Aggregation-dependent turnover of flagellar adhesion molecules in chlamydomonas gametes

Previous studies on flagellar adhesion in chlamydomonas (Snell, W. and S. Roseman. 1979. J. Biol. Chem. 254:10820-10829.) have shown that as gametes adhere to flagella isolated from gametes of the opposite mating type, the adhsiveness of the added flagella but not of the gametes is lost. The studies reported here show that the addition of protein synthesis inhibitors (cycloheximide [CH] or anisomycin) to the medium of such cell- flagella mixtures causes the cells to lose their adhesiveness. This loss, however, occurs only after the cells have interacted with 4-8 flagella/cell and does not occur if the cells are kept in CH (7 h) without aggregating. The availability of an impotent (imp) mating type plus (MT(+)) mutant (provided by U.W. Goodenough), which adheres but is unable to undergo the fusion that normally follows adhesion, made it possible to determine whether a similar loss of adhesiveness occurs in mixtures of matting type minus (mt(-)) and imp mt(+) gametes. In the absence of inhibitor, mt(-) and imp mt(+) gametes adhered to each other (without fusing) for several hours; however, in the presence of CH or anisomycin, the gametes began to de-adhere 35 min after mixing, and, by 90 min, 100 percent of the cells were single again. This effect was reversible, and the rapid turnover of cells were single again. This effect was reversible, and the rapid turnover of molecules involved in adhesion occurred only during adhesion inasmuch as gametes pretreated for 4 h with CH were able to aggregate in CH for the same length of time as nonpretreated cells aggregated in CH. By the addition of CH at various times after the mt(-) and imp mt(+) gametes were mixed, measurements were made of the “pool size” of the molecules involved in adhesion. The pool reached a minimum after 25 min of aggregation, rapidly increased for the next 25 min, and then leveled off at the premixing level. These results suggest that flagellar adhesion in chlamydomonas causes modification of surface molecules (receptors, ligands), which brings about their inactivation and stimulates their replacement.     

A monoclonal antibody that blocks adhesion of Chlamydomonas mt+ gametes

During the mating reaction (fertilization) in the biflagellated alga, Chlamydomonas reinhardtii, mt+ and mt- gametes adhere to each other via their flagella and subsequently fuse to form quadriflagellated zygotes. In the studies reported here, we describe a monoclonal antibody directed against an mt+ flagellar surface molecule. The antibody blocks the adhesiveness of mt+ gametes, isolated mt+ flagella, and detergent extracts thereof. It has no effect on mt- gametes. Cyanogen bromide- activated Sepharose beads derivatized with the antibody bind only mt+ gametes; mt- gametes and mt+ and mt- vegetative cells are unreactive with the derivatized beads. The interaction of mt+ gametes with the beads is dynamic and cells continuously bind, detach, and rebind to the beads. Surprisingly, antibody-derivatized beads that have been incubated with mt+ gametes acquire the ability to bind mt- gametes. Moreover, extraction of the preincubated beads with detergents releases active mt+ adhesion molecules. The evidence suggests that binding of the antibody to the flagellar surface adhesion molecules causes their release from the flagellar surface, possibly mimicking the normal mechanism of flagellar de-adhesion.     

Cell adhesion-dependent inactivation of a soluble protein kinase during fertilization in Chlamydomonas.

Within seconds after the flagella of mt+ and mt- Chlamydomonas gametes adhere during fertilization, their flagellar adenylyl cyclase is activated several fold and preparation for cell fusion is initiated. Our previous studies indicated that early events in this pathway, including control of adenylyl cyclase, are regulated by phosphorylation and dephosphorylation. Here, we describe a soluble, flagellar protein kinase activity that is regulated by flagellar adhesion. A 48-kDa, soluble flagellar protein was consistently phosphorylated in an in vitro assay in flagella isolated from nonadhering mt+ and mt- gametes, but not in flagella isolated from mt+ and mt- gametes that had been adhering for 1 min. Although the 48-kDa protein was present in the flagella isolated from adhering gametes, we demonstrate that its protein kinase was inactivated by flagellar adhesion. Immunoblot analysis and inhibitor studies indicate that the 48-kDa protein in nonadhering gametes is phosphorylated by a protein tyrosine kinase. In vivo experiments showing that the protein tyrosine phosphatase inhibitor sodium orthovanadate inhibits fertilization suggest that protein dephosphorylation may be required for signal transduction. The 48-kDa protein and its protein kinase may be among the first elements of a novel signalling pathway that couples interaction of flagellar adhesion molecules to gamete activation.     

