glycoproteins on the flagellar surface. Purification of these socalled agglutinins was achieved by a three-step procedure consisting of, successively, gel filtration, anion-exchange chromatography, and high-performance gel filtration. The amino-acid and sugar compositions of both agglutinins showed a high degree of similarity; the most prominent amino acids were hydroxyproline, serine and glycine, and the main sugars were arabinose and galactose. The carbohydrate portions represented about half of the molecular mass of both agglutinins. Using high-performance gel filtration, a calibration curve was constructed for high-molecular-mass compounds from which the Stokes' radius of the sexual agglutinins could be estimated. The mt (+) agglutinin had a Stokes' radius of 39 nm and a sedimentation coefficient of 9.3 S. From these data its molecular mass was estimated to be 1.2·10(6). The corresponding data for the mt (-) agglutinin were 38 nm, 9.7 S and 1.3·10(6), respectively. The biological activity of both agglutinins was destroyed by mild periodate treatment. Treatment with specific glycosidases had a differential effect on the biological activity of the agglutinins. These observations indicate that carbohydrate side-chains are needed for biological activity and perhaps are responsible for the specifity of the sexual agglutinins. A comparison of both agglutinins is given and their possible structure is discussed in relation to their amino-acid and sugar compositions.

Experimental analysis of the adhesion reaction between isolated Chlamydomonas flagella

Protocols are presented for producing reproducible in vitro agglutination between Chlamydomonas flagella, based on previous studies by Kohle et al. The results indicate that the adhesive interactions occur between glutaraldehyde cross-linked flagella and can be cross-linked by glutaraldehyde. They are sensitive to divalent cations and, to a lesser extent, to monovalent ions, an inhibition that is fully reversed when the ions are removed. The ambiguities inherent in in vivo studies, particularly in a system where the adhesion molecules have been shown to be in a dynamic state of turnover and inactivation, are eliminated, and numerous studies involving enzyme digestion, antibodies, lectins and the like should now be possible with this adhesion system.

Nature of the mouse egg's receptor for sperm

The zona pellucida is an extracellular coat that surrounds all mammalian eggs. Sperm must penetrate the zona pellucida in order to reach and fuse with the plasma membrane of unfertilized eggs. Penetration is accomplished by a sequence of events involving both egg and sperm. First, sperm must bind to the outer margin of the zona pellucida. Such binding is mediated in a relatively species-specific manner by "sperm receptors" in the zona pellucida. Second, sperm must undergo the "acrosome reaction", a membrane fusion event, in order to traverse the zona pellucida. Here we review results from our own laboratory which demonstrate that, during the course of sperm-egg interaction in mice, zona pellucida glycoprotein ZP3 serves as both receptor for sperm and inducer of the acrosome reaction. Furthermore, we review evidence from our laboratory indicating that the sperm receptor activity of ZP3 is dependent only on its 0-linked carbohydrate components, whereas acrosome reaction-inducing activity is dependent on the polypeptide portion of ZP3 as well.

Intracellular calcium redistribution during mating in Chlamydomonas reinhardii

Gametes of the unicellular green alga Chlamydomonas reinhardii recognize and adhere to cells of the opposite mating type by flagellar contact. Adhesion between these specialized organelles signals a rapid series of mating events which result in gamete fusion. The sequence of morphological changes (flagellar tip activation, cell wall loss, and mating structure elongation), which occur as a consequence of the sexual signalling, have been characterized. The signalling mechanisms have, however, not been defined. Calcium is known to be involved during fertilization of animal species. Increased intracellular free calcium, which can be achieved either by calcium influx or by mobilization of ions from intracellular stores, has been observed during activation of both eggs and sperm. A recent report by Bloodgood & Levin that gametes of C. reinhardii preloaded with 45Ca showed a transient increase in Ca efflux following mating, suggests that intracellular Ca redistribution may also accompany mating in this algal species. We have used X-ray microanalysis to analyze the subcellular distribution of bound calcium during mating in Chlamydomonas reinhardii. X-ray maps reveal that calcium is sequestered in discrete granules within the gamete cell body prior to mating and that during activation and cell fusion, calcium is diffuse throughout the cell. This suggests the possibility that calcium serves as a second messenger in this species.

Cell surface carbohydrates and cell recognition in Dictyostelium

Carbohydrate ligands and complementary receptors have been detected on the surface of Dictyostelium cells, using lectins, monoclonal antibodies, and immobilized sugar probes. They have been implicated in cell recognition processes, such as phagocytosis and intercellular adhesion, and could act as membrane signals for differentiation. Specific glycoproteins have been proposed to mediate intercellular adhesion in Dictyostelium discoideum and Polysphondylium pallidum and to account for the species-specificity of adhesion displayed by these species. Recent studies with the inhibitor of N-glycosylation, tunicamycin, and with glycosylation defective mutants suggest that some carbohydrate groups in these glycoproteins play a role in cell adhesion.