Flagellar adhesion-dependent regulation of Chlamydomonas adenylyl cyclase in vitro: a possible role for protein kinases in sexual signaling

Interactions between adhesion molecules, agglutinins, on the surfaces of the flagella of mt+ and mt- gametes in Chlamydomonas rapidly generate a sexual signal, mediated by cAMP, that prepares the cells for fusion to form a zygote. The mechanism that couples agglutinin interactions to increased cellular levels of cAMP is unknown. In previous studies on the adenylyl cyclase in flagella of a single mating type (i.e., non-adhering flagella) we presented evidence that the gametic form of the enzyme, but not the vegetative form, was regulated by phosphorylation and dephosphorylation (Zhang, Y., E. M. Ross, and W. J. Snell. 1991. J. Biol. Chem. 266:22954-22959; Zhang, Y., and W. J. Snell. 1993. J. Biol. Chem. 268:1786-1791). In the present report we describe studies on regulation of flagellar adenylyl cyclase during adhesion in a cell-free system. The results show that the activity of gametic flagellar adenylyl cyclase is regulated by adhesion in vitro between flagella isolated from mt+ and mt- gametes. After mixing mt+ and mt- flagella together for 15 s in vitro, adenylyl cyclase activity was increased two- to threefold compared to that of the non-mixed (non- adhering), control flagella. This indicates that the regulation of gametic flagellar adenylyl cyclase during the early steps of fertilization is not mediated by signals from the cell body, but is a direct and primary response to interactions between mt+ and mt- agglutinins. By use of this in vitro assay, we discovered that 50 nM staurosporine (a protein kinase inhibitor) blocked adhesion-induced activation of adenylyl cyclase in vitro, while it had no effect on adenylyl cyclase activity of non-adhering gametic flagella. This same low concentration of staurosporine also inhibited adhesion-induced increases in vivo in cellular cAMP and blocked subsequent cellular responses to adhesion. Taken together, our results indicate that flagellar adenylyl cyclase in Chlamydomonas gametes is coupled to interactions between mt+ and mt- agglutinins by a staurosporine- sensitive activity, probably a protein kinase.     

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.     

Regulated targeting of a protein kinase into an intact flagellum. An aurora/Ipl1p-like protein kinase translocates from the cell body into the flagella during gamete activation in chlamydomonas

In the green alga Chlamydomonas reinhardtii flagellar adhesion between gametes of opposite mating types leads to rapid cellular changes, events collectively termed gamete activation, that prepare the gametes for cell-cell fusion. As is true for gametes of most organisms, the cellular and molecular mechanisms that underlie gamete activation are poorly understood. Here we report on the regulated movement of a newly identified protein kinase, Chlamydomonas aurora/Ipl1p-like protein kinase (CALK), from the cell body to the flagella during gamete activation. CALK encodes a protein of 769 amino acids and is the newest member of the aurora/Ipl1p protein kinase family. Immunoblotting with an anti-CALK antibody showed that CALK was present as a 78/80-kDa doublet in vegetative cells and unactivated gametes of both mating types and was localized primarily in cell bodies. In cells undergoing fertilization, the 78-kDa CALK was rapidly targeted to the flagella, and within 5 min after mixing gametes of opposite mating types, the level of CALK in the flagella began to approach levels normally found in the cell body. Protein synthesis was not required for targeting, indicating that the translocated CALK and the cellular molecules required for its movement are present in unactivated gametes. CALK was also translocated to the flagella during flagellar adhesion of nonfusing mutant gametes, demonstrating that cell fusion was not required for movement. Finally, the requirement for flagellar adhesion could be bypassed; incubation of cells of a single mating type in dibutyryl cAMP led to CALK translocation to flagella in gametes but not vegetative cells. These experiments document a new event in gamete activation in Chlamydomonas and reveal the existence of a mechanism for regulated translocation of molecules into an intact flagellum.     

Cell-cell coordination in conjugatingChlamydomonas gametes

Summary During conjugation, complementaryChlamydomonas gametes [mating type plus (mt+) and mating type minus (mt–)] are mutually attached via specific adhesion molecules, called agglutinins, which are located at the surface of the flagella. By these contacts the gametes are stimulated to fuse. It is demonstrated that fusion is preceded by a compulsary sequence of events: first, the flagellar swimming beat is arrested, next the flagellar contact is reinforced and finally, the position of the cell bodies is adjusted to permit fusion. Evidence is presented that each consecutive step of the mating process requires a higher level in cell-cell signalling, which is obtained by the formation of additional agglutinin contacts. It is shown that the mt+ and mt– traverse their conjugation process in synchrony, probably because the two sexes acquire new agglutinin contacts at equal rates. It is proposed that this symmetrical behavior is due to the complementarity of the mt+ and mt– agglutinin molecules. A scenario of the conjugation process inC. eugametos, incorporating the recent findings, is provided.Abbreviations EM electron microscopy - FTA flagellar tip activation - FITC fluorescein isothiocyanate - GA glutaraldehyde - IA vesicle iso-agglutinin vesicle - mAb monoclonal antibody - mt mating type - TRITC tetramethylrhodamine Bisothiocyanate - UrAc uranyl acetate     

Flagellar adhesion between mating type plus and mating type minus gametes activates a flagellar protein-tyrosine kinase during fertilization in Chlamydomonas

When Chlamydomonas gametes of opposite mating type are mixed together, flagellar adhesion through sex-specific adhesion molecules triggers a transient elevation of intracellular cAMP, leading to gamete activation in preparation for cell-cell fusion and zygote formation. Here, we have identified a protein-tyrosine kinase (PTK) activity that is stimulated by flagellar adhesion. We determined that the protein-tyrosine kinase inhibitor genistein inhibited fertilization, and that fertilization was rescued by dibutyryl cAMP, indicating that the genistein-sensitive step was upstream of the increase in cAMP. Incubation with ATP of flagella isolated from non-adhering and adhering gametes followed by SDS-PAGE and immunoblotting with anti-phosphotyrosine antibodies showed that adhesion activated a flagellar PTK that phosphorylated a 105-kDa flagellar protein. Assays using an exogenous protein-tyrosine kinase substrate confirmed that the activated PTK could be detected only in flagella isolated from adhering gametes. Our results indicate that stimulation of the PTK is a very early event during fertilization. Activation of the PTK was blocked when gametes underwent flagellar adhesion in the presence of the protein kinase inhibitor staurosporine, but not in the presence of the cyclic nucleotide-dependent protein kinase inhibitor, H8, which (unlike staurosporine) does not block the increases in cAMP. In addition, incubation of gametes of a single mating type in dibutyryl cAMP failed to activate the PTK. Finally, flagella adhesion between plus and minus fla10-1 gametes, which have a temperature-sensitive lesion in the microtubule motor protein kinesin-II, failed to activate the PTK at elevated temperatures. Our results show that kinesin-II is essential for coupling flagellar adhesion to activation of a flagellar PTK and cAMP generation during fertilization in Chlamydomonas.     

Cell body and flagellar agglutinins in Chlamydomonas reinhardtii: the cell body plasma membrane is a reservoir for agglutinins whose migration to the flagella is regulated by a functional barrier

Fertilization in Chlamydomonas reinhardtii is initiated when gametes of opposite mating types adhere to each other via adhesion molecules (agglutinins) on their flagella. Adhesion leads to loss of active agglutinins from the flagella and recruitment of new agglutinins from a pool associated with the cell body. We have been interested in determining the precise cellular location of the pool and learning more about the relationship between agglutinins in the two domains. In the studies reported here we describe methods for purification of mt+ cell body agglutinins by use of ammonium sulfate precipitation, chromatography (molecular sieve, ion exchange, and hydrophobic interaction), and sucrose gradient centrifugation. About 90% of the total agglutinins were associated with the cell body and the remainder were on the flagella. Cell body agglutinins were indistinguishable from mt+ flagellar agglutinins by SDS-PAGE, elution properties on a hydrophobic interaction column, and in sedimentation properties on sucrose gradients. The nonadhesiveness of cell bodies suggested that the cell body agglutinins would be intracellular, but our results are not consistent with this interpretation. We have demonstrated that brief trypsin treatment of deflagellated gametes destroyed all of the cell body agglutinins and, in addition, we showed that the cell body agglutinins were accessible to surface iodination. These results indicated that C. reinhardtii agglutinins have a novel cellular disposition: active agglutinins, representing approximately 10% of the total cellular agglutinins, are found only on the flagella, whereas the remaining 90% of these molecules are on the external surface of the cell body plasma membrane in a nonfunctional form. This segregation of cell adhesion molecules into distinct membrane domains before gametic interactions has been demonstrated in sperm of multicellular organisms and may be a common mechanism for sequestering these critical molecules until gametes are activated for fusion. In experiments in which surface-iodinated cell bodies were permitted to regenerate new flagella, we found that the agglutinins (as well as the 350,000 Mr, major flagellar membrane protein) on the newly regenerated flagella were iodinated. These results indicate that proteins destined for the flagella can reside on the external surface of the cell body plasma membrane and are recruited onto newly forming flagella as well as onto preexisting flagella during fertilization.     