Structural comparison of purified dynein proteins with in situ dynein arms

Using the quick-freeze deep-etch technique, we describe the structure of outerarm dynein proteins from Chlamydomonas and Tetrahymena after adsorption to a mica surface, after high-salt dissociation, and after glutaraldehyde fixation, and compare these images to the configuration of outer arms bound to microtubules. After adsorption to mica, the extracted dyneins from both organisms look like three-headed "bouquets", as reported for Tetrahymena by Johnson & Wall (1983b). High magnification images demonstrate that each head carries a slender "stalk" and a long "stem", and that small subunits decorate the stems and create a "flowerpot" domain at the base of the bouquet. Exposure to high salt induces this trimer to dissociate into a two-headed species and a single-headed species; it also stimulates the decorative elements to dissociate from the stems. Dynein is thus constructed on the same general plan as myosin, with large globular heads, narrow stems and additional small subunits that associate with the stems. The splayed-out image of the bouquet appears to be a distortion arising during adsorption to mica since, after brief glutaraldehyde fixation, the three heads remain closely associated as vertices of a triangular unit. In situ, the three heads also adopt this trigonal configuration. Two of the three are visible from the exterior of the axoneme and constitute the bilobed rigor head we described previously (Goodenough & Heuser, 1982). The third head faces the interior of the axoneme where, we propose, it forms the "hook" of the outer arm as seen in thin section. We further propose that the decorative elements associated with the stem coalesce to form the two outer-arm "feet" seen in situ, and that at least one of the in vitro stalks is equivalent to the in situ stalk, which extends from the head to the B microtubule. Deep-etch images of stretched axonemes, partially extracted axonemes, and dynein-decorated brain microtubules indicate that each outer arm, as traditionally viewed, is a hybrid of two dynein molecules: its two feet derive from one molecule, whereas its trigonal head derives from the molecule located distally. The resultant overlapping configuration creates the diagonal "linkers" seen in situ, which correspond to the in vitro stems. Thus, a row of dynein arms is essentially a dynein polymer that extends from the tip to the base of a doublet microtubule, each head riding on its neighbor's feet like a row of circus elephants.(ABSTRACT TRUNCATED AT 400 WORDS)

Enzymatic dissection of the functions of the mouse egg's receptor for sperm

During the course of sperm-egg interaction in mice, zona pellucida glycoprotein ZP3 (approximately equal to 80 kDa) serves as both receptor for sperm (J. D. Bleil and P. M. Wassarman, 1980c, Cell 20, 873-882) and inducer of the acrosome reaction (J. D. Bleil and P. M. Wassarman, 1983, Dev. Biol. 95, 317-324). In this investigation, small ZP3 glycopeptides (approximately equal to 1.5-6 kDa), obtained by extensive digestion of the purified glycoprotein with insoluble Pronase, were assayed for both sperm receptor and acrosome reaction-inducing activities. While ZP3 glycopeptides were virtually as effective as intact ZP3 in inhibiting binding of sperm to eggs in vitro ("receptor activity"), unlike intact ZP3, they failed to induce sperm to undergo the acrosome reaction. The latter was determined by indirect immunofluorescence using a monoclonal antibody directed against the acrosomal cap region of sperm. These results suggest that the sperm receptor activity of ZP3 is dependent only on its carbohydrate components, whereas acrosome reaction-inducing activity is dependent on the polypeptide chain of ZP3 as well.

Characterization of the sperm receptor on the surface of eggs of Strongylocentrotus purpuratus

Eggs of the sea urchins Strongylocentrotus purpuratus and Arbacia punctulata bind sperm with a high degree of species specificity. By use of an in vitro assay that utilizes bindin (the protein from sperm that mediates sperm-egg binding) egg surface-derived glycoconjugates that function as receptors in this adhesion process have been identified and purified. These glycoconjugates are of extraordinarily high molecular weight and exhibit some properties expected for a proteoglycan. The isolated receptors from both species bind to sperm and inhibit fertilization species specifically. Both receptors contain active carbohydrate-rich fragments that can be liberated by proteolytic digestion. The carbohydrate-rich receptor fragment from S. purpuratus is a very high-molecular-weight (greater than 10(6)), negatively charged glycosaminoglycan-like polymer containing fucose, galactosamine, iduronic acid, and sulfate esters. By contrast, the carbohydrate-rich fragment derived from the A. punctulata receptor is of defined molecular weight (6000) and has no net charge. Incubation of acrosome-reacted sperm with nanomolar amounts of the carbohydrate-rich fragments from either species results in inhibition of fertilization, indicating that these receptor fragments retain sperm binding activity. However, studies utilizing heterologous gametes show that the carbohydrate-rich receptor fragments are not species specific in binding. Thus, it appears that although the carbohydrate chains of the receptor are an adhesive element of the receptor, the intact glycoconjugate is required for species-specific binding.

Sexual agglutinin of mating-type minus gametes in Chlamydomonas reinhardii. I. Loss and recovery of agglutinability of gametes treated with EDTA

The effect of EDTA on the mating-type-specific agglutinins located on the flagellar surfaces of Chlamydomonas reinhardii gametes was investigated. The mating-type minus (mt-) gametes lost their agglutinability without apparent loss of motility soon after addition of EDTA at low concentrations (1-2 mM). At the same time, the cells released into the medium agglutinins which can elicit agglutinative responses of mating-type plus (mt+) gametes specifically. When EDTA was neutralized with Mg2+ or removed by centrifugation, the mt- cells quickly replaced agglutinins by protein synthesis: the recovery process was sensitive to cycloheximide, but not to tunicamycin. The EDTA-treated mt+ gametes lost their agglutinins much more slowly than the mt- gametes. The replacement of mt+ agglutinins was inhibited by both cycloheximide and tunicamycin.