Tipping and mating-structure activation induced in Chlamydomonas gametes by flagellar membrane antisera

Antisera raised against vegetative and gametic flagella of Chlamydomonas reinhardi have been used to probe dynamic properties of the flagellar membranes. The antisera, which agglutinate cells via their flagella, associate with antigens that are present on both vegetative and gametic membranes and on membranes of both mating types (mt+ and mt-). Gametic cells respond to antibody presentation very differently from vegetative cells, mobilizing even high concentrations of antibody towards the flagellar tips; the possibility is discussed that such "tipping" ability reflects a differentiated gametic property relevant to sexual agglutinability. Gametic cells also respond to antibody agglutination by activating their mating structures, the mt+ reaction involving a rapid polymerization of microfilaments. Several impotent mt+ mutant strains that fail to agglutinate sexually are also activated by the antisera and procede to form zygotes with normal mt- gametes. Fusion does not occur between activated cells of like mating type. Monovalent (Fab) preparations of the antibody fail to activate mt+ gametes, suggesting that the cross-linking properties of the antisera are essential for their ability to mimic, or bypass, sexual agglutination.     

Kinesin-II is required for flagellar sensory transduction during fertilization in Chlamydomonas

The assembly and maintenance of eucaryotic flagella and cilia depend on the microtubule motor, kinesin-II. This plus end-directed motor carries intraflagellar transport particles from the base to the tip of the organelle, where structural components of the axoneme are assembled. Here we test the idea that kinesin-II also is essential for signal transduction. When mating-type plus (mt+) and mating-type minus (mt-) gametes of the unicellular green alga Chlamydomonas are mixed together, binding interactions between mt+ and mt- flagellar adhesion molecules, the agglutinins, initiate a signaling pathway that leads to increases in intracellular cAMP, gamete activation, and zygote formation. A critical question in Chlamydomonas fertilization has been how agglutinin interactions are coupled to increases in intracellular cAMP. Recently, fla10 gametes with a temperature-sensitive defect in FLA10 kinesin-II were found to not form zygotes at the restrictive temperature (32 degrees C). We found that, although the rates and extents of flagellar adhesion in fla10 gametes at 32 degrees C are indistinguishable from wild-type gametes, the cells do not undergo gamete activation. On the other hand, fla10 gametes at 32 degrees C regulated agglutinin location and underwent gamete fusion when the cells were incubated in dibutyryl cAMP, indicating that their capacity to respond to the cAMP signal was intact. We show that the cellular defect in the fla10 gametes at 32 degrees C is a failure to undergo increases in cAMP during flagella adhesion. Thus, in addition to being essential for assembly and maintenance of the structural components of flagella, kinesin-II/intraflagellar transport plays a role in sensory transduction in these organelles.     

Kinetics of adhesion and de-adhesion of Chlamydomonas gametes.

In medium with low nitrogen content, vegetative strains of the unicellular biflagellate alga Chlamydomonas reinhardi form gametes. Mating type plus (mt+) and mating type minus (mt-) gametes adhere via their flagella to give aggregates in which the gametes eventually fuse to form zygotes. A quantitative assay has now been developed which measures aggregation and fusion by use of a Coulter electronic particle counter to determine loss of single gametes as they form aggregates in suspension. Determination of the rate and extent of cell fusion by microscopy agrees with the results obtained with the more rapid and convenient Coulter counter assay. By use of the assay it was found that aggregation and fusion occur at the same rate and to the same extent at 12 degrees C and 25 degrees C. Flagella from one of the mating types can specifically substitute for the corresponding live gametes; more than 70% of the gametes were aggregated and the extent of aggregation was proportional to the number of flagella added, until the ratio of cells to flagella exceeded 2. At 22 degrees C, in the flagella/gamete mixtures, adhesion was complete in less than 5 min, but at 5 to 10 min, gametes began to de-adhere from the clusters and, depending on the number of flagella added, essentially all of the gametes detached from the aggregates in 10 to 50 min. The gametes in such mixtures were fully competent to aggregate again, whereas the flagella recovered from such mixtures were shown by use of a radioactive flagella-binding assay to be inactive with fresh gametes. Inactivation of the flagella was temperature-dependent, was not catalyzed by soluble factors, and required adhesion of flagella to gametes of the opposite mating type. The potential physiological functions of the de-adhesion process are discussed.     

Flagellar membrane agglutination and sexual signaling in the conditional GAM-1 mutant of Chlamydomonas

The temperature-sensitive gametogenesis-defective mutant, gam-1 is sex- limited, expressed only in mating type minus (mt-), and can sexually agglutinate but not fuse at the restrictive temperature (35 degrees C) with gametes of wild type (wt) mt+. Thin-section, freeze-cleave, and scanning electron microscopy reveal that the gam-1 phenotype is dependent on both the temperature at which the cells undergo nitrogen starvation (and therefore gamete formation) and the temperature at which the cells are maintained during the 12 h before mating. Under all conditions of gametogenesis at 35 degrees C, each gam-1 cell produces a normal-appearing membrane-associated mating structure that fails to activate in response to flagellar agglutination. Varying with the conditions of gametogenesis, on the other hand, are the agglutination and signaling properties of the gam-1 flagella. The two mutant phenotypes displayed by gam-1 have been denoted gam-1-I and gam-1-II. An agglutination reaction involving gam-1-I cells does not result in activation of the wt mt+ mating structure. A more stable agglutination reaction, which can result in activation of the wt mt+ mating structure, is characteristic of gam-1-II cells, but because the gam-1 mt- mating sturcture still fails to activate, cell fusion is precluded. We conclude that the gam-1 mutation affects flagellar component(s) involved in establishing an effective, signal-generating agglutination reaction.     

Turnover and transport of agglutinins in conjugatingChlamydomonas gametes

Summary During gamete-gamete adhesion in the unicellular green algaChlamydomonas eugametos, the sexual adhesion molecules or agglutinins that are located on the flagella are subject to tip-oriented migration and rapid inactivation. It is demonstrated that sexual adhesiveness is maintained by incorporation of additional agglutinins, recruited from a cellular pool. The location of this reservoir is unknown but, as indicated by its insensitivity to the chaotropic agent guanidine thiocyanate, it appears to be distinct from the large amount of agglutinins on the plasma membrane of the cell body. By viewing flagella of conjugating gametes in a confocal scanning laser microscope after immuno-labelling of the agglutinins, evidence was obtained for a linear arrangement of the agglutinins in two rows on the flagellar surface. This suggests that after insertion at the base of the flagellum, the agglutinins follow linear tracks to the tip and that the transport system is confined to two longitudinal domains. It is estimated that the half-life of flagellar agglutinins drops from 1–2 h in nonconjugating gametes to 1 min during conjugation, which suggests that after incorporation at the flagellar base, the agglutinins migrate to the tip with a velocity of 100 nm/s. Presumably after arrival at the tip, the molecules are inactivated. It is postulated that rapid turnover and transport of agglutinins are required for optimal signalling between partner gametes.Abbreviations BSA bovine serum albumine - CHI cycloheximide - CSLM confocal scanning laser microscope - GA glutaraldehyde - GTC guanidine thiocyanate - GAM-IgG goat-anti-mouse immuno-globuline - mAb monoclonal antibody - mt mating type - PBS phosphate-buffered saline - SDS sodiumdodecyl sulphate - TRIS tris-(hydroxymethyl)-aminomethane     

Gametic differentiation in Chlamydomonas reinhardtii. III. Cell wall lysis and microfilament-associated mating structure activation in wild- type and mutant strains

Cell fusion between mating type plus (mt+) and minus (mt-) gametes of Chlamydomonas reinhardtii is analyzed structurally and subjected to experimental manipulation. Cell wall lysis, a necessary prelude to fusion, is shown to require flagellar agglutination between competent gametes; glutaraldehyde-fixed gametes ("corpses") of one mating type will elicit both agglutination and cell wall lysis in the opposite mating type, whereas nonagglutinating impotent (imp) mutant strains are without effect. The fusion process is mediated by a narrow fertilization tubule which extends from the mt+ gamete and establishes contact with the mt- gamete. Formation of the tubule requires the "activation" of a specialized mating structure associated with the ml+ cell membrane; activation causes microfilaments to polymerize from the mating structure into the growing fertilization tubule. Mating structure activation is shown to depend on gametic flagellar agglutination; isoagglutination mediated by the lectin concanavalin A has no effect. Gametes carrying the imp-l mt+ mutation are able to agglutinate but not fuse with mt- cells; the imp-l gametes are shown to have structurally defective mating structures that do not generate microfilaments in response to gametic agglutination.     