Sex-specific binding and inactivation of agglutination factor in Chlamydomonas eugametos

Gametes of opposite mating type (mt (+) and mt (-)) of the green alga Chlamydomonas eugametos agglutinate via their flagella as a prelude to sexual fusion. To quantitate sexual agglutination, an in vitro assay has been developed using (35)S-labeled flagella and the isolated mt (-)agglutination factor. It is shown that not only isolated flagella, but also the mt (-)agglutination factor rapidly bind to the flagella of intact gametes of the opposite mating type. This confirms the role of the mt (-)agglutination factor in determining the sexual agglutinability of mt (-)gametes. As a function of binding, the agglutinative power of the flagella of both mating types is destroyed by a temperature-sensitive process. Likewise, the mt (-)agglutination factor can be completely inactivated.

Chlamydomonas agglutinin conjugated to agarose beads as an in vitro probe of adhesion

Flagellar sexual agglutinins are responsible for the primary recognition and adhesion events of mating in Chlamydomonas reinhardi which culminate in zygotic union of plus and minus gametes. Recent studies in this laboratory have shown the plus agglutinin to be an extremely large (greater than 10(6) D) and asymmetric glycoprotein containing a high proportion of hydroxyproline and serine residues [14, 27, 28]. This paper reports an improved method for in vitro investigations of the adhesive nature of this molecule. Purified agglutinin is covalently attached to an insoluble (Affi-gel 15 agarose bead) support and shown to retain potent agglutination activity when presented to living minus gametes, which rapidly and extensively adhere to the coated bead surface by their flagella. The specificity of the response is documented by the lack of interaction of plus gametes with the immobilized plus agglutinin (IA+). Using this simple yet sensitive bioassay, we have subjected IA+ beads to various enzymatic, chemical and physical treatments and assessed the effects on agglutinin activity. These studies reveal that Chlamydomonas plus agglutinin is sensitive to thermolysin or trypsin digestion, alkaline borohydride reduction, periodate oxidation, thiol reduction and heating at 65 degrees C, but unaffected by treatment with chymotrypsin, endo- or exoglycosidases, or incubation with isolated minus agglutinin. The implications of these results for agglutinin structure and possible functional interactions in initial recognition/adhesion events are discussed.

A flagellar surface glycoprotein mediating cell-substrate interaction in Chlamydomonas

The Chlamydomonas flagellar surface exhibits interesting adhesive properties that are associated with flagellar surface motility. This dynamic surface property can be exhibited as the binding and movement of small polystyrene microspheres or as the interaction of the flagellar surface with a solid substrate followed by whole cell locomotion, termed "gliding". In order to identify flagellar surface proteins that mediate substrate interaction during flagellar surface motility, two immobilized iodination systems were employed that mimic the conditions for flagellar surface motility: small polystyrene microspheres derivatized with lactoperoxidase, and large glass beads derivatized with Iodogen. Use of these iodination conditions resulted in preferential iodination of a high-molecular-weight glycoprotein with apparent molecular weight of 300,000-350,000. These results suggest this glycoprotein as a major candidate for the surface-exposed adhesive component that directly interacts with the substrate and couples the substrate to a system of force transduction presumed to be located within the flagellum.

Topography of Chlamydomonas: fine structure and polypeptide components of the gametic flagellar membrane surface and the cell wall

Surface polypeptide components of the flagellar membrane of Chlamydomonas reinhardi Dang. gametes are identified by their accessibility to in-vivo vectoral labeling by glucose oxidase-coupled lactoperoxidase-dependent (125)I iodination. Vectoral labeling is accomplished without observable adverse effects on cell viability or gametic function. Flagella isolated from labeled wild-type cells carry about 3% of the total incorporated label, which is found by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis to be distributed among 16 identifiable polypeptide bands. The most prominent surface-labeled species migrates in the Mr (relative molecular weight) 350 k region of the gel; each of the remaining iodinated polypeptides, which range in Mr from 25 k to 500 k, carries only a small proportion of incorporated label. To determine which polypeptides are unique to the flagellum and which are contaminants from the cell wall, wild-type profiles were compared with those of mutant strains and of mechanically isolated cell walls. Identification of contaminants was also facilitated by two-dimensional peptide mapping. We conclude that only 11 of the labeled bands are contributed by flagellar polypeptides; the remaining five bands are shown to be contaminants from the cell wall, and additional cell-wall polypeptides are found to co-migrate with flagellar species. A polypeptide designated as a possible membrane tubulin in preliminary studies is shown here to be different from tubulin in its peptide map. The 11 polypeptides assigned as specific flagellar surface components are candidate participants in such biological events as sexual adhesion, flagellar surface motility, and sensory signalling.