The iso1 gene of Chlamydomonas is involved in sex determination.

Sexual differentiation in the heterothallic alga Chlamydomonas reinhardtii is controlled by two mating-type loci, mt+ and mt-, which behave as a pair of alleles but contain different DNA sequences. A mutation in the mt minus-linked imp11 gene has been shown previously to convert a minus gamete into a pseudo-plus gamete that expresses all the plus gametic traits except the few encoded by the mt+ locus. Here we describe the iso1 mutation which is unlinked to the mt- locus but is expressed only in minus gametes (sex-limited expression). A population of minus gametes carrying the iso1 mutation behaves as a mixture of minus and pseudo-plus gametes: the gametes isoagglutinate but they do not fuse to form zygotes. Further analysis reveals that individual gametes express either plus or minus traits: a given cell displays one type of agglutinin (flagellar glycoprotein used for sexual adhesion) and one type of mating structure. The iso1 mutation identifies a gene unlinked to the mating-type locus that is involved in sex determination and the repression of plus-specific genes.     

Light dependence of sexual agglutinability in Chlamydomonas eugametos

The mating activity of mating-type plus gametes of Chlamydomonas eugametos depends on light. Cells lost their ability to agglutinate with mating-type minus gametes after a dark period of 30 min. They regained their agglutinability after 10 min exposure to light. Other mating reactions, such as tipping and flagellar tip activation, were not dependent upon light. Since cycloheximide and tunicamycin did not affect the light-induced activation of flagellar agglutinability, no protein synthesis or glycosylation is involved in this process. Equal amounts of biologically active agglutination factor could be extracted from cells placed either in light or in darkness. A minor portion of the active material was found to be located on the flagellar surface of illuminated cells. No active material was found on the flagellar surface of dark-exposed cells, whereas their cell bodies contained the same amount of active material as the cell bodies of illuminated cells. Since a light-induced flow of agglutination factors from the cell body to the flagella could not be detected and dark-exposed cells could be slightly activated by amputation or fixation by glutaraldehyde, we propose that light affects flagellar agglutinability by an in-situ modification of the agglutination factor on the flagella. When mt ⁺ and mt ^- strains were crossed and the progeny examined for light-sensitivity, it was apparent that this phenomenon is not mating type-linked.Abbreviations and symbols FTA flagellar tip activation - mt ^+/- mating type plus or minus - WGA wheat-germ agglutinin     

Elongation of the fertilization tubule in Chlamydomonas: new observations on the core microfilaments and the effect of transient intracellular signals on their structural integrity

Experimental manipulations of gametes of Chlamydomonas reinhardi and ultrastructural observation were used to examine the composition of the microfilaments in the fertilization tubule, their probable mode of formation, and their interaction with intracellular signals. Decoration with myosin subfragment-1 was used to demonstrate that the microfilaments in the fertilization tubule were actin filaments having uniform polarity: Myosin subfragment-1 arrowheads pointed away from the membrane at the tip of the process. Filaments were attached to the cone- shaped "doublet zone" at the base of the process by their pointed ends. Discrete attachment sites for filaments on the surface of the doublet zone were seen in stereo view. To test whether actin polymerization might accompany elongation of the fertilization tubule, mating gametes were exposed to cytochalasin D in an attempt to block actin polymerization. Treatment of mating type "plus" gametes with cytochalasin D prior to and during mating inhibited the appearance of actin filaments in fertilization tubules, suppressed fertilization tubule outgrowth, and lowered mating efficiency from 90 to 15%. The role of signals generated by flagellar adhesion in maintaining the structural integrity of the microfilament-doublet zone complex was examined by correlating flagellar disadhesion with the kinetics of breakdown of the complex. In zygotes, where flagellar disadhesion occurred after cell fusion, the complex disassembled within 3 h after mating. In gametes that had been agglutinated by isolated mating type "minus" flagella, microfilaments and fertilization tubules progressively disassembled over a 3-h time course following flagellar disadhesion. Disassembly of microfilaments was inhibited by maintaining flagellar agglutination, suggesting that signals generated by flagellar adhesion were necessary to maintain microfilaments intact.